LTC3558
Linear USB Battery Charger
with Buck andBuck-Boost Regulators
FEATURESDESCRIPTION

Battery Chargern Standalone USB Charger
n Up to 950mA Charge Current Programmable via
Single Resistor
n HPWR Input Selects 20% or 100% of Programmed
Charge Current
n NTC Input for Temperature Qualiﬁed Charging
n Internal Timer Terminationn Bad Battery Detection
Switching Regulators (Buck and Buck-Boost)n Up to 400mA Output Current per Regulator
n 2.25MHz Constant-Frequency Operationn Power Saving Burst Mode® Operationn Low Proﬁle, 20-Lead, 3mm × 3mm QFN Package

The LTC®3558 is a USB battery charger with dual high efﬁciency switching regulators. The device is ideally suited
to power single-cell Li-Ion/Polymer based handheld applications needing multiple supply rails.
Battery charge current is programmed via the PROG pin
and the HPWR pin with capability up to 950mA of current
at the BAT pin. The CHRG pin allows battery status to bemonitored continuously during the charging process. Aninternal timer controls charger termination.
The part includes monolithic synchronous buck and buckboost regulators that can provide up to 400mA of output
current each and operate at efﬁciencies greater than 90%
over the entire Li-Ion/Polymer battery range. The buckboost regulator can regulate its programmed output voltage
at its rated deliverable current over the entire Li-Ion range
without drop out, increasing battery runtime.

APPLICATIONSThe LTC3558 is offered in a low proﬁle (0.75mm), thermallyenhanced, 20-lead (3mm × 3mm) QFN package.

n

SD/Flash-Based MP3 Players
n Low Power Handheld Applications, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology
Corporation. All other trademarks are the property of their respective owners.

TYPICAL APPLICATIONUSB Charger Plus Buck Regulator and Buck-Boost Regulator
USB (4.3V TO 5.5V)

BAT

VCC

1μF

+

PVIN1
PVIN2

1.74k
PROG

Demo BoardSINGLE
Li-lon CELL
(2.7V TO 4.2V)

10μF
1.2V AT 400mA

NTC

SW1
LTC3558

CHRG

4.7μH

FB1
SWAB2

SUSP

324k

10pF

10μF

649k
2.2μH

DIGITALCONTROL

HPWR

3.3V AT 400mA
SWCD2
VOUT2

MODE
EN1

324k

121k

22μF

33pF
EN2

FB2
105k

GND

EXPOSED
PAD

15k
330pF

10pF

VC2
3558 TA01

3558f

1

LTC3558
ABSOLUTE MAXIMUM RATINGSPIN CONFIGURATION(Note 1)
HPWR

NTC

PROG

CHRG

VCC

TOP VIEW

20 19 18 17 16
15 EN2GND 1

14 VC2BAT 2

13 FB2

21

MODE 312 SUSP

FB1 4

11 VOUT2
7

8

9 10

PVIN2

SWAB2

SWCD2

6
SW1

EN1 5
PVIN1

VCC (Transient);t < 1ms and Duty Cycle < 1%....................... –0.3V to 7V
VCC (Static) .................................................. –0.3V to 6V
BAT, CHRG ................................................... –0.3V to 6V
PROG, SUSP .................................–0.3V to (VCC + 0.3V)
HPWR, NTC................... –0.3V to Max (VCC, BAT) + 0.3V
PROG Pin Current ...............................................1.25mA
BAT Pin Current ..........................................................1A
PVIN1, PVIN2 ..................................–0.3V to (BAT + 0.3V)
EN1, EN2, MODE, VOUT2 .............................. –0.3V to 6V
FB1, SW1 ......................... –0.3V to (PVIN1 + 0.3V) or 6V
FB2, VC2, SWAB2 ............. –0.3V to (PVIN2 + 0.3V) or 6V
SWCD2 ............................–0.3V to (VOUT2 + 0.3V) or 6V
ISW1 ...............................................................600mA DC
ISWAB2, ISWCD2, IVOUT2 ...................................750mA DC
Junction Temperature (Note 2) ............................. 125°C
Operating Temperature Range (Note 3).... –40°C to 85°C
Storage Temperature.............................. –65°C to 125°C

UD PACKAGE
20-LEAD (3mm × 3mm) PLASTIC QFNTJMAX = 125°C, θJA = 68°C/W
EXPOSED PAD (PIN 21) IS GND, MUST BE SOLDERED TO PCBORDER INFORMATIONLEAD FREE FINISH

TAPE AND REELPART MARKINGPACKAGE DESCRIPTIONTEMPERATURE RANGE

LTC3558EUD#PBF

LTC3558EUD#TRPBF

LDCD

20-Lead (3mm × 3mm) Plastic QFN–40°C to 85°C

Consult LTC Marketing for parts speciﬁed with wider operating temperature ranges.
Consult LTC Marketing for information on non-standard lead based ﬁnish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel speciﬁcations, go to: http://www.linear.com/tapeandreel/

3558f

2

LTC3558
ELECTRICAL CHARACTERISTICS The l denotes speciﬁcations that apply over the full operating temperature
range, otherwise speciﬁcations are at TA = 25°C. VCC = 5V, BAT = PVIN1 = PVIN2 = 3.6V, RPROG = 1.74k, unless otherwise noted.

SYMBOLPARAMETERCONDITIONS

MIN

TYP

MAX

UNITS

5.5

V

285
8.5
4.200
4.200
460
92
–3.5
–2.5
–1.5

400
17
4.221
4.235
500
100
–7
–4
–3

μA
μA
V
V
mA
mA
μA
μA
μA

–50
4

–100
4.125

μA
V

Battery Charger
VCC

Input Supply Voltage

IVCC

Battery Charger Quiescent Current(Note 4)
BAT Regulated Output Voltage

VFLOATl

4.3

Standby Mode, Charge TerminatedSuspend Mode, VSUSP = 5V
4.179
4.165
440
84

IBAT

Constant-Current Mode Charge
Current
Battery Drain Current

VUVLO

Undervoltage Lockout Threshold

0°C ≤ TA ≤ 85°Cl
HPWR = 1
HPWR = 0
Standby Mode, Charger Terminated, EN1 = EN2 = 0
Shutdown, VCC < VUVLO, BAT = 4.2V, EN1 = EN2 = 0
Suspend Mode, SUSP = 5V, BAT = 4.2V, EN1 = EN2 = 0
VCC = 0V, EN1 = EN2 = 1, MODE = 1,
FB1 = FB2 = 0.85V, VOUT2 = 3.6V
BAT = 3.5V, VCC Rising

ΔVUVLO

Undervoltage Lockout Hysteresis

BAT = 3.5V

VDUVLOBAT = 4.05V, (VCC – BAT) Falling

VPROG

Differential Undervoltage Lockout
Threshold
Differential Undervoltage Lockout
Hysteresis
PROG Pin Servo Voltage

hPROG

Ratio of IBAT to PROG Pin Current

ITRKL

Trickle Charge Current

BAT < VTRKL

36

VTRKL

Trickle Charge Threshold Voltage

BAT Rising

2.8

ΔVTRKL

Trickle Charge Hysteresis Voltage

ΔVRECHRG

Recharge Battery Threshold Voltage

Threshold Voltage Relative to VFLOATtRECHRG

Recharge Comparator Filter TimeBAT FallingtTERM

Safety Timer Termination Period

BAT = VFLOATtBADBATBad Battery Termination TimeBAT < VTRKL

hC/10

End-of-Charge Indication Current Ratio (Note 5)

tC/10

End-of-Charge Comparator Filter TimeIBAT Falling

2.2

ms

RON(CHG)

Battery Charger Power FET OnResistance (Between VCC and BAT)
Junction Temperature in ConstantTemperature Mode

IBAT = 190mA

500

mΩ

105

°C

VDIS

Cold Temperature Fault Threshold
Voltage
Hot Temperature Fault Threshold
Voltage
NTC Disable Threshold Voltage

INTC

NTC Leakage Current

Rising NTC Voltage
Hysteresis
Falling NTC Voltage
Hysteresis
Falling NTC Voltage
Hysteresis
VNTC = VCC = 5V

ICHG

ΔVDUVLO

TLIM

3.85

200
30

50

mV
70

mV

BAT = 4.05V

130

mV

HPWR = 1
HPWR = 0
BAT < VTRKL

1.000
0.200
0.100
800

V
V
V
mA/mA

46

56

2.9

3

100
–75

–95

mA
V
mV

–115

1.7

mV
ms

3.5

4

4.5

Hour

0.4

0.5

0.6

Hour

0.085

0.1

0.11

mA/mA

NTC
VCOLD
VHOT

75
33.4
l

0.7
–1

76.5
1.6
34.9
1.6
1.7
50

78
36.4
2.7
1

%VCC
%VCC
%VCC
%VCC
%VCC
mV
μA

3558f

3

LTC3558
ELECTRICAL CHARACTERISTICS The l denotes speciﬁcations that apply over the full operating temperature
range, otherwise speciﬁcations are at TA = 25°C. VCC = 5V, BAT = PVIN1 = PVIN2 = 3.6V, RPROG = 1.74k, unless otherwise noted.

SYMBOLPARAMETERCONDITIONS

MIN

TYP

MAX

UNITS

0.4

V

Logic (HPWR, SUSP, CHRG, EN1, EN2, MODE)
VIL

Input Low Voltage

HPWR, SUSP, MODE, EN1, EN2 Pins

VIH

Input High Voltage

HPWR, SUSP, MODE, EN1, EN2 Pins

RDN

Logic Pin Pull-Down Resistance

HPWR, SUSP Pins

VCHRG

CHRG Pin Output Low Voltage

ICHRG

CHRG Pin Input Current

1.2
l

1.9

V
4

6.3

MΩ

ICHRG = 5mA

100

250

mV

BAT = 4.5V, VCHRG = 5V

0

1

μA

4.2

V

400
50
2
8

Buck Switching Regulator
l

PVIN1

Input Supply Voltage

IPVIN1

fOSC

Pulse Skip Input Current
Burst Mode Current
Shutdown Current
Supply Current in UVLO
PVIN1 Falling
PVIN1 Rising
Switching Frequency

ILIMSW1

Peak PMOS Current Limit

VFB1

Feedback Voltage

MODE = 0

IFB1

FB Input Current

FB1 = 0.85V

DMAX1

Maximum Duty Cycle

FB1 = 0V

RPMOS1RDS(ON) of PMOS

ISW1 = 100mA

0.65

Ω

RNMOS1RDS(ON) of NMOS

ISW1 = –100mA

0.75

Ω

RSW1(PD)

SW Pull-Down in Shutdown

13kΩ

PVIN1 UVLO

FB1 = 0.85V, MODE = 0 (Note 6)
FB1 = 0.85V, MODE = 1 (Note 6)
EN1 = 0
PVIN1 = PVIN2 = 2V

2.7

1.91

220
35
0
4
2.45
2.55
2.25

2.70
2.59

μA
μA
μA
μA
V
V
MHz

550

800

1050

mA

780

800

820

mV

50

nA

l
l
l

MODE = 0
l

2.30

–50
l

100

%

Buck-Boost Switching Regulator
l

PVIN2

Input Supply Voltage

IPVIN2

VOUT2(LOW)PWM Input Current
Burst Mode Input Current
Shutdown Current
Supply Current in UVLO
PVIN2 Falling
PVIN2 Rising
Minimum Regulated Buck-Boost VOUT

VOUT2(HIGH)

Maximum Regulated Buck-Boost VOUT

ILIMF2

Forward Current Limit (Switch A)

MODE = 0

IPEAK2(BURST)

Forward Current Limit (Switch A)

ILIMR2

Reverse Current Limit (Switch D)

IZERO2(BURST) Reverse Current Limit (Switch D)

PVIN2 UVLO

2.7

MODE = 0, IOUT = 0A, FB2 = 0.85V (Note 6)
MODE = 1, IOUT = 0A, FB2 = 0.85V (Note 6)
EN2 = 0, IOUT = 0A
PVIN1 = PVIN2 = 2V
l
l

2.30

V

400
30
1
8

μA
μA
μA
μA
V
V
V

2.70
2.75

5.45

5.60

l

580

700

820

mA

MODE = 1

l

180

250

320

mA

MODE = 0

l

325

450

575

mA

MODE = 1

l

–35

0

35

mA

2.7V < PVIN2 < 4.2V
2.75V < VOUT2 < 5.5V

VFB2

Maximum Deliverable Output Current
in Burst Mode Operation
Feedback Servo Voltage

IFB2

FB2 Input Current

FB2 = 0.85V

–50

fOSC

Switching Frequency

MODE = 0

1.91

IMAX2(BURST)

220
20
0
4
2.45
2.55
2.65

4.2

V

50
l

780

mA
800
2.25

820

mV

50

nA

2.59

MHz

3558f

4

LTC3558
ELECTRICAL CHARACTERISTICS The l denotes speciﬁcations that apply over the full operating temperature
range, otherwise speciﬁcations are at TA = 25°C. VCC = 5V, BAT = PVIN1 = PVIN2 = 3.6V, RPROG = 1.74k, unless otherwise noted.

SYMBOLPARAMETERCONDITIONSRDSP(ON)

PMOS RDS(ON)VOUT = 3.6V

MIN

RDSN(ON)NMOS RDS(ON)ILEAK(P)PMOS Switch Leakage

Switches A, D

–1

1

μA

ILEAK(N)NMOS Switch Leakage

Switches B, C

–1

1

μA

DCBUCK(MAX)Maximum Buck Duty Cycle

MODE = 0

DCBOOST(MAX) Maximum Boost Duty Cycle

MODE = 0

l

TYP

MAX

UNITS

0.6

Ω

0.6

Ω

100

%
75

%

tSS2

Soft-Start Time

0.5

ms

ROUT(PD)

VOUT Pull-Down in Shutdown

10kΩ

Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: TJ is calculated from the ambient temperature TA and power
dissipation PD according to the following formula:
TJ = TA + (PD • θJA)
Note 3: The LTC3558E is guaranteed to meet speciﬁcations from 0°C to
85°C. Speciﬁcations over the –40°C to 85°C operating temperature range
are assured by design, characterization and correlation with statisticalprocess controls.

Note 4: VCC supply current does not include current through the PROG pin
or any current delivered to the BAT pin. Total input current is equal to thisspeciﬁcation plus 1.00125 • IBAT where IBAT is the charge current.
Note 5: IC/10 is expressed as a fraction of measured full charge current
with indicated PROG resistor.
Note 6: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.

3558f

5

LTC3558
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.
Suspend State Supply and BAT
Currents vs Temperature
10

4.24

9

4.205

VCC = 5V

4.200

4.23

IVCC

8

Battery Regulation (Float) Voltage
vs Battery Charge Current,
Constant-Voltage Charging

Battery Regulation (Float)
Voltage vs Temperature

4.195
4.22

4.190

5
4

VFLOAT (V)6
VCC = 5V
BAT = 4.2V
SUSP = 5V
EN1 = EN2 = 0V

4.185

4.21

VBAT (V)

CURRENT (μA)

7

4.180

4.20

4.175

4.19

4.170

4.18

4.165

3
2

IBAT

1

4.17

0
–55

4.16
–55 –35

–35

25
5
–15
45
TEMPERATURE (°C)

65

85

4.155
45
25
5
TEMPERATURE (°C)

65

–15

3558 G01

4.150

500

VCC = 5V
495 HPWR= 5V
490 RPROG = 1.74k
485 EN1 = EN2 = 0V

Battery Charge Current vs Ambient
Temperature in Thermal Regulation
500

HPWR = 5V

VCC = 5V
RPROG = 1.74k

450

450

480

350

475

300

300

460

250
200

450

100

445

50
0

4.3 4.4 4.5 4.6 4.7 4.8 4.9 5.0 5.1 5.2 5.3 5.4 5.5
VCC (V)

HPWR = 0V

50
2

2.5

3.5
3
VBAT (V)

4

3.0

1.2
1.0
0.8

3.7
3.6

65

85
3558 G07

VPROG (V)

3.8

25
5
45
–15
TEMPERATURE (°C)

VCC = 5V
HPWR = 5V
RPROG = 1.74k
EN1 = EN2 = 0V

EN1 = EN2 = 0V

2.0
IBAT (μA)

VCC (V)

PROG Voltage
vs Battery Charge Current

BAT = 4.2V

FALLING

5 25 45 65 85 105 125
TEMPERATURE (°C)
3558 G06

2.5

RISING

4.0

3.5–55 –350–55 –35 –15

4.5

Battery Drain Current in Undervoltage
Lockout vs Temperature

BAT = 3.5V

3.9

VCC = 5V
HPWR = 5V
RPROG = 1.74k
EN1 = EN2 = 03558 G05

Battery Charger Undervoltage
Lockout Threshold vs Temperature
4.1

200

100

3558 G04

4.2

250

150

150

455

440

IBAT (mA)

400

350
IBAT (mA)

400

465

0 100 200 300 400 500 600 700 800 900 1000
IBAT (mA)
3558 G03

Battery Charge Current
vs Battery Voltage

500

IBAT (mA)

853558 G02

Battery Charge Current
vs Supply Voltage

470

VCC = 5V
HPWR = 5V
RPROG = 845ΩEN1 = EN2 = 0V

4.160

BAT = 3.6V
1.5

0.6

1.0

0.4

0.5

0.2

0
–55 –35

0
25
5
45
–15
TEMPERATURE (°C)

65853558 G08

050 100 150 200 250 300 350 400 450 500
IBAT (mA)
3558 G093558f

6

LTC3558
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.
Recharge Thresholdvs Temperature
115
111

Battery Charger FET
On-Resistance vs Temperature
1.2

700

VCC = 5V

VCC = 4V
IBAT = 200mA
EN1 = EN2 = 0V

650

107

1.0

95
91

THRESHOLD (V)99

VCC = 5V

1.1

600

103
RDS(ON) (mΩ)

VRECHARGE (mV)

SUSP/HPWR Pin RisingThresholds vs Temperature

550
500
450

0.9
0.8
0.7

87
83
79
75
–55

–35

25
5
–15
45
TEMPERATURE (°C)

65

400

0.6

350

0.5

300
–55

85

–35

–15

5

25

45

65

PERCENT ERROR (%)

1.5

50
ICHRG (mA)

VOLTAGE (mV)

VCC = 5V
BAT = 3.8V

60

100

Timer Accuracy vs Supply Voltage
2.070

VCC = 5V
ICHRG = 5mA

60

40
30

40

20

20

10

0
–55 –35

25
5
45
–15
TEMPERATURE (°C)

65

1

2

4
3
CHRG (V)

3
2
1
0
–1
65

85

3558 G16

4.5

4.7

4.9
VCC (V)

5.1

5.3

5.5

Buck and Buck-Boost Regulator
Switching Frequency vs Temperature

VCC = 5V
RPROG = 0.845k
HPWR = 5V

2.325
FREQUENCY (MHz)BAT (V)

4

CHRG (V)PERCENT ERROR (%)

5

4.3

3558 G15

2.425

1000
800
600
400
200
0
5.0
4.5
4.0
3.5
3.0
5.0
4.0
3.0
2.0
1.0
0

IBAT (mA)

6

–15
5
25
45
TEMPERATURE (°C)

65Complete Charge Cycle
2400mAh Battery

VCC = 5V

–35

03558 G14

Timer Accuracy vs Temperature

–2
–55

0.5

–1.0
0

3558 G13

7

1.0

–0.5

0

85

85
3558 G12CHRG Pin I-V Curve

80

653558 G11

CHRG Pin Output Low Voltage
vs Temperature
120

45
25
5
TEMPERATURE (°C)–15

TEMPERATURE (°C)
3558 G10140

0.4–55 –3585

VCC = 0V, MODE = 0
BAT = PVIN1 = PVIN2
BAT = 4.2V

2.225
BAT = 2.7V

BAT = 3.6V

2.125
2.025
1.925
1.825

0

1

2

4
3
TIME (HOUR)

5

63558 G17

1.725–55 –35 –15

5 25 45 65 85 105 125
TEMPERATURE (°C)
3558 G183558f

7

LTC3558
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.
Buck and Buck-Boost Regulator
Undervoltage Thresholds
vs Temperature
2.700

1200

BAT = PVIN1 = PVIN250

BAT = PVIN1 = PVIN2 = 3.6V

1100

2.650
2.600

RISING

900

2.550
2.500

FALLING

800

2.450

RISING

700

2.400

FALLING

600

2.350

2.250
–55 –35 –15

400–55 –35 –15

5 25 45 65 85 105 125
TEMPERATURE (°C)

PVIN1 = 4.2V
35
PVIN1 = 2.7V
30

20
–55 –35 –15

5 25 45 65 85 105 125
TEMPERATURE (°C)

5 25 45 65 85 105 125
TEMPERATURE (°C)

3558 G20

3558 G19

Buck Regulator Input Current vs
Temperature, Pulse Skip Mode
400

40

25

500

2.300

FB1 = 0.85V

45

1000

VEN (V)INPUT VOLTAGE (V)

Buck Regulator Input Current vs
Temperature, Burst Mode Operation

INPUT CURRENT (μA)

2.750

Buck and Buck-Boost Regulator
Enable Thresholds
vs Temperature

3558 G21

Buck Regulator PMOS RDS(0N)
vs Temperature

Buck Regulator NMOS RDS(0N)
vs Temperature

1300

1300

1200

1200

1100

1100

1000

1000

FB1 = 0.85V

PVIN1 = 4.2V
250
PVIN1 = 2.7V
200
150
100
–55 –35 –15

900
PVIN1 = 2.7V

800

PVIN1 = 4.2V

700

600
500

Buck Regulator Efﬁciency vs ILOAD

1.23

70

1.22

60

1.21

1.230
Burst Mode
OPERATION

1.20

1.220

PULSE SKIP
MODE

1.19

1.210
1.200
1.190

30

1.18

1.180

20

1.17

1.170

1.16

1.160

VOUT = 1.2V
PVIN1 = 2.7V
PVIN1 = 4.2V

0
0.1

1

10
ILOAD (mA)

100

1000

1.15
1

10

100

1000

ILOAD (mA)
3558 G25

ILOAD = 200mA

1.240

VOUT (V)80

VOUT (V)EFFICIENCY (%)

Buck Regulator Line Regulation
1.250

PVIN1 = 3.6V
1.24 VOUT = 1.2V

40

5 25 45 65 85 105 125
TEMPERATURE (°C)
3558 G24

Buck Regulator Load Regulation

Burst Mode
OPERATION

10

8

400
–55 –35 –15

5 25 45 65 85 105 125
TEMPERATURE (°C)

1.25

PULSE SKIP
MODE

PVIN1 = 4.2V

7003558 G23

100

50

800

500

3558 G2290

PVIN1 = 2.7V

900

600

400
–55 –35 –15

5 25 45 65 85 105 125
TEMPERATURE (°C)

RDS(ON) (mΩ)

300

RDS(ON) (mΩ)

INPUT CURRENT (μA)

3503558 G26

1.150
2.700

3.000

3.600
3.300
PVIN1 (V)

3.900

4.200
3558 G27

3558f

LTC3558
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.
Buck Regulator
Pulse Skip Mode Operation

Buck Regulator Start-Up Transient
VOUT
500mV/DIVINDUCTORCURRENT
IL = 200mA/
DIV
EN
2V/DIV

VOUT
20mV/
DIV (AC)

VOUT
20mV/
DIV (AC)SW
2V/DIVSW
2V/DIVINDUCTORCURRENT
IL = 50mA/
DIV

INDUCTORCURRENT
IL = 60mA/
DIV

3558 G28

PVIN1 = 3.8V
LOAD = 10mA

Buck Regulator Transient
Response, Pulse Skip Mode

200ns/DIV

3558 G29

Buck Regulator Transient
Response, Burst Mode Operation

INDUCTORCURRENT
IL = 200mA/
DIV

INDUCTORCURRENT
IL = 200mA/
DIV

VOUT
50mV/
DIV (AC)

VOUT
50mV/
DIV (AC)

LOAD STEP
5mA TO
290mA
50μs/DIV3558 G31

2μs/DIV3558 G30

Buck-Boost Regulator Input
Current vs Temperature
30

Burst Mode OPERATIONFB2 = 0.85V

25

LOAD STEP
5mA TO
290mA
PVIN1 = 3.8V

PVIN1 = 3.8V
LOAD = 60mA

INPUT CURRENT (μA)

PVIN1 = 3.8V
50μs/DIVPULSE SKIP MODE
LOAD = 6Ω

Buck Regulator
Burst Mode Operation

PVIN1 = 3.8V

50μs/DIVPVIN2 = 4.2V
20
PVIN2 = 2.7V
15

103558 G32

5–55 –35 –15

5 25 45 65 85 105 125
TEMPERATURE (°C)
3558 G33

Buck-Boost Regulator Input
Current vs Temperature
500

1100

PVIN2 = 4.2V

250
PVIN2 = 2.7V

600

1000
PVIN2 = 2.7V

900
RDS(ON) (mΩ)

350

550
500
450
400

PVIN2 = 4.2V

3558 G34

600

250

300

200
–55 –35 –15

200–55 –35 –15

5 25 45 65 85 105 125
TEMPERATURE (°C)
3558 G35

PVIN2 = 2.7V

700

400

300

5 25 45 65 85 105 125
TEMPERATURE (°C)

800

500

350

150
100
–55 –35 –15

1200

750
650

400

200

800
700

RDS(ON) (mΩ)

INPUT CURRENT (μA)

PWM MODE
450 FB2 = 0.85V

300

Buck-Boost Regulator NMOSRDS(ON) vs Temperature

Buck-Boost Regulator PMOSRDS(ON) vs Temperature

PVIN2 = 4.2V

5 25 45 65 85 105 125
TEMPERATURE (°C)
3558 G363558f

9

LTC3558
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.
Buck-Boost Efﬁciency
vs Load Current

Buck-Boost Regulator
Efﬁciency vs Input Voltage
100

VOUT = 3.3V

100
ILOAD = 10mA

90

80

85

70

80
ILOAD = 100mA

75
ILOAD = 400mA

70

VOUT = 3.3V

90

ILOAD = 1mA

EFFICIENCY (%)

EFFICIENCY (%)

95

65

3.6V
4.2V

60
50
40
2.7V

30

60

20

Burst Mode
OPERATION
PWM MODE

55
50
2.700

3.000

3.600
3.300
PVIN2 (V)3.900

PVIN2, Burst Mode
OPERATION
PVIN2, PWM MODE

3.6V
4.2V

10
0
0.10

4.200

2.7V

1

10
ILOAD (mA)

1003558 G38

3558 G37

Buck-Boost Regulator
Load Regulation
3.36
3.35

Buck-Boost Regulator
Line Regulation
3.36

PVIN2 = 3.6V

3.35
3.34

3.34

3.33

3.33
3.32

3.32

Burst Mode OPERATION

3.31

PWM MODE

3.30
3.29

VOUT (V)VOUT (V)

1000

3.31
3.30
3.29

3.28

3.28

3.27

3.27

3.26

3.26

3.25

3.25

3.24
0.10

1

10
ILOAD (mA)

100

1000

PWM MODEILOAD = 100mA
Burst Mode
OPERATION
ILOAD = 10mA

3.24
2.700

3.000

3.600
3.300
PVIN2 (V)

4.200

3.900

3558 G40

3558 G39

Buck-Boost Regulator Start-Up
Transient, Burst Mode Operation

Buck-Boost Regulator Start-Up
Transient, PWM Mode

PVIN2 = 3.6V
RLOAD = 332ΩPVIN2 = 3.6V
RLOAD = 16Ω

VOUT
1V/DIV

VOUT
1V/DIVINDUCTORCURRENT
IL = 200mA/DIVINDUCTORCURRENT
IL = 200mA/DIV
EN2
1V/DIV

EN2
1V/DIV
100μs/DIV

3558 G41

100μs/DIV

3558 G42

3558f

10

LTC3558
PIN FUNCTIONSGND (Pin 1): Ground. Connect to Exposed Pad (Pin 21).
BAT (Pin 2): Charge Current Output. Provides charge current to the battery and regulates ﬁnal ﬂoat voltage to 4.2V.

VC2 (Pin 14): Output of the Error Ampliﬁer and Voltage
Compensation Node for the Buck-Boost Regulator. External Type I or Type III compensation (to FB2) connects
to this pin.

MODE (Pin 3): MODE Pin for Switching Regulators. When
held high, both regulators operate in Burst Mode Operation. When held low, the buck regulator operates in pulse
skip mode and the buck-boost regulator operates in PWM
mode. This pin is a high impedance input; do not ﬂoat.

EN2 (Pin 15): Enable Input Pin for the Buck-Boost Regulator. This pin is a high impedance input; do not ﬂoat.
Active high.

FB1 (Pin 4): Buck Regulator Feedback Voltage Pin. Receives feedback by a resistor divider connected acrossthe output.
EN1 (Pin 5): Enable Input Pin for the Buck Regulator. Thispin is a high impedance input; do not ﬂoat. Active high.
SW1 (Pin 6): Buck Regulator Switching Node. External
inductor connects to this node.
PVIN1 (Pin 7): Input Supply Pin for Buck Regulator. Connect to BAT and PVIN2. A single 10μF input decoupling
capacitor to GND is required.
PVIN2 (Pin 8): Input Supply Pin for Buck-Boost Regulator.
Connect to BAT and PVIN1. A single 10μF input decoupling
capacitor to GND is required.
SWAB2 (Pin 9): Switch Node for Buck-Boost Regulator
Connected to the Internal Power Switches A and B. External
inductor connects between this node and SWCD2.
SWCD2 (Pin 10): Switch Node for Buck-Boost Regulator
Connected to the Internal Power Switches C and D. External
inductor connects between this node and SWAB2.
VOUT2 (Pin 11): Regulated Output Voltage for Buck-Boost
Regulator.
SUSP (Pin 12): Suspend Battery Charging Operation. A
voltage greater than 1.2V on this pin puts the battery charger in suspend mode, disables the charger and resets thetermination timer. A weak pull-down current is internallyapplied to this pin to ensure it is low at power-up when
the input is not being driven externally.
FB2 (Pin 13): Buck-Boost Regulator Feedback Voltage
Pin. Receives feedback by a resistor divider connectedacross the output.

HPWR (Pin 16): High Current Battery Charging Enabled.
A voltage greater than 1.2V at this pin programs the
BAT pin current at 100% of the maximum programmed
charge current. A voltage less than 0.4V sets the BAT pin
current to 20% of the maximum programmed charge
current. When used with a 1.74k PROG resistor, this pin
can toggle between low power and high power modes per
USB speciﬁcation. A weak pull-down current is internallyapplied to this pin to ensure it is low at power-up when
the input is not being driven externally.
NTC (Pin 17): Input to the NTC Thermistor Monitoring
Circuit. The NTC pin connects to a negative temperature
coefﬁcient thermistor which is typically co-packaged withthe battery pack to determine if the battery is too hot or too
cold to charge. If the battery temperature is out of range,
charging is paused until the battery temperature re-enters
the valid range. A low drift bias resistor is required from
VCC to NTC and a thermistor is required from NTC to
ground. To disable the NTC function, the NTC pin should
be tied to ground.
PROG (Pin 18): Charge Current Program and Charge
Current Monitor Pin. Charge current is programmed byconnecting a resistor from PROG to ground. When charging in constant-current mode, the PROG pin servos to 1V
if the HPWR pin is pulled high, or 200mV if the HPWR pin
is pulled low. The voltage on this pin always represents
the BAT pin current through the following formula:
IBAT =PROG • 800
RPROG

CHRG (Pin 19): Open-Drain Charge Status Output. The
CHRG pin indicates the status of the battery charger. Fourpossible states are represented by CHRG charging, notcharging (i.e., the charge current is less than one-tenth
3558f

11

LTC3558
PIN FUNCTIONS
of the full-scale charge current), unresponsive battery(i.e., the battery voltage remains below 2.9V after one halfhour of charging) and battery temperature out of range.
CHRG requires a pull-up resistor and/or LED to provide
indication.

VCC (Pin 20): Battery Charger Input. A 1μF decoupling
capacitor to GND is recommended.
Exposed Pad (Pin 21): Ground. The Exposed Pad must
be soldered to PCB ground to provide electrical contact
and rated thermal performance.

BLOCK DIAGRAM
20

VCC

BAT
BODY
MAXER

VCC

1x

800x
BAT

–
16
12

+

19

CHRG
HPWR

TA

CA

LOGIC

SUSP

TDIE
PROG
NTCA

17

5
15
4

18

NTC
NTC REF

3

2

PVIN1

MODE

BATTERY CHARGER

PVIN2
PVIN1

7

EN1
UNDERVOLTAGE
LOCKOUT

EN2

EN

–

FB1
OT
DIE
TEMPERATURE

0.8V

TDIE

+

MODE

CLK

MP
CONTROL
LOGIC

Gm

SW1

6

MN

BUCK REGULATORBANDGAPOSCILLATOR
2.25MHz
13

PVIN2

VREF
= 0.8V

CLK

VOUT2

11

BUCK-BOOST REGULATOR
EN

MODE

CLK

–

FB2

0.8V
14

8

ERROR
+ AMP

VC2

A D
SWAB2

CONTROL
LOGIC

SWCD2

9
10

B C

VC2
GND

EXPOSED PAD

1

213558 BD3558f

12

LTC3558
OPERATION
The LTC3558 is a linear battery charger with a monolithic
synchronous buck regulator and a monolithic synchronous buck-boost regulator. The buck regulator is internally compensated and needs no external compensation
components.
The battery charger employs a constant-current, constantvoltage charging algorithm and is capable of charging asingle Li-Ion battery at charging currents up to 950mA. The
user can program the maximum charging current available
at the BAT pin via a single PROG resistor. The actual BAT
pin current is set by the status of the HPWR pin.

For proper operation, the BAT, PVIN1 and PVIN2 pins
must be tied together, as shown in Figure 1. Current being delivered at the BAT pin is 500mA. Bothswitching regulators are enabled. The sum of theaverage input currents drawn by both switching regulators
is 200mA. This makes the effective battery charging current only 300mA. If the HPWR pin were tied LO, the BAT
pin current would be 100mA. With the switching regulator
conditions unchanged, this would cause the battery todischarge at 100mA.

500mA
USB (5V)

300mA

BAT

VCC

+

200mA
PVIN1

PROG

SUSP
HIGH
HIGH
HIGH
LOW

+

PVIN2
LTC3558

RPROG

10μF
SWAB2
2.2μH

HPWR
EN1
EN2
MODE

SINGLE Li-lon
CELL 3.6V

SWCD2
VOUT2
SW1

VOUT1

3558 F01

Figure 1. For Proper Operation, the BAT, PVIN1 and PVIN2 Pins Must Be Tied Together

APPLICATIONS INFORMATION
Battery Charger Introduction

Input Current vs Charge Current

The LTC3558 has a linear battery charger designed tocharge single-cell lithium-ion batteries. The charger uses
a constant-current/constant-voltage charge algorithmwith a charge current programmable up to 950mA. Additional features include automatic recharge, an internaltermination timer, low-battery trickle charge conditioning,bad-battery detection, and a thermistor sensor input forout of temperature charge pausing.

The battery charger regulates the total current delivered
to the BAT pin; this is the charge current. To calculate thetotal input current (i.e., the total current drawn from the
VCC pin), it is necessary to sum the battery charge current,
charger quiescent current and PROG pin current.

Furthermore, the battery charger is capable of operatingfrom a USB power source. In this application, charge
current can be programmed to a maximum of 100mA or
500mA per USB power speciﬁcations.

Undervoltage Lockout (UVLO)
The undervoltage lockout circuit monitors the input voltage (VCC) and disables the battery charger until VCC rises
above VUVLO (typically 4V). 200mV of hysteresis prevents
oscillations around the trip point. In addition, a differential
undervoltage lockout circuit disables the battery charger
3558f

13

LTC3558
APPLICATIONS INFORMATION
when VCC falls to within VDUVLO (typically 50mV) of theBAT voltage.
Suspend Mode
The battery charger can also be disabled by pulling the
SUSP pin above 1.2V. In suspend mode, the battery
drain current is reduced to 1.5μA and the input current isreduced to 8.5μA.
Charge Cycle Overview
When a battery charge cycle begins, the battery charger
ﬁrst determines if the battery is deeply discharged. If thebattery voltage is below VTRKL, typically 2.9V, an automatic
trickle charge feature sets the battery charge current to
10% of the full-scale value.
Once the battery voltage is above 2.9V, the battery charger
begins charging in constant-current mode. When thebattery voltage approaches the 4.2V required to maintaina full charge, otherwise known as the ﬂoat voltage, thecharge current begins to decrease as the battery charger
switches into constant-voltage mode.
Trickle Charge and Defective Battery Detection
Any time the battery voltage is below VTRKL, the charger
goes into trickle charge mode and reduces the charge
current to 10% of the full-scale current. If the battery
voltage remains below VTRKL for more than 1/2 hour, thecharger latches the bad-battery state, automatically terminates, and indicates via the CHRG pin that the battery wasunresponsive. If for any reason the battery voltage rises
above VTRKL, the charger will resume charging. Since thecharger has latched the bad-battery state, if the battery
voltage then falls below VTRKL again but without rising past
VRECHRG ﬁrst, the charger will immediately assume thatthe battery is defective. To reset the charger (i.e., when
the dead battery is replaced with a new battery), simply
remove the input voltage and reapply it or put the part inand out of suspend mode.
Charge Termination
The battery charger has a built-in safety timer that sets
the total charge time for 4 hours. Once the battery voltage
rises above VRECHRG (typically 4.105V) and the charger

enters constant-voltage mode, the 4-hour timer is started.
After the safety timer expires, charging of the battery will
discontinue and no more current will be delivered.
Automatic Recharge
After the battery charger terminates, it will remain off,
drawing only microamperes of current from the battery.
If the portable product remains in this state long enough,the battery will eventually self discharge. To ensure that thebattery is always topped off, a charge cycle will automatically begin when the battery voltage falls below VRECHRG(typically 4.105V). In the event that the safety timer isrunning when the battery voltage falls below VRECHRG, itwill reset back to zero. To prevent brief excursions below
VRECHRG from resetting the safety timer, the battery voltage
must be below VRECHRG for more than 1.7ms. The charge
cycle and safety timer will also restart if the VCC UVLO or
DUVLO cycles low and then high (e.g., VCC is removed
and then replaced) or the charger enters and then exits
suspend mode.
Programming Charge Current
The PROG pin serves both as a charge current program
pin, and as a charge current monitor pin. By design, thePROG pin current is 1/800th of the battery charge current.
Therefore, connecting a resistor from PROG to groundprograms the charge current while measuring the PROG pin
voltage allows the user to calculate the charge current.
Full-scale charge current is deﬁned as 100% of the constant-current mode charge current programmed by the
PROG resistor. In constant-current mode, the PROG pin
servos to 1V if HPWR is high, which corresponds to charging at the full-scale charge current, or 200mV if HPWR
is low, which corresponds to charging at 20% of the fullscale charge current. Thus, the full-scale charge current
and desired program resistor for a given full-scale charge
current are calculated using the following equations:
ICHG =

800 V
RPROG

RPROG =

800 V
ICHG
3558f

14

LTC3558
APPLICATIONS INFORMATIONIn any mode, the actual battery current can be determinedby monitoring the PROG pin voltage and using the following equation:
IBAT =

PROG
• 800
RPROG

Thermal Regulation
To prevent thermal damage to the IC or surrounding
components, an internal thermal feedback loop will automatically decrease the programmed charge current if thedie temperature rises to approximately 115°C. Thermal
regulation protects the battery charger from excessive
temperature due to high power operation or high ambient
thermal conditions and allows the user to push the limitsof the power handling capability with a given circuit board
design without risk of damaging the LTC3558 or external
components. The beneﬁt of the LTC3558 battery charger
thermal regulation loop is that charge current can be set
according to actual conditions rather than worst-caseconditions with the assurance that the battery charger
will automatically reduce the current in worst-case conditions.
Charge Status Indication
The CHRG pin indicates the status of the battery charger.
Four possible states are represented by CHRG charging,not charging, unresponsive battery and battery temperatureout of range.
The signal at the CHRG pin can be easily recognized as oneof the above four states by either a human or a microprocessor. The CHRG pin, which is an open-drain output, candrive an indicator LED through a current limiting resistor
for human interfacing, or simply a pull-up resistor for
microprocessor interfacing.
To make the CHRG pin easily recognized by both humans
and microprocessors, the pin is either a low for charging,a high for not charging, or it is switched at high frequency(35kHz) to indicate the two possible faults: unresponsive
battery and battery temperature out of range.
When charging begins, CHRG is pulled low and remains
low for the duration of a normal charge cycle. When the

charge current has dropped to below 10% of the full-scalecurrent, the CHRG pin is released (high impedance). If
a fault occurs after the CHRG pin is released, the pin remains high impedance. However, if a fault occurs before
the CHRG pin is released, the pin is switched at 35kHz.
While switching, its duty cycle is modulated between a highand low value at a very low frequency. The low and highduty cycles are disparate enough to make an LED appear
to be on or off thus giving the appearance of “blinking”.
Each of the two faults has its own unique “blink” rate forhuman recognition as well as two unique duty cycles for
microprocessor recognition.
Table 1 illustrates the four possible states of the CHRG
pin when the battery charger is active.
Table 1. CHRG Output Pin

STATUSFREQUENCY

MODULATION(BLINK)
FREQUENCY

Charging

0Hz

0 Hz (Lo-Z)

100%

IBAT < C/10

0Hz

0 Hz (Hi-Z)

0%

NTC Fault

35kHz

1.5Hz at 50%

6.25%, 93.75%

Bad Battery

35kHz

6.1Hz at 50%

12.5%, 87.5%

DUTY CYCLEAn NTC fault is represented by a 35kHz pulse train whose
duty cycle alternates between 6.25% and 93.75% at a
1.5Hz rate. A human will easily recognize the 1.5Hz rate asa “slow” blinking which indicates the out of range battery
temperature while a microprocessor will be able to decode
either the 6.25% or 93.75% duty cycles as an NTC fault.
If a battery is found to be unresponsive to charging (i.e.,
its voltage remains below VTRKL for over 1/2 hour), theCHRG pin gives the battery fault indication. For this fault,
a human would easily recognize the frantic 6.1Hz “fast”
blinking of the LED while a microprocessor would be able
to decode either the 12.5% or 87.5% duty cycles as a badbattery fault.
Although very improbable, it is possible that a duty cycle
reading could be taken at the bright-dim transition (lowduty cycle to high duty cycle). When this happens theduty cycle reading will be precisely 50%. If the duty cycle
reading is 50%, system software should disqualify it andtake a new duty cycle reading.
3558f

15

LTC3558
APPLICATIONS INFORMATIONNTC Thermistor
The battery temperature is measured by placing a negative temperature coefﬁcient (NTC) thermistor close to thebattery pack. The NTC circuitry is shown in Figure 3.
To use this feature, connect the NTC thermistor, RNTC,
between the NTC pin and ground, and a bias resistor, RNOM,
from VCC to NTC. RNOM should be a 1% resistor with avalue equal to the value of the chosen NTC thermistor at
25°C (R25). A 100k thermistor is recommended since
thermistor current is not measured by the battery charger
and its current will have to be considered for compliancewith USB speciﬁcations.
The battery charger will pause charging when the resistance of the NTC thermistor drops to 0.54 times thevalue of R25 or approximately 54k (for a Vishay “Curve1” thermistor, this corresponds to approximately 40°C). If
the battery charger is in constant-voltage mode, the safety
timer will pause until the thermistor indicates a return toa valid temperature.
As the temperature drops, the resistance of the NTC
thermistor rises. The battery charger is also designed
to pause charging when the value of the NTC thermistor
increases to 3.25 times the value of R25. For a Vishay“Curve 1” thermistor, this resistance, 325k, corresponds
to approximately 0°C. The hot and cold comparators eachhave approximately 3°C of hysteresis to prevent oscillation
about the trip point. Grounding the NTC pin disables all
NTC functionality.

DUVLO, UVLO AND SUSPENDDISABLE MODE
NO

POWER
ON

IF SUSP < 0.4V AND
VCC > 4V AND
VCC > BAT + 130mV?

CHRG HIGH IMPEDANCE

YES
FAULT
NTC FAULTSTANDBY MODE

BATTERY CHARGING SUSPENDED
CHRG PULSESNO CHARGE CURRENTCHRG HIGH IMPEDANCENO FAULTBAT b 2.9V

TRICKLE CHARGE MODE
1/10 FULL CHARGE CURRENTCHRG STRONG PULL-DOWN
30 MINUTE TIMER BEGINS

2.9V < BAT < 4.105V
BAT > 2.9V

CONSTANT CURRENT MODE
FULL CHARGE CURRENTCHRG STRONG PULL-DOWN

4-HOUR
TIMEOUT

30 MINUTE
TIMEOUT

DEFECTIVE BATTERYNO CHARGE CURRENTCHRG PULSESCONSTANT VOLTAGE MODE
4-HOUR TERMINATION TIMER
BEGINSBAT DROPS BELOW 4.105V
4-HOUR TERMINATION TIMER RESETS
3558 F02

Figure 2. State Diagram of Battery Charger Operation
3558f

16

LTC3558
APPLICATIONS INFORMATION
Alternate NTC Thermistors and Biasing
The battery charger provides temperature qualiﬁed
charging if a grounded thermistor and a bias resistor areconnected to the NTC pin. By using a bias resistor whose
value is equal to the room temperature resistance of thethermistor (R25) the upper and lower temperatures are
pre-programmed to approximately 40°C and 0°C, respectively (assuming a Vishay “Curve 1” thermistor).
The upper and lower temperature thresholds can be adjusted by either a modiﬁcation of the bias resistor value
or by adding a second adjustment resistor to the circuit.
If only the bias resistor is adjusted, then either the upperor the lower threshold can be modiﬁed but not both. The
other trip point will be determined by the characteristicsof the thermistor. Using the bias resistor in addition to an
adjustment resistor, both the upper and the lower temperature trip points can be independently programmed withthe constraint that the difference between the upper andlower temperature thresholds cannot decrease. Examplesof each technique are given below.
NTC thermistors have temperature characteristics which
are indicated on resistance-temperature conversion tables.
The Vishay-Dale thermistor NTHS0603N011-N1003F, usedin the following examples, has a nominal value of 100k
and follows the Vishay “Curve 1” resistance-temperature
characteristic.
In the explanation below, the following notation is used.
R25 = Value of the thermistor at 25°C
RNTC|COLD = Value of thermistor at the cold trip point
RNTC|HOT = Value of the thermistor at the hot trip point
rCOLD = Ratio of RNTC|COLD to R25
rHOT = Ratio of RNTC|HOT to R25
RNOM = Primary thermistor bias resistor (see Figure 3)
R1 = Optional temperature range adjustment resistor (seeFigure 4)
The trip points for the battery charger’s temperature qualiﬁcation are internally programmed at 0.349 • VCC for the
hot threshold and 0.765 • VCC for the cold threshold.

Therefore, the hot trip point is set when:
RNTCHOT|RNOM + RNTCHOT|• VCC = 0.349 • VCC

and the cold trip point is set when:
RNTC|COLD
RNOM + RNTC|COLD• VCC = 0.765 • VCC

Solving these equations for RNTC|COLD and RNTC|HOT
results in the following:
RNTC|HOT = 0.536 • RNOMandRNTC|COLD = 3.25 • RNOM
By setting RNOM equal to R25, the above equations result
in rHOT = 0.536 and rCOLD = 3.25. Referencing these ratios
to the Vishay Resistance-Temperature Curve 1 chart gives
a hot trip point of about 40°C and a cold trip point of about
0°C. The difference between the hot and cold trip points
is approximately 40°C.
By using a bias resistor, RNOM, different in value from
R25, the hot and cold trip points can be moved in either
direction. The temperature span will change somewhat due
to the nonlinear behavior of the thermistor. The following
equations can be used to easily calculate a new value forthe bias resistor:r
RNOM = HOT • R25
0.536
r
RNOM = COLD • R25
3.25
where rHOT and rCOLD are the resistance ratios at the desired hot and cold trip points. Note that these equations
are linked. Therefore, only one of the two trip points canbe chosen, the other is determined by the default ratios
designed in the IC. Consider an example where a 60°C
hot trip point is desired.
From the Vishay Curve 1 R-T characteristics, rHOT is 0.2488
at 60°C. Using the above equation, RNOM should be set
3558f

17

LTC3558
APPLICATIONS INFORMATION
to 46.4k. With this value of RNOM, the cold trip point isabout 16°C. Notice that the span is now 44°C rather thanthe previous 40°C.
The upper and lower temperature trip points can be independently programmed by using an additional bias resistor
as shown in Figure 4. The following formulas can be used
to compute the values of RNOM and R1:r–r
RNOM = COLD HOT • R25
2.714

0.765 • VCC(NTC RISING)

17

R1 = 0.536 • 105k – 0.4368 • 100k = 12.6k

20

–
+

NTC

3.266 – 0.4368• 100k = 104.2k
2.714

the nearest 1% value is 105k.

NTC BLOCK

VCC

RNOM
100k

RNOM =

the nearest 1% value is 12.7k. The ﬁnal solution is shownin Figure 4 and results in an upper trip point of 45°C anda lower trip point of 0°C.

R1 = 0.536 • RNOM – rHOT • R25

20

For example, to set the trip points to 0°C and 45°C witha Vishay Curve 1 thermistor choose:

VCC
0.765 • VCC(NTC RISING)

RNOM
105k
TOO_COLD
17

–
+

NTC

TOO_COLD

R1
12.7k

RNTC
100k

–

–
0.349 • VCC(NTC FALLING)

+

RNTC
100k

TOO_HOT

0.349 • VCC(NTC FALLING)

+

+

+

NTC_ENABLE

NTC_ENABLE
0.017 • VCC(NTC FALLING)

TOO_HOT–

0.017 • VCC(NTC FALLING)
3558 F03

Figure 3. Typical NTC Thermistor Circuit–
3558 F04

Figure 4. NTC Thermistor Circuit with Additional Bias Resistor

3558f

18

LTC3558
APPLICATIONS INFORMATION
USB and Wall Adapter Power
Although the battery charger is designed to draw powerfrom a USB port to charge Li-Ion batteries, a wall adapter
can also be used. Figure 5 shows an example of how to
combine wall adapter and USB power inputs. A P-channel
MOSFET, MP1, is used to prevent back conduction into
the USB port when a wall adapter is present and Schottky
diode, D1, is used to prevent USB power loss through the
1k pull-down resistor.
Typically, a wall adapter can supply signiﬁcantly morecurrent than the 500mA-limited USB port. Therefore, anN-channel MOSFET, MN1, and an extra program resistor areused to increase the maximum charge current to 950mA
when the wall adapter is present.

5V WALL
ADAPTER
950mA ICHG
USB
POWER
500mA ICHG

BAT
BATTERY
CHARGER
VCC

MP1

PROG
MN1 1.65k

TA = 105°C – PDθ JA

TA = 105°C – ( VCC – VBAT ) • IBAT • θ JA
Example: Consider an LTC3558 operating from a USB port
providing 500mA to a 3.5V Li-Ion battery. The ambient
temperature above which the LTC3558 will begin to reducethe 500mA charge current is approximately:
TA = 105°C – ( 5V – 3.5V ) • ( 500mA ) • 68°C / W
TA = 105°C – 0.75W • 68°C / W = 105°C – 51°C
TA = 54°C

IBAT
D1

current. It is not necessary to perform any worst-casepower dissipation scenarios because the LTC3558 willautomatically reduce the charge current to maintain thedie temperature at approximately 105°C. However, theapproximate ambient temperature at which the thermal
feedback begins to protect the IC is:+

Li-IonBATTERY

The LTC3558 can be used above 70°C, but the charge current will be reduced from 500mA. The approximate current
at a given ambient temperature can be calculated:
IBAT =

1.74k

1k3558 F05

Figure 5. Combining Wall Adapter and USB PowerPower Dissipation
The conditions that cause the LTC3558 to reduce charge
current through thermal feedback can be approximatedby considering the power dissipated in the IC. For highcharge currents, the LTC3558 power dissipation isapproximately:
PD = ( VCC – VBAT ) • IBAT
where PD is the power dissipated, VCC is the input supply
voltage, VBAT is the battery voltage, and IBAT is the charge

105°C – TA
( VCC – VBAT ) • θJA

Using the previous example with an ambient temperature of 88°C, the charge current will be reduced toapproximately:
IBAT =

105°C – 88°C
17°C=
(5V – 3.5V ) • 68°C / W 102°C / A

IBAT = 167mA
Furthermore, the voltage at the PROG pin will change
proportionally with the charge current as discussed in
the Programming Charge Current section.
It is important to remember that LTC3558 applications do
not need to be designed for worst-case thermal conditions
since the IC will automatically reduce power dissipation
when the junction temperature reaches approximately
105°C.

3558f

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LTC3558
APPLICATIONS INFORMATION
Battery Charger Stability Considerations
The LTC3558 battery charger contains two control loops: theconstant-voltage and constant-current loops. The constantvoltage loop is stable without any compensation when abattery is connected with low impedance leads. Excessive
lead length, however, may add enough series inductanceto require a bypass capacitor of at least 1.5μF from BAT
to GND. Furthermore, a 4.7μF capacitor with a 0.2Ω to 1Ω
series resistor from BAT to GND is required to keep ripple
voltage low when the battery is disconnected.
High value capacitors with very low ESR (especiallyceramic) reduce the constant-voltage loop phase margin,
possibly resulting in instability. Ceramic capacitors up to
22μF may be used in parallel with a battery, but larger
ceramics should be decoupled with 0.2Ω to 1Ω of series
resistance.
In constant-current mode, the PROG pin is in the feedback
loop, not the battery. Because of the additional pole createdby the PROG pin capacitance, capacitance on this pin must
be kept to a minimum. With no additional capacitance on
the PROG pin, the charger is stable with program resistor
values as high as 25K. However, additional capacitance on
this node reduces the maximum allowed program resistor. The pole frequency at the PROG pin should be kept
above 100kHz. Therefore, if the PROG pin is loaded with acapacitance, CPROG, the following equation should be used
to calculate the maximum resistance value for RPROG:
RPROG ≤

1

Average, rather than instantaneous, battery current may beof interest to the user. For example, if a switching power
supply operating in low-current mode is connected inparallel with the battery, the average current being pulled
out of the BAT pin is typically of more interest than the
instantaneous current pulses. In such a case, a simple RC
ﬁlter can be used on the PROG pin to measure the average
battery current as shown in Figure 6. A 10k resistor has
been added between the PROG pin and the ﬁlter capacitor
to ensure stability.
USB Inrush Limiting
When a USB cable is plugged into a portable product,
the inductance of the cable and the high-Q ceramic input
capacitor form an L-C resonant circuit. If there is not much
impedance in the cable, it is possible for the voltage at
the input of the product to reach as high as twice theUSB voltage (~10V) before it settles out. In fact, due tothe high voltage coefﬁcient of many ceramic capacitors(a nonlinearity), the voltage may even exceed twice theUSB voltage. To prevent excessive voltage from damaging the LTC3558 during a hot insertion, the soft connect
circuit in Figure 7 can be employed.
In the circuit of Figure 7, capacitor C1 holds MP1 off
when the cable is ﬁrst connected. Eventually C1 begins
to charge up to the USB input voltage applying increasinggate support to MP1. The long time constant of R1 andC1 prevents the current from building up in the cable too
fast thus dampening out any resonant overshoot.

5

2π • 10 • CPROG
MP1
Si2333VCC
LTC3558
10k
PROG
GND

RPROG

CFILTERCHARGE
CURRENTMONITORCIRCUITRY

5V USB
INPUT

C1
100nF
USB CABLE
R1
40k

C2
10μF

LTC3558

GND
3558 F06
3558 F07

Figure 6. Isolated Capacitive Load on PROG Pin and Filtering

Figure 7. USB Soft Connect Circuit

3558f

20

LTC3558
APPLICATIONS INFORMATION
Buck Switching Regulator General Information

Buck Switching Regulator
Output Voltage Programming

The LTC3558 contains a 2.25MHz constant-frequency
current mode buck switching regulator that can provide
up to 400mA. The switcher can be programmed for aminimum output voltage of 0.8V and can be used to powera microcontroller core, microcontroller I/O, memory orother logic circuitry. The regulator supports 100% duty
cycle operation (dropout mode) when the input voltage
drops very close to the output voltage and is also capableof operating in Burst Mode operation for highest efﬁciencies at light loads (Burst Mode operation is pin selectable).
The buck switching regulator also includes soft-start tolimit inrush current when powering on, short-circuit current protection, and switch node slew limiting circuitry to
reduce radiated EMI.

The buck switching regulator can be programmed for
output voltages greater than 0.8V. The output voltage
for the buck switching regulator is programmed using a
resistor divider from the switching regulator output connected to its feedback pin (FB1), as shown in Figure 8,
such that:
VOUT = 0.8(1 + R1/R2)
Typical values for R1 are in the range of 40k to 1M. The
capacitor CFB cancels the pole created by feedback resistors and the input capacitance of the FB pin and also
helps to improve transient response for output voltages
much greater than 0.8V. A variety of capacitor sizes canbe used for CFB but a value of 10pF is recommended formost applications. Experimentation with capacitor sizes
between 2pF and 22pF may yield improved transient
response if so desired by the user.

A MODE pin sets the buck switching regulator in Burst
Mode operation or pulse skip operating mode. The regulator is enabled individually through its enable pin. The buckregulator input supply (PVIN1) should be connected to thebattery pin (BAT) and PVIN2. This allows the undervoltage
lockout circuit on the BAT pin to disable the buck regulators
when the BAT voltage drops below 2.45V. Do not drive thebuck switching regulator from a voltage other than BAT.
A 10μF decoupling capacitor from the PVIN1 pin to GND
is recommended.

Buck Switching Regulator Operating Modes
The buck switching regulator includes two possible operating modes to meet the noise/power needs of a variety
of applications.
In pulse skip mode, an internal latch is set at the start ofevery cycle, which turns on the main P-channel MOSFET

PVIN
EN

MP
PWM
CONTROL

MODE

SW

MN

L

VOUT
CO

CFB
R1

FB
0.8V
GND

R2

3558 F08

Figure 8. Buck Converter Application Circuit

3558f

21

LTC3558
APPLICATIONS INFORMATION
switch. During each cycle, a current comparator compares
the peak inductor current to the output of an error ampliﬁer.
The output of the current comparator resets the internallatch, which causes the main P-channel MOSFET switch toturn off and the N-channel MOSFET synchronous rectiﬁer
to turn on. The N-channel MOSFET synchronous rectiﬁer
turns off at the end of the 2.25MHz cycle or if the current
through the N-channel MOSFET synchronous rectiﬁer
drops to zero. Using this method of operation, the error
ampliﬁer adjusts the peak inductor current to deliver therequired output power. All necessary compensation isinternal to the buck switching regulator requiring only asingle ceramic output capacitor for stability. At light loads
in pulse skip mode, the inductor current may reach zeroon each pulse which will turn off the N-channel MOSFET
synchronous rectiﬁer. In this case, the switch node (SW1)goes high impedance and the switch node voltage will“ring”. This is discontinuous operation, and is normal behavior for a switching regulator. At very light loads in pulse
skip mode, the buck switching regulator will automaticallyskip pulses as needed to maintain output regulation. At
high duty cycle (VOUT > PVIN1 /2) in pulse skip mode, it ispossible for the inductor current to reverse causing thebuck converter to switch continuously. Regulation andlow noise operation are maintained but the input supply
current will increase to a couple mA due to the continuous
gate switching.
During Burst Mode operation, the buck switching regulator automatically switches between ﬁxed frequency PWM
operation and hysteretic control as a function of the load
current. At light loads the buck switching regulator controls
the inductor current directly and use a hysteretic control
loop to minimize both noise and switching losses. During
Burst Mode operation, the output capacitor is charged to a
voltage slightly higher than the regulation point. The buckswitching regulator then goes into sleep mode, during
which the output capacitor provides the load current. In
sleep mode, most of the switching regulator’s circuitry is

powered down, helping conserve battery power. Whenthe output voltage drops below a pre-determined value,
the buck switching regulator circuitry is powered on andanother burst cycle begins. The sleep time decreases as theload current increases. Beyond a certain load current point(about 1/4 rated output load current) the buck switchingregulator will switch to a low noise constant-frequency
PWM mode of operation, much the same as pulse skip
operation at high loads. For applications that can toleratesome output ripple at low output currents, Burst Modeoperation provides better efﬁciency than pulse skip atlight loads.
The buck switching regulator allows mode transition onthe-ﬂy, providing seamless transition between modes evenunder load. This allows the user to switch back and forth
between modes to reduce output ripple or increase low
current efﬁciency as needed. Burst Mode operation is set
by driving the MODE pin high, while pulse skip mode isachieved by driving the MODE pin low.
Buck Switching Regulator in Shutdown
The buck switching regulator is in shutdown when notenabled for operation. In shutdown, all circuitry in the buckswitching regulator is disconnected from the regulator input
supply, leaving only a few nanoamps of leakage pulled to
ground through a 13k resistor on the switch (SW1) pin
when in shutdown.
Buck Switching Regulator Dropout Operation
It is possible for the buck switching regulator’s input voltage to approach its programmed output voltage (e.g., abattery voltage of 3.4V with a programmed output voltageof 3.3V). When this happens, the PMOS switch duty cycle
increases until it is turned on continuously at 100%. In this
dropout condition, the respective output voltage equals theregulator’s input voltage minus the voltage drops acrossthe internal P-channel MOSFET and the inductor.

3558f

22

LTC3558
APPLICATIONS INFORMATION
Buck Switching Regulator Soft-Start Operation

Buck Switching Regulator Inductor Selection

Soft-start is accomplished by gradually increasing the peak
inductor current for each switching regulator over a 500μs
period. This allows an output to rise slowly, helping minimize the battery in-rush current required to charge up theregulator’s output capacitor. A soft-start cycle occurs when
the buck switcher ﬁrst turns on, or after a fault condition
has occurred (thermal shutdown or UVLO). A soft-start
cycle is not triggered by changing operating modes using
the MODE pin. This allows seamless output operation whentransitioning between operating modes.

The buck switching regulator is designed to work withinductors in the range of 2.2μH to 10μH, but for mostapplications a 4.7μH inductor is suggested. Larger value
inductors reduce ripple current which improves output
ripple voltage. Lower value inductors result in higher
ripple current which improves transient response time.
To maximize efﬁciency, choose an inductor with a low DC
resistance. For a 1.2V output efﬁciency is reduced about 2%
for every 100mΩ series resistance at 400mA load current,
and about 2% for every 300mΩ series resistance at 100mA
load current. Choose an inductor with a DC current rating
at least 1.5 times larger than the maximum load current toensure that the inductor does not saturate during normal
operation. If output short-circuit is a possible condition
the inductor should be rated to handle the maximum peakcurrent speciﬁed for the buck regulators.

Buck Switching Regulator
Switching Slew Rate Control
The buck switching regulator contains circuitry to limit
the slew rate of the switch node (SW1). This circuitry isdesigned to transition the switch node over a period of acouple of nanoseconds, signiﬁcantly reducing radiatedEMI and conducted supply noise while maintaining highefﬁciency.
Buck Switching Regulator Low Supply Operation
An undervoltage lockout (UVLO) circuit on PVIN1 shuts
down the step-down switching regulators when BAT drops
below 2.45V. This UVLO prevents the buck switching regulator from operating at low supply voltages where loss ofregulation or other undesirable operation may occur.

Different core materials and shapes will change the size/current and price/current relationship of an inductor. Toroid orshielded pot cores in ferrite or permalloy materials are small
and don’t radiate much energy, but generally cost morethan powdered iron core inductors with similar electricalcharacteristics. Inductors that are very thin or have a verysmall volume typically have much higher DCR losses, andwill not give the best efﬁciency. The choice of which styleinductor to use often depends more on the price vs size,
performance, and any radiated EMI requirements than onwhat the buck regulator requires to operate.
The inductor value also has an effect on Burst Modeoperation. Lower inductor values will cause Burst Modeswitching frequency to increase.

3558f

23

LTC3558
APPLICATIONS INFORMATION
Table 2 shows several inductors that work well with the
LTC3558 buck switching regulator. These inductors offera good compromise in current rating, DCR and physicalsize. Consult each manufacturer for detailed informationon their entire selection of inductors.
Buck Switching Regulator
Input/Output Capacitor Selection
Low ESR (equivalent series resistance) ceramic capacitors
should be used at switching regulator outputs as well as
the switching regulator input supply. Ceramic capacitor
dielectrics are a compromise between high dielectric
constant and stability versus temperature and versus
DC bias voltage. The X5R/X7R dielectrics offer the bestcompromise with high dielectric constant and acceptableperformance over temperature and under bias. Do not
use Y5V dielectrics. A 10μF output capacitor is sufﬁcientfor most applications. For good transient response andstability the output capacitor should retain at least 4μF
of capacitance over operating temperature and bias voltage. The buck switching regulator input supply should be
bypassed with a 10μF capacitor. Consult manufacturer
for detailed information on their selection and speciﬁcations of ceramic capacitors. Many manufacturers now
offer very thin (< 1mm tall) ceramic capacitors ideal foruse in height-restricted designs. Table 3 shows a list of
several ceramic capacitor manufacturers.
Table 3: Recommended Ceramic Capacitor Manufacturers
AVX(803) 448-9411

www.avxcorp.com

Murata(714) 852-2001

www.murata.com

Taiyo Yuden(408) 537-4150

www.t-yuden.com

TDK(888) 835-6646

www.tdk.com

Table 2. Recommended Inductors for Buck Switching Regulators
INDUCTOR TYPE

L
(μH)

MAX IDC(A)MAX DCR(mΩ)

SIZE IN mm(L × W × H)

MANUFACTURER

DE2818C
DE2812C

4.7
4.7

1.25
1.15

72*
130*

3 × 2.8 × 1.8
3 × 2.8 × 1.2

Toko
www.toko.com

CDRH3D16

4.7

0.9

110

4 × 4 × 1.8

Sumidawww.sumida.com

SD3118SD3112

4.7
4.7

1.3
0.8

162
246

3.1 × 3.1 × 1.8
3.1 × 3.1 × 1.2

Cooper
www.cooperet.com

LPS3015

4.7

1.1

200

3 × 3 × 1.5

Coilcraftwww.coilcraft.com*Typical DCR

3558f

24

LTC3558
APPLICATIONS INFORMATION
Buck-Boost Switching Regulator
The LTC3558 contains a 2.25MHz constant-frequency,
voltage mode, buck-boost switching regulator. The regulator provides up to 400mA of output load current. The
buck-boost switching regulator can be programmed for aminimum output voltage of 2.75V and can be used to powera microcontroller core, microcontroller I/O, memory, disk
drive, or other logic circuitry. To suit a variety of applications, different mode functions allow the user to trade off
noise for efﬁciency. Two modes are available to control theoperation of the buck-boost regulator. At moderate to heavyloads, the constant-frequency PWM mode provides theleast noise switching solution. At lighter loads, Burst Modeoperation may be selected. Regulation is maintained by anerror ampliﬁer that compares the divided output voltage
with a reference and adjusts the compensation voltage
accordingly until the FB2 voltage has stabilized at 0.8V. The
buck-boost switching regulator also includes soft-start tolimit inrush current and voltage overshoot when powering
on, short-circuit current protection, and switch node slew
limiting circuitry for reduced radiated EMI.
Buck-Boost Regulator PWM Operating ModeIn PWM mode, the voltage seen at the feedback node iscompared to a 0.8V reference. From the feedback voltage,
an error ampliﬁer generates an error signal seen at theVC2 pin. This error signal controls PWM waveforms thatmodulate switches A (input PMOS), B (input NMOS), C(output NMOS), and D (output PMOS). Switches A and
B operate synchronously, as do switches C and D. If the
input voltage is signiﬁcantly greater than the programmed
output voltage, then the regulator will operate in buckmode. In this case, switches A and B will be modulated,with switch D always on (and switch C always off), to stepdown the input voltage to the programmed output. If the
input voltage is signiﬁcantly less than the programmed
output voltage, then the converter will operate in boost
mode. In this case, switches C and D are modulated, withswitch A always on (and switch B always off), to step up
the input voltage to the programmed output. If the input
voltage is close to the programmed output voltage, thenthe converter will operate in four-switch mode. While
operating in four-switch mode, switches turn on as per
the following sequence: switches A and D → switches A
and C → switches B and D → switches A and D.
Buck-Boost Regulator Burst Mode Operation
In Burst Mode operation, the switching regulator uses a
hysteretic feedback voltage algorithm to control the output
voltage. By limiting FET switching and using a hysteretic
control loop switching losses are greatly reduced. In
this mode, output current is limited to 50mA. While in
Burst Mode operation, the output capacitor is charged
to a voltage slightly higher than the regulation point. The
buck-boost converter then goes into a SLEEP state, during which the output capacitor provides the load current.
The output capacitor is charged by charging the inductor
until the input current reaches 250mA typical, and thendischarging the inductor until the reverse current reaches
0mA typical. This process of bursting current is repeateduntil the feedback voltage has charged to the reference
voltage plus 6mV (806mV typical). In the SLEEP state,most of the regulator’s circuitry is powered down, helping
to conserve battery power. When the feedback voltage
drops below the reference voltage minus 6mV (794mV
typical), the switching regulator circuitry is powered on
and another burst cycle begins. The duration for which theregulator operates in SLEEP depends on the load current
and output capacitor value. The SLEEP time decreases
as the load current increases. The maximum deliverable
load current in Burst Mode operation is 50mA typical.
The buck-boost regulator may not enter SLEEP if the load
current is greater than 50mA. If the load current increases
beyond this point while in Burst Mode operation, the output may lose regulation. Burst Mode operation provides asigniﬁcant improvement in efﬁciency at light loads at theexpense of higher output ripple when compared to PWM
mode. For many noise-sensitive systems, Burst Modeoperation might be undesirable at certain times (i.e., during a transmit or receive cycle of a wireless device), buthighly desirable at others (i.e., when the device is in low
power standby mode).

3558f

25

LTC3558
APPLICATIONS INFORMATION
Buck-Boost Switching Regulator Output Voltage
Programming

The output ﬁlter zero is given by:
f FILTER _ ZERO =The buck-boost switching regulator can be programmed
for output voltages greater than 2.75V and less than 5.45V.
To program the output voltage, a resistor divider is connected between VOUT2 and the feedback node (FB2) asshown in Figure 9. The output voltage is given by VOUT2= 0.8(1 + R1/R2).

2 • π • RESR • COUT

Hz

where RESR is the capacitor equivalent series resistance.
A troublesome feature in boost mode is the right-halfplane zero (RHP), and is given by:
f RHPZ =LTC3558
VOUT2
R1

PVIN22Hz
2 • π • IOUT • L • VOUT2

The loop gain is typically rolled off before the RHP zerofrequency.

FB2
R2

A simple Type I compensation network, as shown in Figure
10, can be incorporated to stabilize the loop, but at thecost of reduced bandwidth and slower transient response.
To ensure proper phase margin, the loop requires to be
crossed over a decade before the LC double pole.

3558 F09

Figure 9. Programming the Buck-Boost Output Voltage Requiresa Resistor Divider Connected Between VOUT2 and FB2

The unity-gain frequency of the error ampliﬁer with theType I compensation is given by:

Closing the Feedback Loop
The LTC3558 incorporates voltage mode PWM control. The
control to output gain varies with operation region (buck,boost, buck-boost), but is usually no greater than 20. The
output ﬁlter exhibits a double pole response given by:
f FILTER _ POLE =1

f UG =

1Hz
2 • π • R1• CP1

1Hz
2 • π • L • COUT

where COUT is the output ﬁlter capacitor.

VOUT2

+
ERROR
AMP

0.8V

R1

FB2

–
VC2

CP1

R2

3558 F10

Figure 10. Error Ampliﬁer with Type I Compensation

3558f

26

LTC3558
APPLICATIONS INFORMATION
Most applications demand an improved transient response
to allow a smaller output ﬁlter capacitor. To achieve a higher
bandwidth, Type III compensation is required. Two zeros
are required to compensate for the double-pole response.
Type III compensation also reduces any VOUT2 overshootseen during a start-up condition. A Type III compensation circuit is shown in Figure 11 and yields the following
transfer function:
VC2
1=
VOUT 2 R1 (C1 + C2)•(1 + sR2C2) [1 + s (R1 + R3)C3 ]s ⎣⎡1 + sR2(C1|| C2)⎤⎦ (1 + sR3C3)

A Type III compensation network attempts to introduce
a phase bump at a higher frequency than the LC double
pole. This allows the system to cross unity gain after the
LC double pole, and achieve a higher bandwidth. Whileattempting to cross over after the LC double pole, thesystem must still cross over before the boost right-halfplane zero. If unity gain is not reached sufﬁciently before
the right-half plane zero, then the –180° of phase lag fromthe LC double pole combined with the –90° of phase lagfrom the right-half plane zero will result in negating thephase bump of the compensator.

at the ﬁlter double pole. If they are placed at too low of afrequency, they will introduce too much gain to the systemand the crossover frequency will be too high. The two highfrequency poles should be placed such that the systemcrosses unity gain during the phase bump introducedby the zeros and before the boost right-half plane zeroand such that the compensator bandwidth is less thanthe bandwidth of the error amp (typically 900kHz). If thegain of the compensation network is ever greater than
the gain of the error ampliﬁer, then the error ampliﬁer nolonger acts as an ideal op amp, and another pole will be
introduced at the same point.
Recommended Type III compensation components for a
3.3V output are:R1: 324kΩ
RFB: 105kΩ
C1: 10pF
R2: 15kΩ
C2: 330pF
R3: 121kΩ
C3: 33pF
COUT : 22μF

The compensator zeros should be placed either before
or only slightly after the LC double pole such that theirpositive phase contributions offset the –180° that occurs

LOUT : 2.2μH

VOUT2

+
ERROR
AMP

R3
0.8V

R1
C3

FB2

–
VC2

RFB

C2

R2
C1

3558 F11

Figure 11. Error Ampliﬁer with Type III Compensation

3558f

27

LTC3558
APPLICATIONS INFORMATION
Input Current Limit

Buck-Boost Switching Regulator Inductor Selection

The input current limit comparator will shut the input PMOS
switch off once current exceeds 700mA typical. Before theswitch current limit, the average current limit amp (620mA
typical) will source current into the feedback pin to drop
the output voltage. The input current limit also protects
against a short-circuit condition at the VOUT2 pin.

The buck-boost switching regulator is designed to workwith inductors in the range of 1μH to 5μH. For mostapplications, a 2.2μH inductor will sufﬁce. Larger value
inductors reduce ripple current which improves output
ripple voltage. Lower value inductors result in higher
ripple current and improved transient response time.
To maximize efﬁciency, choose an inductor with a low
DC resistance and a DC current rating at least 1.5 times
larger than the maximum load current to ensure that theinductor does not saturate during normal operation. If
output short-circuit is a possible condition, the inductor
current should be rated to handle up to the peak current
speciﬁed for the buck-boost regulator.

Reverse Current Limit
The reverse current limit comparator will shut the output
PMOS switch off once current returning from the output
exceeds 450mA typical.
Output Overvoltage Protection
If the feedback node were inadvertently shorted to ground,then the output would increase indeﬁnitely with the maximum current that could be sourced from the input supply.
The buck-boost regulator protects against this by shutting
off the input PMOS if the output voltage exceeds a 5.75V
maximum.
Buck-Boost Regulator Soft-Start Operation
Soft-start is accomplished by gradually increasing thereference voltage over a 500μs typical period. A softstart cycle occurs whenever the buck-boost is enabled,
or after a fault condition has occurred (thermal shutdown
or UVLO). A soft-start cycle is not triggered by changing
operating modes. This allows seamless output operation
when transitioning between Burst Mode operation and
PWM mode operation.

The inductor value also affects Burst Mode operation.
Lower inductor values will cause Burst Mode switchingfrequencies to increase.
Different core materials and shapes will change the size/current and price/current relationship of an inductor. Toroid
or shielded pot cores in ferrite or permalloy materials
are small and do not radiate much energy, but cost morethan powdered iron core inductors with similar electricalcharacteristics. Inductors that are very thin or have a verysmall volume typically have much higher core and DCR
losses and will not give the best efﬁciency.
Table 4 shows some inductors that work well with the
buck-boost regulator. These inductors offer a good compromise in current rating, DCR and physical size. Consult
each manufacturer for detailed information on their entire
selection of inductors.

Table 4. Recommended Inductors for the Buck-Boost Switching Regulator.
INDUCTOR TYPE

L
(μH)

MAX IDC(A)MAX DCR(mΩ)

SIZE IN mm(L × W × H)

DB3018C
D312C
DE2812C
DE2812C

2.4
2.2
2
2.7

1.31
1.14
1.4
1.2

80
140
81
87

3.8 × 3.8 × 1.4
3.6 × 3.6 × 1.2
3 × 3.2 × 1.2
3 × 3.2 × 1.2

Toko
www.toko.com

CDRH3D16

2.2

1.2

72

4 × 4 × 1.8

Sumidawww.sumida.com

SD12

2.2

1.8

74

5.2 × 5.2 × 1.2

Cooper
www.cooperet.com

MANUFACTURER*Typical DCR
3558f

28

LTC3558
APPLICATIONS INFORMATION
Buck-Boost Switching Regulator Input/Output
Capacitor Selection
Low ESR (equivalent series resistance) ceramic capacitors
should be used at both the buck-boost regulator input(PVIN2) and the output (VOUT2). It is recommended that the
input be bypassed with a 10μF capacitor. The output shouldbe bypassed with at least a 10μF capacitor if using Type I
compensation and 22μF if using Type III compensation.
The same selection criteria apply for the buck-boost
regulator input and output capacitors as described in the
Buck Switching Regulator Input/Output Capacitor Selection section.

PCB Layout Considerations
In order to deliver maximum charge current under all
conditions, it is critical that the backside of the LTC3558
be soldered to the PC board ground.
The LTC3558 has dual switching regulators. As with all
switching regulators, care must be taken while laying out
a PC board and placing components. The input decoupling
capacitors, the output capacitor and the inductors must all
be placed as close to the pins as possible and on the sameside of the board as the LTC3558. All connections mustalso be made on the same layer. Place a local unbroken
ground plane below these components. Avoid routing
noisy high frequency lines such as those that connect toswitch pins over or parallel to lines that drive high impedance inputs.

3558f

29

LTC3558
TYPICAL APPLICATIONSUP TO 500mAUSB(4.3V TO 5.5V)
OR AC ADAPTER

VCC
110k

BAT
PVIN1
PVIN2

10μF
NTC

+
1
10μF
4.7μF

28.7K
100k (NTC)
NTH50603NO1

510Ω

SINGLE
Li-lon CELL
(2.7V TO 4.2V)

LTC3558
1.8V AT 400mA

4.7μH
SW1
CHRG

806k

1.74k
PROG

FB1

SUSP

SWAB2

EN1
EN2

10μF

649k
2.2μH

HPWR
DIGITALCONTROL

10pF

3.3V AT 400mA

SWCD2
VOUT2
619k

MODE

10μF

FB2

GND2
(EXPOSED
GND
PAD)

200k

15k
150pF

VC2
3558 TA02

Figure 12. Li-Ion to 3.3V at 400mA, 1.8V at 400mA and USB-Compatible Battery Charger

As shown in Figure 12, the LTC3558 can be operated
with no battery connected to the BAT pin. A 1Ω resistor
in series with a 4.7μF capacitor at the BAT pin ensures
battery charger stability. 10μF VCC decoupling capacitors
are required for proper operation of the DC/DC converters.
A three-resistor bias network for NTC sets hot and cold
trip points at approximately 55°C and 0°C.
The battery can be charged with up to 950mA of charge
current when powered from a 5V wall adaptor, as shownin Figure 13. CHRG has a LED to provide a user with avisual indication of battery charge status. The buck-boost
regulator starts up only after VOUT1 is up to approximately
0.7V. This provides a sequencing function which may bedesirable in applications where a microprocessor needs tobe powered up before peripherals. A Type III compensation
network improves the transient response of the buck-boost
switching regulator.

3558f

30

LTC3558
PACKAGE DESCRIPTIONUD Package
20-Lead Plastic QFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1720 Rev A)

0.70 ±0.05
3.50 ± 0.05
(4 SIDES)

1.65 ± 0.05

2.10 ± 0.05

PACKAGEOUTLINE
0.20 ±0.05
0.40 BSC
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONSAPPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED

3.00 ± 0.10
(4 SIDES)

BOTTOM VIEW—EXPOSED PAD

R = 0.115
TYP

0.75 ± 0.05
R = 0.05
TYP

PIN 1
TOP MARK(NOTE 6)

PIN 1 NOTCH
R = 0.20 TYP
OR 0.25 × 45°
CHAMFER
19 20
0.40 ± 0.10
12

1.65 ± 0.10
(4-SIDES)(UD20) QFN 0306 REV A

0.200 REF
0.00 – 0.05
NOTE:
1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATIONON THE TOP AND BOTTOM OF PACKAGE

0.20 ± 0.05
0.40 BSC

3558f

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

31

LTC3558
TYPICAL APPLICATIONSUP TO 950mA
5V WALL
ADAPTER
100k

510Ω

VCC

BAT
PVIN1

NTC

PVIN2

1μF

10μF
1.2V AT 400mA

4.7μH

LTC3558

100k (NTC)

SW1
324k

CHRG

FB1

PROG

SWAB2

887Ω

DIGITALCONTROL

SUSP
HPWR
MODE
EN1

SINGLE
Li-lon CELL
(2.7V TO 4.2V)

+

10pF

10μF

649k
2.2μH
3.3V AT 400mA

SWCD2
VOUT2

121k
324k

22μF

33pF
EN2

FB2
105k

15k
330pF

10pF

VC2
3558 TA03

GND2(EXPOSED
PAD)
GND

Figure 13. Battery Charger Can Charge a Battery with Up to 950mA When Powered From a Wall Adapter

RELATED PARTS
PART NUMBER DESCRIPTIONCOMMENTS

LTC3550

Dual Input USB/AC Adapter Li-Ion Battery
Charger with Adjustable Output 600mA
Buck Converter

Synchronous Buck Converter, Efﬁciency: 93%, Adjustable Output at 600mA, Charge Current:
950mA Programmable, USB Compatible, Automatic Input Power Detectionand Selection

LTC3552

Standalone Linear Li-Ion Battery Charger
with Adjustable Output Dual Synchronous
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Synchronous Buck Converter, Efﬁciency: >90%, Adjustable Outputs at 800mA and
400mA, Charge Current Programmable Up to 950mA, USB Compatible, 5mm × 3mm
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LTC3552-1

Standalone Linear Li-Ion Battery Charger
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Synchronous Buck Converter, Efﬁciency: >90%, Outputs 1.8V at 800mA and 1.575 at
400mA, Charge Current Programmable up to 950mA, USB Compatible

LTC3455

Dual DC/DC Converter with USB PowerManager and Li-Ion Battery Charger

Seamless Transition Between Input Power Sources: Li-Ion Battery, USB and 5V Wall
Adapter, Two High Efﬁciency DC/DC Converters: Up to 96%, Full Featured Li-Ion Battery
Charger with Accurate USB Current Limiting (500mA/100mA) Pin-Selectable Burst Mode
Operation, Hot SwapTM Output for SDIO and Memory Cards, 4mm × 4mm QFN-24 Package

LTC3456

2-Cell, Multi-Output DC/DC Converter with Seamless Transition Between 2-Cell Battery, USB and AC Wall Adapter Input Power Sources,
USB Power Manager
Main Output: Fixed 3.3V Output, Core Output: Adjustable from 0.8V to VBATT(MIN), Hot Swap
Output for Memory Cards, Power Supply Sequencing: Main and Hot Swap Accurate USB
Current Limiting, High Frequency Operation: 1MHz, High Efﬁciency: Up to 92%, 4mm ×
4mm QFN-24 Package

LTC3559

USB Charger with Dual Buck Regulators

Adjustable, Synchronous Buck Converters, Efﬁciency >90%, Outputs: Down to 0.8V at
400mA Each, Charge Current Programmable Up to 950mA, USB-Compatible, 3mm × 3mm
QFN-16 Package

LTC4080

500mA Standalone Charger with 300mA
Synchronous Buck

Charges Single-Cell Li-Ion Batteries, Timer Termination + C/10, Thermal Regulation, Buck
Output: 0.8V to VBAT, Buck Input VIN: 2.7V to 5.5V, 3mm × 3mm DFN-10 Package

Hot Swap is a trademark of Linear Technology Corporation.
3558f

32 Linear Technology Corporation

LT 0408 • PRINTED IN USA

1630 McCarthy Blvd., Milpitas, CA 95035-7417(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com© LINEAR TECHNOLOGY CORPORATION 2008