LT8640/LT8640-1
42V, 5A Synchronous
Step-Down Silent Switcher
with 2.5µA Quiescent Current
Description

Features

Silent Switcher® Architecture
nn Ultralow EMI/EMC Emissions
nn Spread Spectrum Frequency Modulation
nn High Efficiency at High Frequency
nn Up to 96% Efficiency at 1MHz
nn Up to 95% Efficiency at 2MHz
nn Wide Input Voltage Range: 3.4V to 42V
nn 5A Maximum Continuous Output, 7A PeakTransient Output
nn Ultralow Quiescent Current Burst Mode® Operation
nn 2.5µA I Regulating 12V to 3.3V
Q
IN
OUT
nn Output Ripple < 10mV
P-P
nn Fast Minimum Switch-On Time: 35ns
nn Low Dropout Under All Conditions: 100mV at 1A
nn Forced Continuous Mode (LT8640-1 Only)
nn Safely Tolerates Inductor Saturation in Overload
nn Adjustable and Synchronizable: 200kHz to 3MHz
nn Peak Current Mode Operation
nn Output Soft-Start and Tracking
nn Small 18-Lead 3mm × 4mm QFN
nn

Applications
Automotive and Industrial Supplies
General Purpose Step-Down
nn GSM Power Supplies
nn
nn

The LT®8640/LT8640-1 step-down regulator features Silent
Switcher architecture designed to minimize EMI/EMC emissions while delivering high efficiency at frequencies up to
3MHz. Assembled in a 3mm × 4mm QFN, the monolithic
construction with integrated power switches and inclusionof all necessary circuitry yields a solution with a minimal
PCB footprint. An ultralow 2.5µA quiescent current—with
the output in full regulation—enables applications requiring
highest efficiency at very small load currents. Transient
response remains excellent and output voltage ripple isbelow 10mVP-P at any load, from zero to full current.
The LT8640/LT8640-1 allows high VIN to low VOUT conversion athigh frequency with a fast minimum top switch on-time of 35ns.
Operation is safe in overload even with a saturated inductor.
Essential features are included and easy to use: An opendrain PG pin signals when the output is in regulation. The
SYNC/MODE pin selects between Burst Mode operation,
spread spectrum mode, synchronization to an external
clock, and either pulse-skipping (LT8640) or forced
continuous mode (LT8640-1). Soft-start and tracking
functionality is accessed via the TR/SS pin. An accurate
enable threshold can be set using the EN/UV pin and a
resistor at the RT pin programs switch frequency.
L, LT, LTC, LTM, Linear Technology, the Linear logo, Silent Switcher and Burst Mode areregistered trademarks of Linear Technology Corporation. All other trademarks are the propertyof their respective owners.

Typical Application

100

5V 5A Step-Down Converter

3.00
2.63

95
4.7µF
1µF

GND1
PG

10nF

VIN2
GND2

LT8640/LT8640-1 BSTSYNC/MODE
TR/SS

fSW = 1MHz

1µF

0.1µF 3.3µH

4.7pF
INTVCC
RT

VOUT
5V
5A

SW
BIAS

1µF
41.2k

90

1M
47µF

FB
GND

243k

2.25

EFFICIENCY

85

1.88

80

1.50
1.13

75
70

POWER LOSS

65
60
0.5

8640 TA01a

1

POWER LOSS (W)

EN/UV

VIN1

EFFICIENCY (%)

VIN
5.5V TO 42V

12VIN to 5VOUT Efficiency

0.75
1MHz, L = 3.3µH
2MHz, L = 2.2µH 0.38
3MHz, L = 1µH
01.5 2 2.5 3 3.5 4 4.5 5
LOAD CURRENT (A)8640 TA01b

8640faFor more information www.linear.com/LT8640

1

LT8640/LT8640-1
Pin Configuration

VIN, EN/UV, PG...........................................................42V
BIAS...........................................................................25V
FB, TR/SS ...................................................................4V
SYNC Voltage ..............................................................6V
Operating Junction Temperature Range (Note 2)
LT8640E/LT8640-1E........................... –40°C to 125°C
LT8640I/LT8640-1I............................. –40°C to 125°C
LT8640H............................................. –40°C to 150°C
Storage Temperature Range.......................–65 to 150°C

SYNC/MODE
17

PG

20 19 18FB

GND

TOP VIEW

BIAS 1

16 TR/SS

INTVCC 2

15 RT

BST 3

21
SW

13 VIN2

11 GND2

7

8

9

10
GND2

GND1 614 EN/UVSW

VIN1 422
SW

SW(Note 1)GND1Absolute Maximum Ratings

UDC PACKAGE
18-LEAD (3mm × 4mm) PLASTIC QFNθJA = 40°C/W, θJC(PAD) = 12°C/W
EXPOSED PAD (PINS 21, 22) ARE SW, SHOULD BE SOLDERED TO PCBNOTE: PINS 5 AND 12 ARE REMOVED. CONFIGURATION DOES NOT MATCH
JEDEC 20-LEAD PACKAGE OUTLINE

Order Information
LEAD FREE FINISH

TAPE AND REELPART MARKING*PACKAGE DESCRIPTIONTEMPERATURE RANGE

LT8640EUDC#PBF

LT8640EUDC#TRPBF

LGNJ

18-Lead (3mm × 4mm) Plastic QFN–40°C to 125°C

LT8640IUDC#PBF

LT8640IUDC#TRPBF

LGNJ

18-Lead (3mm × 4mm) Plastic QFN–40°C to 125°C

LT8640HUDC#PBF

LT8640HUDC#TRPBF

LGNJ

18-Lead (3mm × 4mm) Plastic QFN–40°C to 150°C

LT8640EUDC-1#PBF

LT8640EUDC-1#TRPBF

LGVT

18-Lead (3mm × 4mm) Plastic QFN–40°C to 125°C

LT8640IUDC-1#PBF

LT8640IUDC-1#TRPBF

LGVT

18-Lead (3mm × 4mm) Plastic QFN–40°C to 125°C

Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
®

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/2

8640faFor more information www.linear.com/LT8640

LT8640/LT8640-1
Electrical Characteristics The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C.

PARAMETERCONDITIONSMinimum Input Voltage
VIN Quiescent Current

TYP

MAX

l

MIN

2.9

3.4

V

l

0.75
0.75

3
10

µA
µA

l

1.7
1.7

4
10

µA
µA

0.3

0.5

mA

21
220

50
350

µA
µA

0.970
0.970

0.976
0.982

V
V

0.004

0.02

%/V

VEN/UV = 0V
VEN/UV = 2V, Not Switching, VSYNC = 0V
VEN/UV = 2V, Not Switching, VSYNC = 2V (LT8640 Only)

VIN Current in Regulation

VOUT = 0.97V, VIN = 6V, Output Load = 100µA
VOUT = 0.97V, VIN = 6V, Output Load = 1mAll

Feedback Reference Voltage

VIN = 6V, ILOAD = 0.5A
VIN = 6V, ILOAD = 0.5Al

VIN = 4.0V to 42V, ILOAD = 0.5AlFeedback Voltage Line Regulation
Feedback Pin Input Current

VFB = 1V

BIAS Pin Current Consumption

VBIAS = 3.3V, ILOAD = 1A, 2MHz

Minimum On-Time

ILOAD = 1A, SYNC = 0V
ILOAD = 1A, SYNC = 3.3V

0.964
0.958
–20

l
l

Minimum Off-Time
Oscillator Frequency

RT = 221k, ILOAD = 1A
RT = 60.4k, ILOAD = 1A
RT = 18.2k, ILOAD = 1A

Top Power NMOS On-Resistance

ISW = 1A

l
l
l

180
665
1.85

l

Bottom Power NMOS On-Resistance

VINTVCC = 3.4V, ISW = 1A

SW Leakage Current

VIN = 42V, VSW = 0V, 42V

EN/UV Pin Threshold

EN/UV Rising

7.5

35
35

50
50

ns
ns

80

110

ns

210
700
2.00

240
735
2.15

kHz
kHz
MHz

10

mΩ
12.5

28
–15
l

0.94

EN/UV Pin Hysteresis

1.0

VEN/UV = 2V

–20

PG Upper Threshold Offset from VFB

VFB Fallingl5

PG Lower Threshold Offset from VFB

VFB Rising

l

–5.25

PG Hysteresis

mΩ
µA

1.06

V
mV

20

nA

7.5

10.25

%

–8

–10.75%0.2

PG Leakage

VPG = 3.3V

PG Pull-Down Resistance

VPG = 0.1V

SYNC/MODE Threshold

SYNC/MODE DC and Clock Low Level Voltage
SYNC/MODE Clock High Level Voltage
SYNC/MODE DC High Level Voltage

Spread Spectrum Modulation
Frequency Range

RT = 60.4k, VSYNC = 3.3V

Spread Spectrum Modulation Frequency

VSYNC = 3.3V

TR/SS Source Current

–40
l

0.7
1.0
2.3

Fault Condition, TR/SS = 0.1V

%
40

nA

700

2000

Ω

0.9
1.2
2.6

1.1
1.4
2.9

V
V
V

22

%

3
l

1.2

A

15
40

EN/UV Pin Current

nA
mA

67

Top Power NMOS Current Limit

TR/SS Pull-Down Resistance

20
11

UNITS

1.9
200

kHz
2.6

µA
Ω

8640faFor more information www.linear.com/LT8640

3

LT8640/LT8640-1
Electrical Characteristics The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C.
PARAMETERCONDITIONS

MIN

TYP

MAX

UNITS

LT8640-1 Output Sink Current in Forced
Continuous Mode

VFB = 1.01V, L = 6.8µH, RT = 60.4k

0.25

0.6

1

A

LT8640-1 VIN to Disable Forced Continuous
Mode

VIN Rising

35

37

39

V

LT8640-1 VFB Offset from Feedback
Reference Voltage to Disable Forced
Continuous Mode

VFB Rising

7

9.5

12

%

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: The LT8640E/LT8640-1E is guaranteed to meet performance
specifications from 0°C to 125°C junction temperature. Specifications
over the –40°C to 125°C operating junction temperature range are assured
by design, characterization, and correlation with statistical processcontrols. The LT8640I/LT8640-1I is guaranteed over the full –40°C to
125°C operating junction temperature range. The LT8640H/LT8640-1H isguaranteed over the full –40°C to 150°C operating junction temperature
range. High junction temperatures degrade operating lifetimes. Operating

4

lifetime is derated at junction temperatures greater than 125°C.
The junction temperature (TJ, in °C) is calculated from the ambient
temperature (TA in °C) and power dissipation (PD, in Watts) according to
the formula:
TJ = TA + (PD • θJA)
where θJA (in °C/W) is the package thermal impedance.
Note 3: This IC includes overtemperature protection that is intended toprotect the device during overload conditions. Junction temperature will
exceed 150°C when overtemperature protection is active. Continuous
operation above the specified maximum operating junction temperature
will reduce lifetime.

8640faFor more information www.linear.com/LT8640

LT8640/LT8640-1
Typical Performance Characteristics
12VIN to 3.3VOUT Efficiency
vs Frequency

EFFICIENCY

95

2.25

90

1.13

75
POWER LOSS

70
65
60
0.5

1

1.75

85

1.40

80

1.05

75
POWER LOSS

65
60
0.5

3MHz, L = 1μH

1

1.5

90

L = WE–LHMI1040 0.70
1MHz, L = 2.2μH
2MHz, L = 1μH 0.35

2 2.5 3 3.5
LOAD CURRENT (A)48640 G01

4.5

5

1.8

75
70

55
0

0.5

Efficiency at 3.3VOUT

Efficiency at 5VOUT

90

90

80

80

2.1

70

70

1.8

75

1.5

70
65
60
55
50

0

0.5

1

1.2

VIN = 12V 0.9
VIN = 24V
0.6
VIN = 36V
fSW = 1MHz 0.3
L = IHLP3232DZ-01, 2.2μH
0
1.5 2 2.5 3 3.5 4 4.5 5
LOAD CURRENT (A)

100

60
50
40
fSW = 1MHz
L = IHLP3232DZ-01, 4.7μH
VIN = 12V
VIN = 24V
VIN = 36V

30
20
10
0
0.01

0.1

1

10

100

0.1

1
10
100
LOAD CURRENT (mA)

8640 G06

Reference Voltage

VIN = 12V

0.977

EFFICIENCY (%)

88
86
VIN = 12V
VOUT = 3.3V
ILOAD = 2A
L = IHLP3232DZ-01, 4.7μH

VIN = 24V

85
80
75

VOUT = 5V
ILOAD = 10mA
L = IHLP3232DZ-01

70

38640 G07

REFERENCE VOLAGE (V)

90

90

65

1000

0.979

92
EFFICIENCY (%)

00.01

Burst Mode Operation Efficiency
vs Inductor Value

94

2
1.5
2.5
0.5
1
SWITCHING FREQUENCY (MHz)fSW = 1MHz
L = IHLP3232DZ-01, 4.7μH
VIN = 12V
VIN = 24V
VIN = 36V

30
10

95

0

40

8640 G05

96

82

50

LOAD CURRENT (mA)

Efficiency vs Frequency

84

60

20

1000

8640 G04

80

EFFICIENCY (%)

80

EFFICIENCY (%)

2.7
2.4
POWER LOSS (W)95
90

POWER LOSS

18640 G03

100

EFFICIENCY

1.2

VIN = 12V 0.9
VIN = 24V
0.6
VIN = 36V
fSW = 1MHz 0.3
L = IHLP3232DZ-01, 3.3μH
0
1.5 2 2.5 3 3.5 4 4.5 5
LOAD CURRENT (A)

65

50

1.5

POWER LOSS

3.0

85

2.1

80

60

0

2.4

EFFICIENCY

858640 G02

Efficiency at 3.3VOUT
100

EFFICIENCY (%)

2.7

2.10

EFFICIENCY70

L = WE–LHMI1040 0.75
1MHz, L = 3.3µH
2MHz, L = 2.2µH 0.38
3MHz, L = 1µH
01.5 2 2.5 3 3.5 4 4.5 5
LOAD CURRENT (A)

3.0

95

POWER LOSS (W)

1.50

80

100

2.45

POWER LOSS (W)

1.88

85

POWER LOSS (W)EFFICIENCY (%)

90

2.63

EFFICIENCY (%)

95

Efficiency at 5VOUT
2.80

100

3.00

100

EFFICIENCY (%)

12VIN to 5VOUT Efficiency
vs Frequency

1

2

3
4
5
6
INDUCTOR VALUE (µH)

0.973
0.971
0.969
0.967
0.965
0.963

8

7

0.975

8640 G08

0.961–50 –250

25 50 75 100 125 150
TEMPERATURE (°C)
8640 G09

8640faFor more information www.linear.com/LT8640

5

LT8640/LT8640-1
Typical Performance Characteristics
EN Pin Thresholds

0.12
0.10

1.02

0.10

1.00
0.99
0.98

0.08
CHANGE IN VOUT (%)

EN RISING

1.01

CHANGE IN VOUT (%)

EN THRESHOLD (V)Line Regulation

Load Regulation
0.15

1.03

0.05
0

–0.05

0.97

EN FALLING

0.95–50 –250

25 50 75 100 125 150
TEMPERATURE (°C)

–0.15

0.02
0

–0.04

VOUT = 5V
VIN = 12V
R1/R2 = 100k/24.3k
0

0.5

1.5 2 2.5 3 3.5
LOAD CURRENT (A)

1

4

4.5

5

–0.08

5

10

15

20 25 30 35
INPUT VOLTAGE (V)40

458640 G12

Top FET Current Limit vs Duty Cycle

No-Load Supply Current
4.0

Top FET Current Limit

10.0

12.0

9.5

3.5

2.5
2.0

CURRENT LIMIT (A)CURRENT LIMIT (A)

9.0
3.0

8.5
8.0
7.5

11.0

VOUT = 3.3V
L = 4.7µH
IN REGULATION
0

5

10

15 20 25 30 35
INPUT VOLTAGE (V)

40

5% DC

10.0

9.0

7.0
1.5
1.0

VOUT = 5V
ILOAD = 1A
R1/R2 = 100k/24.3k–0.068640 G11

8640 G10INPUT CURRENT (µA)

0.04

–0.02

–0.10

0.96

0.06

6.5
6.0

45

0

0.2

0.4
0.6
DUTY CYCLE

8640 G13

0.8

8.0–50 –25

1

0

25 50 75 100 125 150
TEMPERATURE (°C)

8640 G14
8640 G15

Switch Drop
SWITCH CURRENT = 1A

SWITCH DROP (mV)

SWITCH DROP (mV)TOP SWITCH
75
50
BOTTOM SWITCH

25

025 50 75 100 125 150
TEMPERATURE (°C)
8640 G16350
300
TOP SWITCH

250
200
150
100

37
34
31
28

50
0

VSYNC = FLOATVSYNC = 0V

40

400

100

6

43

450

125

0
–50 –25

Minimum On-Time

Switch Drop500

MINIMUM ON-TIME (s)150

BOTTOM SWITCH
0

1

4
2
3
SWITCH CURRENT (A)

5

25
–50

ILOAD = 2A
–25

0
25
50
75
TEMPERATURE (°C)

100

125

8640 G18
8640 G17

8640faFor more information www.linear.com/LT8640

LT8640/LT8640-1
Typical Performance Characteristics
Switching Frequency

Dropout Voltage
740

600
VIN = 5V
VOUT SET TO REGULATE AT 5V
L = IHLP3232DZ-01, 1µH

FRONT PAGE APPLICATION
VIN = 12V
1000 VOUT = 5V

300
200
100

SWITCHING FREQUENCY (kHz)400

0RT = 60.4k

730
SWITCHING FREQUENCY (kHz)DROPOUT VOLTAGE (mV)

500

Burst Frequency
1200

720
710
700
690
680
670

0

0.5

1.5 2 2.5 3 3.5
LOAD CURRENT (A)

1

4

4.5

660
–50 –25

5

0

20

600

1.0

500
400
300

20 25
30 35
INPUT VOLTAGE (V)

40

0

45

0

0.2

0.4

0.6
FB VOLTAGE (V)PG THRESHOLD OFFSET FROM VREF (%)

SS PIN CURRENT (µA)

0

1

1.9
1.8
1.7
1.6
1.5
25 50 75 100 125 150
TEMPERATURE (°C)
8640 G25

0

0.2

1.0
0.4 0.6 0.8
TR/SS VOLTAGE (V)1.4

PG Low Thresholds
–6.0

9.5
9.0
8.5
8.0
7.5

1.2

8640 G24

PG High Thresholds

2.0

0

0.8

10.0

VSS = 0.5V

1.4
–50 –250.48640 G23

Soft-Start Current
2.1

0.6

0.2

8640 G22

2.2

0.8

200

PG THRESHOLD OFFSET FROM VREF (%)

15

400

Soft-Start Tracking

100
10

100
200
300
LOAD CURRENT (mA)

1.2

FB VOLTAGE (V)40

08640 G21

VOUT = 3.3V
VIN = 12V
VSYNC = 0V
RT = 60.4k

700
SWITCHING FREQUENCY (kHz)LOAD CURRENT (mA)

800

FRONT PAGE APPLICATIONVOUT = 5V
fSW = 1MHz

5200

Frequency Foldback

60

0

400

8640 G20

Minimum Load to Full Frequency(Pulse-Skipping Mode)

80

600

0

25 50 75 100 125 150
TEMPERATURE (°C)

8640 G19

100800

FB RISINGFB FALLING

7.0
6.5
6.0
–50 –250

25 50 75 100 125 150
TEMPERATURE (°C)
8640 G26

–6.5
–7.0
–7.5
FB RISING

–8.0
–8.5

FB FALLING

–9.0
–9.5

–10.0
–50 –250

25 50 75 100 125 150
TEMPERATURE (°C)
8640 G27

8640faFor more information www.linear.com/LT8640

7

LT8640/LT8640-1
Typical Performance Characteristics
RT Programmed Switching
Frequency
3.6

225

8.5

3.4

200

125
100
75

BIAS PIN CURRENT (mA)

150

3.0
2.8
2.6
2.4

50

0
0.2

0.6
1.4 1.8 2.2 2.6
1
SWITCHING FREQUENCY (MHz)2.0–55 –25

3

95
65
35
TEMPERATURE (°C)

5125Bias Pin Current
80

10

5

2.6
1.4 1.8
2.2
0.6
1
SWITCHING FREQUENCY (MHz)390

DC2202A DEMO BOARD
VIN = 12V, fSW = 1MHz
VIN = 24V, fSW = 1MHz
VIN = 12V, fSW = 2MHz
VIN = 24V, fSW = 2MHz

70
60

40
30
20

0

0

1

2

3
LOAD CURRENT (A)

4

5

70
60

40

45

50
40
30
20

0

0

0.2

0.4
0.6
DUTY CYCLE OF 7A LOAD

0.8
8640 G33

8640 G32

Switching Waveforms, Burst
Mode Operation

Switching Waveforms
IL
1A/DIV

VSW
10V/DIV

VSW
5V/DIV8

20 25 30 35
INPUT VOLTAGE (V)10IL
500mA/DIV

8640 G34

15

DC2202A DEMO BOARD
VIN = 12V
VOUT = 5V
fSW = 2MHz
STANDBY LOAD = 0.25A
PULSED LOAD = 7A

80

50

Switching Waveforms, Full
Frequency Continuous Operation

VSW
5V/DIV

10

Case Temperature Rise vs 7A
Pulsed Load

8640 G31

IL
1A/DIV58640 G30

10500ns/DIVFRONT PAGE APPLICATION
12VIN TO 5VOUT AT 1A

5.5

155

CASE TEMPERATURE RISE (°C)

VBIAS = 5V
VOUT = 5V
VIN = 12V
ILOAD = 1A

0
0.2

6.5

Case Temperature Rise

CASE TEMPERATURE RISE (°C)

BIAS PIN CURRENT (mA)

15

7.0

8640 G29

8640 G28

20

7.5

6.0

2.2

25

VBIAS = 5V
VOUT = 5V
ILOAD = 1A
fSW = 1MHz

8.0

3.2

175

INPUT VOLTAGE (V)RT PIN RESISTOR (kΩ)

Bias Pin Current

VIN UVLO

250

5µs/DIVFRONT PAGE APPLICATION
12VIN TO 5VOUT AT 10mA
VSYNC = 0V

8640 G35

500ns/DIVFRONT PAGE APPLICATION
36VIN TO 5VOUT AT 1A

8640 G36

8640faFor more information www.linear.com/LT8640

LT8640/LT8640-1
Typical Performance Characteristics
Transient Response; Load Current
Stepped from 100mA (Burst Mode
Operation) to 1.1A

Transient Response; Load Current
Stepped from 1A to 2A
IL
1A/DIVIL
1A/DIVVOUT
100mV/DIV

VOUT
200mV/DIV

50µs/DIVFRONT PAGE APPLICATION
1A TO 2A TRANSIENT
12VIN, 5VOUT
COUT = 47µF

50µs/DIVFRONT PAGE APPLICATION
100mA (Burst Mode OPERATION) TO
1.1A TRANSIENT
12VIN, 5VOUT
COUT = 47µF

8640 G37

Start-Up Dropout Performance

Start-Up Dropout Performance

VIN

VIN
2V/DIV

VIN

VIN
2V/DIV

VOUT

VOUT
2V/DIV

VOUT

VOUT
2V/DIV

100ms/DIV
2.5Ω LOAD(2A IN REGULATION)

8640 G38

100ms/DIV
20Ω LOAD(250mA IN REGULATION)

8640 G39

8640 G40

Conducted EMI Performance
60
50

AMPLITUDE (dBµV)

40
30
20
10
0
-10
-20
FIXED FREQUENCY MODESPREAD SPECTRUM MODE

-30
-40
0

3

6

9

12

15

18

21

24

FREQUENCY (MHz)2730
8640 G41

DC2202A DEMO BOARD(WITH EMI FILTER INSTALLED)
14V INPUT TO 5V OUTPUT AT 4A, fSW = 2MHz

8640faFor more information www.linear.com/LT8640

9

LT8640/LT8640-1
Typical Performance Characteristics
Radiated EMI Performance(CISPR25 Radiated Emission Test with Class 5 Peak Limits)
50
VERTICAL POLARIZATIONPEAK DETECTOR45

AMPLITUDE (dBµV/m)

40
35
30
25
20
15
10
5

CLASS 5 PEAK LIMITFIXED FREQUENCY MODESPREAD SPECTRUM MODE

0
-5
0

100

200

300

400

500

600

700

800

900

1000

FREQUENCY (MHz)50
HORIZONTAL POLARIZATIONPEAK DETECTOR45

AMPLITUDE (dBµV/m)

40
35
30
25
20
15
10
5

CLASS 5 PEAK LIMITFIXED FREQUENCY MODESPREAD SPECTRUM MODE

0
-5
0

100

200

300

400

500

600

700

800

900

1000

FREQUENCY (MHz)
DC2202A DEMO BOARD(WITH EMI FILTER INSTALLED)
14V INPUT TO 5V OUTPUT AT 4A, fSW = 2MHz

108640 G42

8640faFor more information www.linear.com/LT8640

LT8640/LT8640-1
Pin Functions
BIAS (Pin 1): The internal regulator will draw current
from BIAS instead of VIN when BIAS is tied to a voltage
higher than 3.1V. For output voltages of 3.3V to 25V thispin should be tied to VOUT. If this pin is tied to a supply
other than VOUT use a 1µF local bypass capacitor on thispin. If no supply is available, tie to GND.
INTVCC (Pin 2): Internal 3.4V Regulator Bypass Pin. Theinternal power drivers and control circuits are powered fromthis voltage. INTVCC maximum output current is 20mA.
Do not load the INTVCC pin with external circuitry. INTVCC
current will be supplied from BIAS if BIAS > 3.1V, otherwise
current will be drawn from VIN. Voltage on INTVCC willvary between 2.8V and 3.4V when BIAS is between 3.0V
and 3.6V. Decouple this pin to power ground with at leasta 1µF low ESR ceramic capacitor placed close to the IC.
BST (Pin 3): This pin is used to provide a drive voltage,
higher than the input voltage, to the topside power switch.
Place a 0.1µF boost capacitor as close as possible to the IC.
VIN1 (Pin 4): The LT8640/LT8640-1 requires two 1µF
small input bypass capacitors. One 1µF capacitor shouldbe placed between VIN1 and GND1. A second 1µF capacitor
should be placed between VIN2 and GND2. These capacitors must be placed as close as possible to the LT8640/
LT8640-1. A third larger capacitor of 2.2µF or more shouldbe placed close to the LT8640/LT8640-1 with the positive
terminal connected to VIN1 and VIN2, and the negative
terminal connected to ground. See applications section
for sample layout.
GND1 (6, 7): Power Switch Ground. These pins are thereturn path of the internal bottom side power switch andmust be tied together. Place the negative terminal of the
input capacitor as close to the GND1 pins as possible. Also
be sure to tie GND1 to the ground plane. See the Applications Information section for sample layout.
SW (Pins 8, 9): The SW pins are the outputs of the internal
power switches. Tie these pins together and connect themto the inductor and boost capacitor. This node should be
kept small on the PCB for good performance and low EMI.
GND2 (10, 11): Power Switch Ground. These pins are thereturn path of the internal bottom side power switch andmust be tied together. Place the negative terminal of the
input capacitor as close to the GND2 pins as possible. Also

be sure to tie GND2 to the ground plane. See the Applications Information section for sample layout.
VIN2 (Pin 13): The LT8640/LT8640-1 requires two 1µF
small input bypass capacitors. One 1µF capacitor shouldbe placed between VIN1 and GND1. A second 1µF capacitor
should be placed between VIN2 and GND2. These capacitors must be placed as close as possible to the LT8640/
LT8640-1. A third larger capacitor of 2.2µF or more shouldbe placed close to the LT8640/LT8640-1 with the positive terminal connected to VIN1 and VIN2, and the negative terminal connected to ground. See the Applications
Information section for sample layout.
EN/UV (Pin 14): The LT8640/LT8640-1 is shut downwhen this pin is low and active when this pin is high. The
hysteretic threshold voltage is 1.00V going up and 0.96V
going down. Tie to VIN if the shutdown feature is not used.
An external resistor divider from VIN can be used to program a VIN threshold below which the LT8640/LT8640-1
will shut down.
RT (Pin 15): A resistor is tied between RT and ground to
set the switching frequency.
TR/SS (Pin 16): Output Tracking and Soft-Start Pin. Thispin allows user control of output voltage ramp rate during start-up. A TR/SS voltage below 0.97V forces theLT8640/LT8640-1 to regulate the FB pin to equal the TR/
SS pin voltage. When TR/SS is above 0.97V, the tracking
function is disabled and the internal reference resumes
control of the error amplifier. An internal 1.9µA pull-up
current from INTVCC on this pin allows a capacitor toprogram output voltage slew rate. This pin is pulled to
ground with an internal 200Ω MOSFET during shutdown
and fault conditions; use a series resistor if driving froma low impedance output. This pin may be left floating if
the tracking function is not needed.
SYNC/MODE (Pin 17, LT8640 Only): This pin programs
four different operating modes: 1) Burst Mode operation.
Tie this pin to ground for Burst Mode operation at low
output loads—this will result in ultralow quiescent current.
2) Pulse-skipping mode. This mode offers full frequency
operation down to low output loads before pulse skipping
occurs. Float this pin for pulse-skipping mode. When
floating, pin leakage currents should be <1µA. 3) Spread
8640faFor more information www.linear.com/LT8640

11

LT8640/LT8640-1
Pin Functions
spectrum mode. Tie this pin high to INTVCC (~3.4V) for
pulse-skipping mode with spread spectrum modulation. 4)
Synchronization mode. Drive this pin with a clock source tosynchronize to an external frequency. During synchronization the part will operate in pulse-skipping mode.

GND (Pins 18): LT8640/LT8640-1 Ground Pin. Connect
this pin to system ground and to the ground plane.
PG (Pin 19): The PG pin is the open-drain output of aninternal comparator. PG remains low until the FB pin iswithin ±8% of the final regulation voltage, and there areno fault conditions. PG is valid when VIN is above 3.4V,
regardless of EN/UV pin state.

SYNC/MODE (Pin 17, LT8640-1 Only): For the LT8640-1,
this pin programs four different operating modes: 1)
Burst Mode operation. Tie this pin to ground for Burst
Mode operation at low output loads—this will result inultralow quiescent current. 2) Forced Continuous mode
(FCM). This mode offers fast transient response andfull frequency operation over a wide load range. Float
this pin for FCM. When floating, pin leakage currents
should be <1µA. 3) Spread spectrum mode. Tie this pin
high to INTVCC (~3.4V) for forced continuous mode withspread-spectrum modulation. 4) Synchronization mode.
Drive this pin with a clock source to synchronize to an
external frequency. During synchronization the part will
operate in forced continuous mode.

FB (Pin 20): The LT8640/LT8640-1 regulates the FB pin to0.970V. Connect the feedback resistor divider tap to thispin. Also, connect a phase lead capacitor between FB and
VOUT. Typically, this capacitor is 4.7pF to 22pF.
SW (Exposed Pad Pins 21, 22): The exposed pads should
be connected and soldered to the SW trace for good thermal
performance. If necessary due to manufacturing limitations Pins 21 and 22 may be left disconnected, however
thermal performance will be degraded.

Block Diagram
VIN

4
CIN3

VIN2

VIN1

CIN1

R3
OPT

14

R4
OPT

19

INTERNAL 0.97V REF

EN/UV

1V

PG

+
–

SHDN

±8%

+
+
–

VOUT
C1
R2

R1
20

CSS
OPT
16
RT

ERROR
AMP

15
17

FB

TR/SS

BIAS

3.4V
REG
SLOPE COMPINTVCCOSCILLATOR
200kHz TO 3MHz

BST

VC

BURST
DETECTSHDN
THERMAL SHDNINTVCC UVLO
VIN UVLO
1.9µA

13
CIN2

–
+

SWITCHLOGIC
AND
ANTISHOOT
THROUGH

1

2

CVCC

3
CBST

M1

L

SW
8, 9, 21, 22

VOUT

COUT

M2
GND1

SHDN
THERMAL SHDNVIN UVLO

6, 7
GND2
10, 11RT
SYNC/MODE
GND
18

12

8640 BD

8640faFor more information www.linear.com/LT8640

LT8640/LT8640-1
Operation
The LT8640/LT8640-1 is a monolithic, constant frequency,
current mode step-down DC/DC converter. An oscillator,
with frequency set using a resistor on the RT pin, turns
on the internal top power switch at the beginning of eachclock cycle. Current in the inductor then increases until
the top switch current comparator trips and turns off the
top power switch. The peak inductor current at which
the top switch turns off is controlled by the voltage on
the internal VC node. The error amplifier servos the VC
node by comparing the voltage on the VFB pin with aninternal 0.97V reference. When the load current increases
it causes a reduction in the feedback voltage relative tothe reference leading the error amplifier to raise the VC
voltage until the average inductor current matches the newload current. When the top power switch turns off, thesynchronous power switch turns on until the next clock
cycle begins or inductor current falls to zero. If overload
conditions result in more than 10A flowing through thebottom switch, the next clock cycle will be delayed until
switch current returns to a safe level.
If the EN/UV pin is low, the LT8640/LT8640-1 is shut
down and draws 1µA from the input. When the EN/UV pin
is above 1V, the switching regulator will become active.
To optimize efficiency at light loads, the LT8640/LT8640-1
operates in Burst Mode operation in light load situations.
Between bursts, all circuitry associated with controllingthe output switch is shut down, reducing the input supply
current to 1.7µA. In a typical application, 2.5µA will be
consumed from the input supply when regulating with noload. The SYNC/MODE pin is tied low to use Burst Modeoperation and can be floated to use pulse-skipping mode
(LT8640) or forced continuous mode (FCM) (LT8640-1).
If a clock is applied to the SYNC/MODE pin, the part will
synchronize to an external clock frequency and operate ineither pulse-skipping mode (LT8640) or FCM (LT8640-1).
While in pulse-skipping mode (LT8640 only), the oscillator operates continuously and positive SW transitions arealigned to the clock. During light loads, switch pulses are
skipped to regulate the output and the quiescent current
will be several hundred µA.

The LT8640-1 can operate in forced continuous mode
(FCM) for fast transient response and full frequency operation over a wide load range. When in FCM the oscillator
operates continuously and positive SW transitions arealigned to the clock. Negative inductor current is allowed.
The LT8640-1 can sink current from the output and return
this charge to the input in this mode, improving load steptransient response.
To improve EMI/EMC, the LT8640/LT8640-1 can operate inspread spectrum mode. This feature varies the clock witha triangular frequency modulation of +20%. For example,
if the LT8640/LT8640-1’s frequency is programmed toswitch at 2MHz, spread spectrum mode will modulate
the oscillator between 2MHz and 2.4MHz. The SYNC/
MODE pin should be tied high to INTVCC (~3.4V) to enablespread spectrum modulation with either pulse-skipping
mode (LT8640) or forced continuous mode (LT8640-1).
To improve efficiency across all loads, supply current tointernal circuitry can be sourced from the BIAS pin whenbiased at 3.3V or above. Else, the internal circuitry willdraw current from VIN. The BIAS pin should be connectedto VOUT if the LT8640/LT8640-1 output is programmed
at 3.3V to 25V.
Comparators monitoring the FB pin voltage will pull thePG pin low if the output voltage varies more than ±8%(typical) from the set point, or if a fault condition is present.
The oscillator reduces the LT8640/LT8640-1’s operating
frequency when the voltage at the FB pin is low. Thisfrequency foldback helps to control the inductor current
when the output voltage is lower than the programmed
value which occurs during start-up or overcurrent conditions. When a clock is applied to the SYNC/MODE pin, theSYNC/MODE pin is floated, or held DC high, the frequency
foldback is disabled and the switching frequency will slow
down only during overcurrent conditions.

8640faFor more information www.linear.com/LT8640

13

LT8640/LT8640-1
Applications Information
Low EMI PCB Layout

Note that large, switched currents flow in the LT8640/
LT8640-1 VIN1, VIN2, GND1, and GND2 pins and the input
capacitors (CIN1, CIN2). The loops formed by the input
capacitors should be as small as possible by placing thecapacitors adjacent to the VIN1/2 and GND1/2 pins. Capacitors with small case size such as 0603 are optimal due tolowest parasitic inductance.

The LT8640/LT8640-1 is specifically designed to minimize
EMI/EMC emissions and also to maximize efficiency whenswitching at high frequencies. For optimal performancethe LT8640/LT8640-1 requires the use of multiple VIN
bypass capacitors.
Two small 1µF capacitors should be placed as close as
possible to the LT8640/LT8640-1: One capacitor should
be tied to VIN1/GND1; a second capacitor should be tied
to VIN2/GND2. A third capacitor with a larger value, 2.2µF
or higher, should be placed near VIN1 or VIN2.

The input capacitors, along with the inductor and output
capacitors, should be placed on the same side of thecircuit board, and their connections should be made onthat layer. Place a local, unbroken ground plane under theapplication circuit on the layer closest to the surface layer.
The SW and BOOST nodes should be as small as possible.
Finally, keep the FB and RT nodes small so that the groundtraces will shield them from the SW and BOOST nodes.

See Figure 1 for a recommended PCB layout.
For more detail and PCB design files refer to the Demo
Board guide for the LT8640/LT8640-1.

GROUND PLANE
ON LAYER 2
C1

R1

RPG

V

R2
CVCC

CSS

V
V

1

V

16
17

20

V
V

22

CIN1

6

21
7

RT

10

11

CIN2
CIN3

CBST

L

GROUND VIA

VIN VIA

VOUT VIACOUT

V OTHER SIGNAL VIAS

8640 F01

Figure 1. Recommended PCB Layout for the LT8640/LT8640-1

148640faFor more information www.linear.com/LT8640

LT8640/LT8640-1
Applications Information
The exposed pad on the bottom of the package should besoldered to SW to reduce thermal resistance to ambient. To
keep thermal resistance low, extend the ground plane from
GND1 and GND2 as much as possible, and add thermal
vias to additional ground planes within the circuit board
and on the bottom side.
Achieving Ultralow Quiescent Current (Burst Mode
Operation)of time the LT8640/LT8640-1 is in sleep mode increases,
resulting in much higher light load efficiency than for typical converters. By maximizing the time between pulses,
the converter quiescent current approaches 2.5µA for atypical application when there is no output load. Therefore,
to optimize the quiescent current performance at lightloads, the current in the feedback resistor divider must
be minimized as it appears to the output as load current.

As the output load decreases, the frequency of single current pulses decreases (see Figure 2a) and the percentage

In order to achieve higher light load efficiency, moreenergy must be delivered to the output during the single
small pulses in Burst Mode operation such that theLT8640/LT8640-1 can stay in sleep mode longer between each pulse. This can be achieved by using a larger
value inductor (i.e., 4.7µH), and should be considered
independent of switching frequency when choosing
an inductor. For example, while a lower inductor value
would typically be used for a high switching frequency
application, if high light load efficiency is desired, ahigher inductor value should be chosen. See curve in
Typical Performance Characteristics.

Burst Frequency

Minimum Load to Full Frequency (Pulse-Skipping Mode)

To enhance efficiency at light loads, the LT8640/LT8640-1
operates in low ripple Burst Mode operation, which keeps
the output capacitor charged to the desired output voltage
while minimizing the input quiescent current and minimizing output voltage ripple. In Burst Mode operation theLT8640/LT8640-1 delivers single small pulses of current
to the output capacitor followed by sleep periods where
the output power is supplied by the output capacitor. Whilein sleep mode the LT8640/LT8640-1 consumes 1.7µA.

1200

100

800
600
400

60

40

20

200
0

FRONT PAGE APPLICATIONVOUT = 5V
fSW = 1MHz

80
LOAD CURRENT (mA)

SWITCHING FREQUENCY (kHz)FRONT PAGE APPLICATION
VIN = 12V
1000 VOUT = 5V

0

100
200
300
LOAD CURRENT (mA)

(2a)

400

0

5

10

15

20 25
30 35
INPUT VOLTAGE (V)8640 F02a

40458640 F02b(2b)

Figure 2. SW Frequency vs Load Information in Burst Mode Operation (2a) and Pulse-Skipping Mode (2b)8640faFor more information www.linear.com/LT8640

15

LT8640/LT8640-1
Applications Information
While in Burst Mode operation the current limit of the top
switch is approximately 900mA (as shown in Figure 3),
resulting in low output voltage ripple. Increasing the output
capacitance will decrease output ripple proportionally. As
load ramps upward from zero the switching frequency
will increase but only up to the switching frequency
programmed by the resistor at the RT pin as shown in
Figure 2a.
The output load at which the LT8640/LT8640-1 reaches
the programmed frequency varies based on input voltage,
output voltage and inductor choice. To select low ripple
Burst Mode operation, tie the SYNC/MODE pin below 0.4V
(this can be ground or a logic low output).

IL
500mA/DIV
VSW
5V/DIV
5µs/DIVFRONT PAGE APPLICATION
12VIN TO 5VOUT AT 10mA
VSYNC = 0V

8640 F03

Figure 3. Burst Mode Operation

Forced Continuous Mode (LT8640-1 Only)
The LT8640-1 can operate in forced continuous mode
(FCM) for fast transient response and full frequency operation over a wide load range. When in FCM, the oscillator
operates continuously and positive SW transitions arealigned to the clock. Negative inductor current is allowed
at light loads or under large transient conditions. The
LT8640-1 can sink current from the output and return
this charge to the input in this mode, improving load steptransient response (see Figure 4). At light loads, FCM
operation is less efficient than Burst Mode operation or
pulse-skipping mode, but may be desirable in applications
where it is necessary to keep switching harmonics outof the signal band. FCM must be used if the output isrequired to sink current. To enable FCM (LT8640-1 only),
float the SYNC/MODE pin. Leakage current on this pinshould be <1µA.
FCM is disabled if the VIN pin is held above 37V or if the FB
pin is held greater than 9.5% above the feedback reference
voltage. FCM is also disabled during soft-start until thesoft-start capacitor is fully charged. When FCM is disabledin these ways, negative inductor current is not allowed andthe LT8640-1 operates in pulse-skipping mode.
For robust operation over a wide VIN and VOUT range, use
an inductor value greater than LMIN:

Pulse-Skipping Mode (LT8640 Only)
For some applications, it is desirable for the LT8640 tooperate in pulse-skipping mode, offering two major differences from Burst Mode operation. First is the clock stays
awake at all times and all switching cycles are aligned tothe clock. In this mode, much of the internal circuitry isawake at all times, increasing quiescent current to several
hundred µA. Second is that full switching frequency isreached at lower output load than in Burst Mode operation(see Figure 2b). To enable pulse-skipping mode (LT8640
only), float the SYNC/MODE pin. Leakage current on thispin should be <1µA.

LMIN =

VOUT ⎛ VOUT ⎞• ⎜ 1–⎟
2 • fSW ⎝40 ⎠FRONT PAGE APPLICATION
100mA to 1.1A TRANSIENT
BURST MODE

VOUT
200mA/DIVFCM
ILOAD
1A/DIV
12VIN, 5VOUT
COUT = 47µF
50µs/DIV

8640 F04

Figure 4. Load Step Transient Response withand without Forced Continuous Mode

16

8640faFor more information www.linear.com/LT8640

LT8640/LT8640-1
Applications Information
Spread Spectrum ModeFB Resistor Network

The LT8640/LT8640-1 features spread spectrum operation
to further reduce EMI/EMC emissions. To enable spread
spectrum operation, the SYNC/MODE pin should be tiedhigh to INTVCC (~3.4V). In this mode, triangular frequency
modulation is used to vary the switching frequency
between the value programmed by RT to approximately
20% higher than that value. The modulation frequency
is approximately 3kHz. For example, when the LT8640/
LT8640-1 is programmed to 2MHz, the frequency willvary from 2MHz to 2.4MHz at a 3kHz rate. When spread
spectrum operation is selected, Burst Mode operation isdisabled, and the part will run in either pulse-skipping
mode (LT8640) or forced continuous mode (LT8640-1).

The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the resistor
values according to:Synchronization
To synchronize the LT8640/LT8640-1 oscillator to an
external frequency, connect a square wave (with 20%
to 80% duty cycle) to the SYNC/MODE pin. The squarewave amplitude should have valleys that are below 0.4V
and peaks above 1.5V (up to 6V).
The LT8640/LT8640-1 will not enter Burst Mode operation at low output loads while synchronized to an external
clock, but instead will pulse-skip (LT8640) or run forced
continuous mode (LT8640-1) to maintain regulation. The
LT8640/LT8640-1 may be synchronized over a 200kHz to
3MHz range. The RT resistor should be chosen to set theLT8640/LT8640-1 switching frequency equal to or below
the lowest synchronization input. For example, if thesynchronization signal will be 500kHz and higher, the RTshould be selected for 500kHz. The slope compensation isset by the RT value, while the minimum slope compensation
required to avoid subharmonic oscillations is establishedby the inductor size, input voltage and output voltage.
Since the synchronization frequency will not change theslopes of the inductor current waveform, if the inductor
is large enough to avoid subharmonic oscillations at thefrequency set by RT, then the slope compensation will be
sufficient for all synchronization frequencies.

⎛ V⎞
R1= R2 ⎜ OUT – 1⎟
⎝ 0.970V ⎠(1)

Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain output voltage
accuracy.
If low input quiescent current and good light-load efficiency
are desired, use large resistor values for the FB resistor
divider. The current flowing in the divider acts as a load
current, and will increase the no-load input current to theconverter, which is approximately:⎛ V⎞ ⎛ V ⎞ ⎛ 1⎞
IQ = 1.7µA + ⎜ OUT ⎟ ⎜ OUT ⎟ ⎜ ⎟⎝ R1+R2 ⎠ ⎝ VIN ⎠ ⎝ n ⎠(2)

where 1.7µA is the quiescent current of the LT8640/
LT8640-1 and the second term is the current in the feedback divider reflected to the input of the buck operating
at its light load efficiency n. For a 3.3V application with
R1 = 1M and R2 = 412k, the feedback divider draws 2.3µA.
With VIN = 12V and n = 80%, this adds 0.8µA to the 1.7µA
quiescent current resulting in 2.5µA no-load current fromthe 12V supply. Note that this equation implies that the
no-load current is a function of VIN; this is plotted in the
Typical Performance Characteristics section.
When using large FB resistors, a 4.7pF to 22pF phase-lead
capacitor should be connected from VOUT to FB.

8640faFor more information www.linear.com/LT8640

17

LT8640/LT8640-1
Applications Information
Setting the Switching Frequency
The LT8640/LT8640-1 uses a constant frequency PWM
architecture that can be programmed to switch from
200kHz to 3MHz by using a resistor tied from the RT pin
to ground. A table showing the necessary RT value for adesired switching frequency is in Table 1.
The RT resistor required for a desired switching frequency
can be calculated using:
RT =

46.5
– 5.2
fSW

(3)

where RT is in kΩ and fSW is the desired switching frequency in MHz.
Table 1. SW Frequency vs RT ValuefSW (MHz)RT (kΩ)

0.2

232

0.3

150

0.4

110

0.5

88.7

0.6

71.5

0.7

60.4

0.8

52.3

1.0

41.2

1.2

33.2

1.4

28.0

1.6

23.7

1.8

20.5

2.0

18.2

2.2

15.8

3.0

10.7

Operating Frequency Selection and Trade-Offs
Selection of the operating frequency is a trade-off between
efficiency, component size, and input voltage range. The
advantage of high frequency operation is that smaller inductor and capacitor values may be used. The disadvantages
are lower efficiency and a smaller input voltage range.

18

The highest switching frequency (fSW(MAX)) for a given
application can be calculated as follows:fSW(MAX) =(VOUT + VSW(BOT)tON(MIN) VIN – VSW(TOP) + VSW(BOT))(4)

where VIN is the typical input voltage, VOUT is the output
voltage, VSW(TOP) and VSW(BOT) are the internal switch
drops (~0.3V, ~0.15V, respectively at maximum load)
and tON(MIN) is the minimum top switch on-time (see theElectrical Characteristics). This equation shows that aslower switching frequency is necessary to accommodatea high VIN/VOUT ratio.
For transient operation, VIN may go as high as the absolute
maximum rating of 42V regardless of the RT value, however
the LT8640/LT8640-1 will reduce switching frequency
as necessary to maintain control of inductor current toassure safe operation.
The LT8640/LT8640-1 is capable of a maximum duty cycleof approximately 99%, and the VIN-to-VOUT dropout islimited by the RDS(ON) of the top switch. In this mode theLT8640/LT8640-1 skips switch cycles, resulting in a lower
switching frequency than programmed by RT.
For applications that cannot allow deviation from the programmed switching frequency at low VIN/VOUT ratios use
the following formula to set switching frequency:
VIN(MIN) =VOUT + VSW(BOT)
1– fSW • tOFF(MIN)– VSW(BOT) + VSW(TOP) (5)where VIN(MIN) is the minimum input voltage withoutskipped cycles, VOUT is the output voltage, VSW(TOP) andVSW(BOT) are the internal switch drops (~0.3V, ~0.15V,
respectively at maximum load), fSW is the switching frequency (set by RT), and tOFF(MIN) is the minimum switch
off-time. Note that higher switching frequency will increase
the minimum input voltage below which cycles will be
dropped to achieve higher duty cycle.

8640faFor more information www.linear.com/LT8640

LT8640/LT8640-1
Applications Information
Inductor Selection and Maximum Output Current
The LT8640/LT8640-1 is designed to minimize solution sizeby allowing the inductor to be chosen based on the output
load requirements of the application. During overload or
short-circuit conditions the LT8640/LT8640-1 safely tolerates operation with a saturated inductor through the useof a high speed peak-current mode architecture.
A good first choice for the inductor value is:L=

VOUT + VSW(BOT)fSW

IOUT(MAX) =ILIM –(6)

where fSW is the switching frequency in MHz, VOUT is
the output voltage, VSW(BOT) is the bottom switch drop(~0.15V) and L is the inductor value in µH.
To avoid overheating and poor efficiency, an inductor must
be chosen with an RMS current rating that is greater thanthe maximum expected output load of the application. In
addition, the saturation current (typically labeled ISAT)
rating of the inductor must be higher than the load current
plus 1/2 of in inductor ripple current:
1
IL(PEAK) =ILOAD(MAX) + ΔIL
2

The LT8640/LT8640-1 limits the peak switch current inorder to protect the switches and the system from overload
faults. The top switch current limit (ILIM) is 10A at low
duty cycles and decreases linearly to 7A at DC = 0.8. The
inductor value must then be sufficient to supply the desired
maximum output current (IOUT(MAX)), which is a functionof the switch current limit (ILIM) and the ripple current.

(7)

where ∆IL is the inductor ripple current as calculated in
Equation 9 and ILOAD(MAX) is the maximum output load
for a given application.
As a quick example, an application requiring 3A output
should use an inductor with an RMS rating of greater than
3A and an ISAT of greater than 4A. During long duration
overload or short-circuit conditions, the inductor RMS
rating requirement is greater to avoid overheating of theinductor. To keep the efficiency high, the series resistance(DCR) should be less than 0.04Ω, and the core material
should be intended for high frequency applications.

ΔIL
2

(8)

The peak-to-peak ripple current in the inductor can becalculated as follows:
ΔIL =⎞VOUT ⎛V• ⎜ 1– OUT ⎟L • fSW ⎝ VIN(MAX) ⎠(9)

where fSW is the switching frequency of the LT8640/
LT8640-1, and L is the value of the inductor. Therefore,
the maximum output current that the LT8640/LT86401 will deliver depends on the switch current limit, theinductor value, and the input and output voltages. The
inductor value may have to be increased if the inductor
ripple current does not allow sufficient maximum output
current (IOUT(MAX)) given the switching frequency, andmaximum input voltage used in the desired application.
In order to achieve higher light load efficiency, more energy
must be delivered to the output during the single small
pulses in Burst Mode operation such that the LT8640/
LT8640-1 can stay in sleep mode longer between eachpulse. This can be achieved by using a larger value inductor (i.e., 4.7µH), and should be considered independentof switching frequency when choosing an inductor. For
example, while a lower inductor value would typically be
used for a high switching frequency application, if highlight load efficiency is desired, a higher inductor value
should be chosen. See curve in Typical Performance
Characteristics.

8640faFor more information www.linear.com/LT8640

19

LT8640/LT8640-1
Applications Information
The optimum inductor for a given application may differ
from the one indicated by this design guide. A larger
value inductor provides a higher maximum load current
and reduces the output voltage ripple. For applications
requiring smaller load currents, the value of the inductor
may be lower and the LT8640/LT8640-1 may operatewith higher ripple current. This allows use of a physicallysmaller inductor, or one with a lower DCR resulting inhigher efficiency. Be aware that low inductance may resultin discontinuous mode operation, which further reduces
maximum load current.
For more information about maximum output current
and discontinuous operation, see Linear Technology’s
Application Note 44.
For duty cycles greater than 50% (VOUT/VIN > 0.5), a
minimum inductance is required to avoid sub-harmonic
oscillation. See Application Note 19.
Input Capacitors
The VIN of the LT8640/LT8640-1 should be bypassed withat least three ceramic capacitors for best performance. Twosmall ceramic capacitors of 1µF should be placed close to
the part; one at the VIN1/GND1 pins and a second at VIN2/
GND2 pins. These capacitors should be 0402 or 0603 insize. For automotive applications requiring 2 series input
capacitors, two small 0402 or 0603 may be placed ateach side of the LT8640/LT8640-1 near the VIN1/GND1
and VIN2/GND2 pins.
A third, larger ceramic capacitor of 2.2µF or larger shouldbe placed close to VIN1 or VIN2. See layout section for moredetail. X7R or X5R capacitors are recommended for best performance across temperature and input voltage variations.
Note that larger input capacitance is required when a lower
switching frequency is used. If the input power source hashigh impedance, or there is significant inductance due tolong wires or cables, additional bulk capacitance may benecessary. This can be provided with a low performanceelectrolytic capacitor.

20

A ceramic input capacitor combined with trace or cableinductance forms a high quality (under damped) tankcircuit. If the LT8640/LT8640-1 circuit is plugged into alive supply, the input voltage can ring to twice its nominal
value, possibly exceeding the LT8640/LT8640-1’s voltage
rating. This situation is easily avoided (see Linear Technology Application Note 88).
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Alongwith the inductor, it filters the square wave generated by
the LT8640/LT8640-1 to produce the DC output. In thisrole it determines the output ripple, thus low impedance atthe switching frequency is important. The second function
is to store energy in order to satisfy transient loads andstabilize the LT8640/LT8640-1’s control loop. Ceramic
capacitors have very low equivalent series resistance (ESR)
and provide the best ripple performance. For good starting
values, see the Typical Applications section.
Use X5R or X7R types. This choice will provide low output
ripple and good transient response. Transient performancecan be improved with a higher value output capacitor andthe addition of a feedforward capacitor placed between
VOUT and FB. Increasing the output capacitance will also
decrease the output voltage ripple. A lower value of output
capacitor can be used to save space and cost but transient
performance will suffer and may cause loop instability. See
the Typical Applications in this data sheet for suggested
capacitor values.
When choosing a capacitor, special attention should begiven to the data sheet to calculate the effective capacitanceunder the relevant operating conditions of voltage bias andtemperature. A physically larger capacitor or one with ahigher voltage rating may be required.

8640faFor more information www.linear.com/LT8640

LT8640/LT8640-1
Applications Information
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT8640/LT8640-1 due to theirpiezoelectric nature. When in Burst Mode operation, theLT8640/LT8640-1’s switching frequency depends on theload current, and at very light loads the LT8640/LT86401 can excite the ceramic capacitor at audio frequencies,
generating audible noise. Since the LT8640/LT8640-1
operates at a lower current limit during Burst Mode operation, the noise is typically very quiet to a casual ear. If
this is unacceptable, use a high performance tantalum orelectrolytic capacitor at the output. Low noise ceramic
capacitors are also available.
A final precaution regarding ceramic capacitors concerns the
maximum input voltage rating of the LT8640/LT8640-1. As
previously mentioned, a ceramic input capacitor combinedwith trace or cable inductance forms a high quality (underdamped) tank circuit. If the LT8640/LT8640-1 circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT8640/
LT8640-1’s rating. This situation is easily avoided (seeLinear Technology Application Note 88).
Enable Pin
The LT8640/LT8640-1 is in shutdown when the EN pin is
low and active when the pin is high. The rising thresholdof the EN comparator is 1.0V, with 40mV of hysteresis.
The EN pin can be tied to VIN if the shutdown feature is not
used, or tied to a logic level if shutdown control is required.
Adding a resistor divider from VIN to EN programs theLT8640/LT8640-1 to regulate the output only when VIN isabove a desired voltage (see the Block Diagram). Typically,
this threshold, VIN(EN), is used in situations where the input
supply is current limited, or has a relatively high source
resistance. A switching regulator draws constant powerfrom the source, so source current increases as source
voltage drops. This looks like a negative resistance loadto the source and can cause the source to current limit orlatch low under low source voltage conditions. The VIN(EN)threshold prevents the regulator from operating at source
voltages where the problems might occur. This threshold
can be adjusted by setting the values R3 and R4 such thatthey satisfy the following equation:⎛ R3 ⎞
VIN(EN) = ⎜ +1⎟ •1.0V⎝ R4 ⎠(10)

where the LT8640/LT8640-1 will remain off until VIN isabove VIN(EN). Due to the comparator’s hysteresis, switching will not stop until the input falls slightly below VIN(EN).
When operating in Burst Mode operation for light load
currents, the current through the VIN(EN) resistor network
can easily be greater than the supply current consumed
by the LT8640/LT8640-1. Therefore, the VIN(EN) resistors
should be large to minimize their effect on efficiency at
low loads.
INTVCC Regulator
An internal low dropout (LDO) regulator produces the 3.4V
supply from VIN that powers the drivers and the internalbias circuitry. The INTVCC can supply enough current for
the LT8640/LT8640-1’s circuitry and must be bypassed
to ground with a minimum of 1µF ceramic capacitor.
Good bypassing is necessary to supply the high transientcurrents required by the power MOSFET gate drivers. To
improve efficiency the internal LDO can also draw current
from the BIAS pin when the BIAS pin is at 3.1V or higher.
Typically the BIAS pin can be tied to the output of theLT8640/LT8640-1, or can be tied to an external supply of
3.3V or above. If BIAS is connected to a supply other than
VOUT, be sure to bypass with a local ceramic capacitor. If
the BIAS pin is below 3.0V, the internal LDO will consume
current from VIN. Applications with high input voltage andhigh switching frequency where the internal LDO pulls
current from VIN will increase die temperature becauseof the higher power dissipation across the LDO. Do notconnect an external load to the INTVCC pin.

8640faFor more information www.linear.com/LT8640

21

LT8640/LT8640-1
Applications Information
Output Voltage Tracking and Soft-Start
The LT8640/LT8640-1 allows the user to program its output
voltage ramp rate by means of the TR/SS pin. An internal
1.9µA pulls up the TR/SS pin to INTVCC. Putting an external
capacitor on TR/SS enables soft starting the output to prevent current surge on the input supply. During the soft-start
ramp the output voltage will proportionally track the TR/SS
pin voltage. For output tracking applications, TR/SS canbe externally driven by another voltage source. From 0V to
0.97V, the TR/SS voltage will override the internal 0.97V
reference input to the error amplifier, thus regulating the
FB pin voltage to that of TR/SS pin. When TR/SS is above
0.97V, tracking is disabled and the feedback voltage will
regulate to the internal reference voltage. The TR/SS pin
may be left floating if the function is not needed.
An active pull-down circuit is connected to the TR/SS pin
which will discharge the external soft-start capacitor in
the case of fault conditions and restart the ramp when thefaults are cleared. Fault conditions that clear the soft-startcapacitor are the EN/UV pin transitioning low, VIN voltage
falling too low, or thermal shutdown.
Output Power Good
When the LT8640/LT8640-1’s output voltage is within the±8% window of the regulation point, the output voltage
is considered good and the open-drain PG pin goes highimpedance and is typically pulled high with an external
resistor. Otherwise, the internal pull-down device will pull
the PG pin low. To prevent glitching both the upper andlower thresholds include 0.2% of hysteresis.
The PG pin is also actively pulled low during several fault
conditions: EN/UV pin is below 1V, INTVCC has fallen too
low, VIN is too low, or thermal shutdown.

control. Second, the bottom switch current is monitored
such that if inductor current is beyond safe levels switching of the top switch will be delayed until such time as theinductor current falls to safe levels.
Frequency foldback behavior depends on the state of the
SYNC pin: If the SYNC pin is low the switching frequency
will slow while the output voltage is lower than the programmed level. If the SYNC pin is connected to a clock
source, floated or tied high, the LT8640/LT8640-1 will stayat the programmed frequency without foldback and only
slow switching if the inductor current exceeds safe levels.
There is another situation to consider in systems where
the output will be held high when the input to the LT8640/
LT8640-1 is absent. This may occur in battery charging applications or in battery-backup systems wherea battery or some other supply is diode ORed with theLT8640/LT8640-1’s output. If the VIN pin is allowed tofloat and the EN pin is held high (either by a logic signal
or because it is tied to VIN), then the LT8640/LT8640-1’s
internal circuitry will pull its quiescent current throughits SW pin. This is acceptable if the system can tolerate
several µA in this state. If the EN pin is grounded the SW
pin current will drop to near 1µA. However, if the VIN pin
is grounded while the output is held high, regardless of
EN, parasitic body diodes inside the LT8640/LT8640-1
can pull current from the output through the SW pin andthe VIN pin. Figure 5 shows a connection of the VIN and
EN/UV pins that will allow the LT8640/LT8640-1 to run
only when the input voltage is present and that protects
against a shorted or reversed input.
D1
VIN

Shorted and Reversed Input Protection
The LT8640/LT8640-1 will tolerate a shorted output.
Several features are used for protection during output
short-circuit and brownout conditions. The first is theswitching frequency will be folded back while the output
is lower than the set point to maintain inductor current

22

VIN
LT8640/LT8640-1
EN/UV
GND
8640 F05

Figure 5. Reverse VIN Protection

8640faFor more information www.linear.com/LT8640

LT8640/LT8640-1
Applications Information
Thermal Considerations and Peak Output Current
For higher ambient temperatures, care should be takenin the layout of the PCB to ensure good heat sinking of
the LT8640/LT8640-1. The ground pins on the bottom of
the package should be soldered to a ground plane. Thisground should be tied to large copper layers below withthermal vias; these layers will spread heat dissipated by
the LT8640/LT8640-1. Placing additional vias can reduce
thermal resistance further. The maximum load current
should be derated as the ambient temperature approaches
the maximum junction rating. Power dissipation within theLT8640/LT8640-1 can be estimated by calculating the total
power loss from an efficiency measurement and subtracting the inductor loss. The die temperature is calculated bymultiplying the LT8640/LT8640-1 power dissipation by thethermal resistance from junction to ambient.
The internal overtemperature protection monitors the junction temperature of the LT8640/LT8640-1. If the junction
temperature reaches approximately 170°C, the LT8640/

60

DC2202A DEMO BOARD
VIN = 12V
VOUT = 5V
fSW = 2MHz
STANDBY LOAD = 0.25A
PULSED LOAD = 7A

80

50
40
30
20
10
0

The LT8640/LT8640-1’s internal power switches arecapable of safely delivering up to 7A of peak output current. However, due to thermal limits, the package canonly handle 7A loads for short periods of time. This timeis determined by how quickly the case temperature approaches the maximum junction rating. Figure 6 shows
an example of how case temperature rise changes withthe duty cycle of a 1kHz pulsed 7A load.

90

DC2202A DEMO BOARD
VIN = 12V, fSW = 1MHz
VIN = 24V, fSW = 1MHz
VIN = 12V, fSW = 2MHz
VIN = 24V, fSW = 2MHz70

Temperature rise of the LT8640/LT8640-1 is worst when
operating at high load, high VIN, and high switching frequency. If the case temperature is too high for a given
application, then either VIN, switching frequency, or load
current can be decreased to reduce the temperature toan acceptable level. Figure 6 shows examples of how
case temperature rise can be managed by reducing VIN,
switching frequency, or load.

CASE TEMPERATURE RISE (°C)

CASE TEMPERATURE RISE (°C)

80

LT8640-1 will stop switching and indicate a fault condition
until the temperature drops about 10°C cooler.

70
60
50
40
30
20
10

0

1

2

3
LOAD CURRENT (A)

4

0

5

0.2

0.4
0.6
DUTY CYCLE OF 7A LOAD

0.8
8640 F07

8640 F06

Figure 6. Case Temperature Rise0

Figure 7. Case Temperature Rise vs 7A Pulsed Load

8640faFor more information www.linear.com/LT8640

23

LT8640/LT8640-1
Typical Applications
5V 5A Step-Down Converter
VIN
5.5V TO 42V

4.7µF

EN/UV

VIN1

1µF
0603

GND1
PG

10nF

VIN2

1µF
0603

GND2

LT8640/LT8640-1 BSTSYNC/MODE
TR/SS

0.1µF 3.3µH

1µF

4.7pF

1M

FB

INTVCC

41.2k

VOUT
5V
5A

SW
BIAS

RT

243k

GND

fSW = 1MHz
L: VISHAY IHLP2525EZ-01

47µF
1210
X7R

8640 TA08

3.3V, 5A Step-Down Converter
VIN
3.8V TO 42V

4.7µF

EN/UV

VIN1

1µF
0603

GND1
PG

10nF

VIN2
GND2

LT8640/LT8640-1 BSTSYNC/MODE
TR/SS

0.1µF 2.2µH

VOUT
3.3V
5A

SW
BIAS

1µF

4.7pF

1M

FB

INTVCC

41.2k

1µF
0603

RT

412k

GND

fSW = 1MHz
L: VISHAY IHLP2525EZ-01

47µF
1210
X7R

8640 TA05

Ultralow EMI 5V, 5A Step-Down Converter
VIN
5.6V TO 42V

FB1
BEAD

L2
6.8µH

10µF
1210

10µF
1210

10µF
1206
1µF
0603

EN/UV

VIN1
GND1
PG

10nF

VIN2
GND2

LT8640/LT8640-1 BSTSYNC/MODE
TR/SS

0.1µF 1.5µH

4.7pF
INTVCC
RT

1M

FB
GND

fSW = 2MHzFB1 BEAD: WE-MPSB 100Ω 8A 1812
L: WE-LHMI7030
L2: COILCRAFT XAL6060

24

VOUT
5V
5A

SW
BIAS

1µF
18.2k

1µF
0603

243k

47µF
1210
X7R

8640 TA02

8640faFor more information www.linear.com/LT8640

LT8640/LT8640-1
Typical Applications
2MHz 5V, 5A Step-Down Converter
VIN
5.5V TO 42V

4.7µF

EN/UV

VIN1

1µF
0603

GND1
PG

10nF

VIN2
GND2

LT8640/LT8640-1 BSTSYNC/MODE
TR/SS

0.1µF 1.5µH

VOUT
5V
5A

SW
BIAS

1µF

4.7pF

1M

FB

INTVCC

18.2k

1µF
0603

RT

243k

GND

fSW = 2MHz
L: VISHAY IHLP2525CZ-01

47µF
1210
X7R

8640 TA03

2MHz 3.3V, 5A Step-Down Converter
VIN
3.8V TO 42V

4.7µF

EN/UV

VIN1

1µF
0603

GND1
PG

10nF

VIN2
GND2

LT8640/LT8640-1 BSTSYNC/MODE
TR/SS

0.1µF

1µH

VOUT
3.3V
5A

SW
BIAS

1µF

4.7pF

1M

FB

INTVCC

18.2k

1µF
0603

RT

412k

GND

fSW = 2MHz
L: VISHAY IHLP2525CZ-01

47µF
1210
X7R

8640 TA06

12V, 5A Step-Down Converter
VIN
12.5V TO 42V

4.7µF

EN/UV

VIN1

1µF
0603

GND1
PG

10nF

VIN2
GND2

LT8640/LT8640-1 BSTSYNC/MODE
TR/SS

0.1µF 4.7µH

4.7pF
INTVCC
RT

VOUT
12V
5A

SW
BIAS

1µF
41.2k

1µF
0603

1M

FB
GND

fSW = 1MHz
L: VISHAY IHLP2525EZ-01

47µF
1210

88.7k

8640 TA04

8640faFor more information www.linear.com/LT8640

25

LT8640/LT8640-1
Package Description

Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
UDC Package
18-Lead Plastic QFN (3mm × 4mm)

(Reference LTC DWG # 05-08-1956 Rev B)

Exposed Pad Variation AA
0.055 BSC
0.70 ±0.05
3.50 ±0.05
2.10 ±0.05
1.50 REF

0.770
BSC

0.220 ±0.05
0.356 ±0.05

0.400 ±0.05
PACKAGE OUTLINE
0.25 ±0.05
0.50 BSC
2.50 REF
3.10 ±0.05
4.50 ±0.05
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONSAPPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
3.00 ±0.10

0.75 ±0.05

1.50 REF

PIN 1 ID
0.12 × 45°

0.40 ±0.10
1

PIN 1
TOP MARK(NOTE 5)4.00 ±0.10

0.220 ±0.05

2.127 ±0.10

22.50 REF
0.770
BSC

0.356 ±0.05
0.400 ±0.05(UDC18) QFN 1213 REV B

0.200 REF
0.00 – 0.05

R = 0.110
TYP

0.25 ±0.05
0.50 BSC

BOTTOM VIEW—EXPOSED PAD
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. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATIONON THE TOP AND BOTTOM OF PACKAGE

26

8640faFor more information www.linear.com/LT8640

LT8640/LT8640-1
Revision History
REV

DATE

DESCRIPTION

A

10/15

Added LT8640-1 Version to Title

PAGE NUMBERAll

Clarified Minimum On-Time to 35ns

1, 3

Added LT8640-1 Version

1, 2, 4

Clarified SYNC/MODE Threshold

3

Added LT8640-1 Version Specifications

4

Clarified Minimum Load Graph to Pulse Skipping7

Added LT8640-1 Version

11 to 25

8640faInformation 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 interconnectionof its circuits
as describedherein will not infringe on existing patent rights.
For more
informationwww.linear.com/LT8640

27

LT8640/LT8640-1
Typical Applications
VIN
3.4V TO 22V(42V TRANSIENT)

2MHz 1.8V, 5A Step-Down Converter
4.7µF

EN/UV

VIN1

1µF
0603

GND1
PG

10nF

VIN2
GND2

LT8640/LT8640-1 BSTSYNC/MODE
TR/SS

SW
BIAS

1µF
18.2k

INTVCC
RT

FB

1µF
0603
0.1µF

1µH

EXTERNAL
1µF SOURCE >3.1V
OR GND

VOUT
1.8V
5A
10pF

866k

100µF
1210

1M

GND

fSW = 2MHz
L: VISHAY IHLP2525CZ-01

8640 TA07

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Converter with IQ = 2.7uA

VIN(MIN) = 4.3V, VIN(MAX) = 42V, VOUT(MIN) = 1.2V, IQ = 2.7µA,
ISD < 1µA, MSOP-16E

LT3976

40V, 5A, High Efficiency, 2MHz MicroPower Step-Down DC/DC
Converter with IQ = 3.3uA

VIN(MIN) = 4.3V, VIN(MAX) = 40V, VOUT(MIN) = 1.2V, IQ = 3.3µA,
ISD < 1µA, MSOP-16E

LT8612

42V, 6A, 96% Efficiency, 2.2MHz Synchronous MicroPower Step-Down
DC/DC Converter with IQ = 2.5µA

VIN(MIN) = 3.4V, VIN(MAX) = 42V, VOUT(MIN) = 0.97V, IQ = 3.0µA,
ISD < 1µA, 3mm × 6mm QFN-28

LT8613

42V, 6A, 96% Efficiency, 2.2MHz Synchronous MicroPower StepDown DC/DC Converter with Current Limiting

VIN(MIN) = 3.4V, VIN(MAX) = 42V, VOUT(MIN) = 0.97V, IQ = 3.0µA,
ISD < 1µA, 3mm × 6mm QFN-28

LT8602

42V, Quad Output (2.5A + 1.5A + 1.5A + 1.5A) 95% Efficiency, 2.2MHz
Synchronous MicroPower Step-Down DC/DC Converter with IQ = 25µA

VIN(MIN) = 3V, VIN(MAX) = 42V, VOUT(MIN) = 0.8V, IQ = 2.5µA,
ISD < 1µA, 6mm × 6mm QFN-40

28 Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417
For more information www.linear.com/LT8640(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com/LT8640

8640faLT 1015 • PRINTED IN USA

 LINEAR TECHNOLOGY CORPORATION 2015