LTC3453
Synchronous Buck-Boost
High Power White LED DriverU

FEATURESDESCRIPTIO■The LTC®3453 is a synchronous buck-boost DC/DC converter optimized for driving up to 4 white LEDs at a
combined current of up to 500mA from a single Li-Ion
battery input. The regulator operates in either synchronous buck, synchronous boost, or buck-boost mode,
depending on input voltage and LED maximum forward
voltage. Optimum efficiency is achieved using a proprietary architecture that determines which LED requires thelargest forward voltage drop at its programmed current,
and regulates the common output rail for lowest dropout.
Efficiency of 90% can be achieved over the entire usable
range of a Li-Ion battery (2.7V to 4.2V).

High Efficiency: 90% Typical Over Entire
Li-Ion Battery Range■ Wide V Range: 2.7V to 5.5V
IN
■ Up to 500mA Continuous Output Current■ Internal Soft-Start■ Open/Shorted LED Protection■ LED Current Matching Typically <2%■ Constant Frequency 1MHz Operation■ Low Shutdown Current: 6µA■ Overtemperature Protection■ Small Thermally Enhanced 16-Lead (4mm x 4mm)
QFN PackageU
APPLICATIO S

LED current is programmable to one of four levels (including shutdown) with dual current setting resistors and dual
enable pins. In shutdown, the supply current is only 6µA.

Cell Phones
Digital Cameras■ PDAs■ Portable Devices■■

A high constant operating frequency of 1MHz allows theuse of a small external inductor. The LTC3453 is offeredin a low profile (0.75mm) thermally enhanced 16-lead(4mm x 4mm) QFN package.

, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.

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TYPICAL APPLICATIOHigh Efficiency Torch/Flash LED Driver
L1
4.7µH
VIN
1-CELL
Li-Ion2.7V to 4.2V

Torch Mode Efficiency vs VIN

VIN

2.2µF

PVIN

SW1

SW2

VOUT

150mA/500mA

4.7µF

LED2
VC
0.1µF

EN1 (TORCH)
EN2 (FLASH)

LED3

1MHz
BUCK-BOOST

LED4

EN1

D1: LUMILEDS LXCL-PWF1
L1: VISHAY DALE IDCS-2512

EN2
ISET1

8.25k
1%

ISET2
3.48k
1%

LTC3453
GND

GND

PGND

3453 TA01a

EN1

EN2

ILED

0
1
0
1

0
0
1
1

0 (SHUTDOWN)
150mA
350mA
500mA

180

90

160
EFFICIENCY

80

140

70

120
IIN

60

INPUT CURRENT (mA)

D1
LED1

LED POWER EFFICIENCY PLED/PIN (%)

100

100
ILED = 150mA
TA = 25°C

50
2.7

3.1

3.5

80
3.9 4.3
VIN (V)

4.7

5.1

5.5

3453 TA01b

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LTC3453

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ABSOLUTEAXI U RATI GS

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PACKAGE/ORDER I FOR ATIO

(Note 1)
VOUT

SW2

PVIN

SW1

TOP VIEW

VIN, PVIN, SW1, SW2, VOUT Voltage ............ –0.3V to 6V
LED1 to LED4 Voltage ...... –0.3V to (VOUT + 0.3V) or 6V
VC, EN1, EN2,
ISET1, ISET2 Voltage .......... –0.3V to (VIN + 0.3V) or 6V
LED1 to LED4 Peak Current ................................ 250mA
Storage Temperature Range .................. –65°C to 125°C
Operating Temperature Range (Note 2) ... –40°C to 85°C
Junction Temperature (Note 3) ............................. 125°C

16 15 14 13
VIN 1

12 VC

EN1 2

11 EN2

17

ISET1 3

10 ISET2

6

7

8

GND

LED3

GND

9 LED4
5

LED2

LED1 4

UF PACKAGE
16-LEAD (4mm × 4mm) PLASTIC QFNTJMAX = 125°C, θJA = 40°C/W, θJC = 2.6°C/W
EXPOSED PAD (PIN 17) IS PGND, MUST BE SOLDERED TO PCBORDER PART NUMBER
LTC3453EUF

UF PART MARKING
3453

Order Options Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
Lead Free Part Marking: http://www.linear.com/leadfree/Consult LTC Marketing for parts specified with wider operating temperature ranges.

ELECTRICAL CHARACTERISTICS

The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C, VIN = VOUT = 3.6V unless otherwise noted. (Note 2)
PARAMETERCONDITIONS●

Input Supply Voltage
Input DC Supply Current
Normal Operation
Shutdown
UVLO
Undervoltage Lockout Threshold

MIN

MAX

UNITS

5.5

V

0.6
6
3

1
18
5

mA
µA
µA

2
1.9

2.3

1.6

V
V

0.65

1

0.63

2.7

2.7V ≤ VIN ≤ 5.5V, RISET1||RISET2 = 51.1k, ILEDx = 0 (Note 4)
2.7V ≤ VIN ≤ 5.5V; VEN1 = VEN2 = 0V
VIN < UVLO Threshold
VIN Rising
VIN Falling●

2.7V ≤ VIN ≤ 5.5V, VEN1,2 Rising●

EN1,2 DC Threshold for Shutdown (ILEDx = 0) 2.7V ≤ VIN ≤ 5.5V, VEN1,2 Falling●

0.2

EN1,2 Input Current

VEN1,2 = 3.6V●

–1

ISET1,2 Servo Voltage

RISET1,2 = 4.12k, 0°C ≤ TA ≤ 85°C
RISET1,2 = 4.12k, –40°C ≤ TA ≤ 85°C

●

788
780

ILED/(ISET1 + ISET2), ILEDx = 75mA, VLEDx = 300mV,
2.7V ≤ VIN ≤ 5.5V

●

365
357

EN1,2 DC Threshold for Normal Operation

LED Output Current Ratio

TYP

V
V

1

µA

800
800

812
812

mV
mV

384
384

403
403

mA/mA
mA/mA

6

%

LED Output Current Matching(MAX – MIN)/[(MAX + MIN)/2] • 100%, ILEDx = 75mA
VLEDx = 300mV2

LED Pin Drain Voltage

ILEDx = 75mA

130Regulated Maximum VOUT

VLEDx = 0V

PMOS Switch RON

Switches A and D, @ 100mA

0.3Ω

NMOS Switch RON

Switches B and C, @ 100mA

0.25

Ω

Forward Current Limit

Switch A

Reverse Current Limit

Switch D

●

4.4

1125

4.5

1612
200

mV
4.6

2100

V

mA
mA
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LTC3453

ELECTRICAL CHARACTERISTICS

The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C, VIN = VOUT = 3.6V unless otherwise noted. (Note 2)
PARAMETERCONDITIONS

MAX

UNITS

PMOS Switch Leakage

Switches A and D

●

MIN

1

µA

NMOS Switch Leakage

Switches B and C

●

1

µA

1.1

MHz

Oscillator Frequency

TYP

0.9

Soft-Start Time1
0.65

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 LTC3453E is guaranteed to meet specifications from 0°C to
70°C. Specifications over the –40°C to 85°C operating temperature range
are assured by design, characterization and correlation with statisticalprocess controls.

ms

Note 3: TJ is calculated from the ambient temperature TA and power
dissipation PD according to the following formula:
TJ = TA + (PD • θJA °C/W).
Note 4: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.

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TYPICAL PERFOR A CE CHARACTERISTICS
Input DC Supply Current in
Shutdown vs Temperature
20

ISET1,2 Servo Voltage vs
Temperature

Undervoltage Lockout Thresholdvs Temperature
2.2

FRONT PAGE APPLICATION

812

18

VIN = 4.2V

1210

VIN = 3.6V

8
VIN = 2.7V

6

UVLO THRESHOLD (V)IIN (µA)

14

808

2.1

VIN = 5.5V

VIN RISINGVISET1,2 (mV)

16

2.0

1.9

VIN FALLING

804
800
796

1.8

4

VIN = 3.6V
RISET1,2 = 8.25k

792

2
0
–55 –35 –15

1.7–55 –35 –15

5 25 45 65 85 105 125
TEMPERATURE (°C)
3453 G01

1050

VIN = 3.6V
4.54 ALL LED PINS OPEN

VOUT (V)

804

796

4.53

1030

4.52

1020

4.51
4.50
4.49
4.48

792

3.1

3.5

3.9 4.3
VIN (V)

4.7

5.1

5.5

3453 G05

1000

980
970
960

3453 G06

VIN = 4.2V

VIN = 3.6V

990

4.46
5 25 45 65 85 105 125
TEMPERATURE (°C)

VIN = 5.5V

1010

4.47

4.45
–55 –35 –15

VOUT = 3V

1040

FREQUENCY (kHz)808

VISET1,2 (mV)

Oscillator Frequency vsTemperature

4.55

TA = 25°C
RISET1,2 = 8.25k

788
2.7

3453 G04

Regulated Maximum VOUT vs
Temperature800

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

3453 G02

ISET1,2 Servo Voltage vs VIN
812

788–55 –35 –15

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

950
–55 –35 –15

VIN = 2.7V

5 25 45 65 85 105 125
TEMPERATURE (°C)
3453 G07

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LTC3453
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TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency vs LED Current
100

Output Voltage Ripple

Startup Transient

FRONT PAGE APPLICATION
PLED/PIN, VIN = 3.6V, TA = 25°C
CH1, VOUT
2V/DIVEFFICIENCY (%)

90
20mV/DIVAC COUPLED

80

0V
CH2, EN1
1V/DIV

70

0V

60
5µs/DIV

50
100 150 200 250 300 350 400 450 500
LED CURRENT (mA)

FRONT PAGE APPLICATION
VIN = 3.6V
ILED = 150mA

3453 G08

1ms/DIV

3453 G09

FRONT PAGE APPLICATION
VIN = 3.6V
ILED = 150mA

3453 G07

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PI FU CTIO S
VIN (Pin 1): Signal Voltage Input Supply Pin [2.7V ≤ VIN ≤5.5V]. Recommended bypass capacitor to GND is 2.2µF
ceramic or larger. Connect to PVIN (Pin 16).
EN1 (Pin 2): Enable Input Pin for ISET1 Current.
ISET1 (Pin 3): White Led Current Programming Pin. A
resistor to ground programs each current source output to
ILED = 384(0.8V/RISET1). This amount of current adds toany amount set by EN2/ISET2 if also used.
LED1 to LED4 (Pins 4, 6, 7, 9): Individual Low Dropout
Current Source Outputs for White LED Current Biasing.
Connect each white LED between VOUT and an individual
LEDx pin. Unused LEDx outputs should be connected to
VOUT.
GND (Pins 5 and 8): Signal Ground Pin. Connect to PGND
(Exposed Pad).
ISET2 (Pin 10): White Led Current Programming Pin. A
resistor to ground programs each current source output to
ILED = 384(0.8V/RISET2). This amount of current adds toany amount set by EN1/ISET1 if also used.

EN2 (Pin 11): Enable Input Pin for ISET2 Current.
VC (Pin 12): Compensation Point for the Internal Error
Amplifier Output. Recommended compensation capacitor
to GND is 0.1µF ceramic or larger.
VOUT (Pin 13): Buck-Boost Output Pin. Recommended
bypass capacitor to GND is 4.7µF ceramic.
SW2 (Pin 14): Switching Node Pin. Connected to internal
power switches C and D. External inductor connects
between SW1 and SW2. Recommended value is 4.7µH.
SW1 (Pin 15): Switching Node Pin. Connected to internal
power switches A and B. External inductor connects
between SW1 and SW2. Recommended value is 4.7µH.
PVIN (Pin 16): Power Voltage Input Supply Pin. Connect to
VIN (Pin 1).
Exposed Pad (Pin 17): Power Ground Pin. Connect to
GND (Pin 8) and solder to PCB ground for optimum
thermal performance.

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LTC3453

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BLOCK DIAGRAOPTIONAL

OPTIONAL

15
16

VIN
2.7V TO 5.5V

1

PVIN

SWITCH A

SWITCH B
UVVOUT

SWITCH D

GATEDRIVERSAND
ANTICROSSCONDUCTION

VIN

UNDERVOLTAGE
LOCKOUT

14

SW2

SW1

13

VOUT

SWITCH C

LED1
4

BANDGAPREFERENCE

1.23V
1612mALED
DETECT+––OTREVERSECURRENTLIMIT+OVERTEMPPROTECTIONFORWARD
CURRENT
LIMIT

200mA

LED2
6

LOGIC

–

–
AB PWM
COMPARATOR

CD PWM
COMPARATOR

UV

12

LED
DETECT

+

+

LED3
7

OT
LED
DETECT

1MHz
OSCILLATOR

VC

VBIAS

–

MAIN
ERROR AMPSAFETYERROR AMP

VOUT

–

LED4
9

1.23V
327k

VFB

+

LED CURRENTSETTING AMP 1

ILED
384

800mV

+
–

10

6

ISET1

RISET1

RISET2

4

∑

+
–

3

OR

123k

SOFT
START
CLAMP

1.23V

800mV

LED
DETECT+

7

LED CURRENTSETTING AMP 2
ILED
384

9

ISET2

EN1
2
EN2
11

SHUTDOWN
5

GND

8

GND

17

EXPOSED PAD (PGND)
3453 BD3453fa5

LTC3453U
OPERATIO
Buck-Boost DC-DC Converter

Buck Mode (VIN > VOUT)

The LTC3453 employs an LTC proprietary buck-boost
DC/DC converter to generate the output voltage required
to drive the LEDs. This architecture permits high-efficiency, low noise operation at input voltages above, below
or equal to the output voltage by properly phasing fourinternal power switches. The error amp output voltage on
the VC pin determines the duty cycle of the switches. Since
the VC pin is a filtered signal, it provides rejection of
frequencies well below the factory trimmed switchingfrequency of 1MHz. The low RDS(ON), low gate charge
synchronous switches provide high frequency pulse widthmodulation control at high efficiency. Schottky diodes
across synchronous rectifier switch B and synchronousrectifier switch D are not required, but if used do provide
a lower voltage drop during the break-before-make time
(typically 20ns), which improves peak efficiency by typically 1% to 2% at higher loads.

In buck mode, switch D is always on and switch C is always
off. Referring to Figure 2, when the control voltage VC isabove voltage V1, switch A begins to turn on each cycle.
During the off time of switch A, synchronous rectifierswitch B turns on for the remainder of the cycle. Switches
A and B will alternate conducting similar to a typicalsynchronous buck regulator. As the control voltage increases, the duty cycle of switch A increases until the
maximum duty cycle of the converter in buck mode
reaches DCBUCK|max given by:

Figure 1 shows a simplified diagram of how the fourinternal power switches are connected to the inductor,
VIN, VOUT and GND. Figure 2 shows the regions of operation of the buck-boost as a function of the control voltage
VC. The output switches are properly phased so transitions between regions of operation are continuous, filtered and transparent to the user. When VIN approaches
VOUT, the buck-boost region is reached where the conduction time of the four switch region is typically 150ns.
Referring to Figures 1 and 2, the various regions ofoperation encountered as VC increases will now be
described.

DCBUCK|max = 100% – DC4SW
where DC4SW equals the duty cycle in % of the “fourswitch” range.
DC4SW = (150ns • f) • 100%
where f is the operating frequency in Hz.
Beyond this point the “four switch” or buck-boost region
is reached.
Buck-Boost or Four-Switch Mode (VIN ≈ VOUT)
Referring to Figure 2, when the control voltage VC is above
voltage V2, switch pair AD continue to operate for duty
cycle DCBUCK|max, and the switch pair AC begins to phasein. As switch pair AC phases in, switch pair BD phases out
accordingly. When the VC voltage reaches the edge of the
buck-boost range at voltage V3, switch pair AC completely
phases out switch pair BD and the boost region begins at
75%
DMAX
BOOST

PVIN

VOUT

16

13

SW1

SW2

15

14

NMOS B

A ON, B OFF
BOOST REGIONPWM CD SWITCHES
DMIN
BOOST

PMOS DPMOS AV4 (≈2.1V)

DMAX
BUCK

V3 (≈1.65V)
FOUR SWITCH PWM

BUCK/BOOST REGION
V2 (≈1.55V)

D ON, C OFF
PWM AB SWITCHES BUCK REGIONNMOS C3453 F01

Figure 1. Simplified Diagram of Internal Power Switches

V1 (≈0.9V)

0%
DUTY
CYCLE

3453 F02

CONTROLVOLTAGE, VC

Figure 2. Switch Control vs Control Voltage, VC
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LTC3453U
OPERATIO
duty cycle DC4SW. The input voltage VIN where the four
switch region begins is given by:
VIN = VOUT/[1 – (150ns • f)]
and the input voltage VIN where the four switch region
ends is given by
VIN = VOUT • (1 – DC4SW) = VOUT • [1 – (150ns • f)]
Boost Mode (VIN < VOUT)
In boost mode, switch A is always on and switch B isalways off. Referring to Figure 2, when the control voltage
VC is above voltage V3, switches C and D will alternate
conducting similar to a typical synchronous boost regulator. The maximum duty cycle of the converter is limited to
88% typical and is reached when VC is above V4.

Soft-Start
The LTC3453 includes an internally fixed soft-start whichis active when powering up or coming out of shutdown.
The soft-start works by clamping the voltage on the VC
node and gradually releasing it such that it requires
0.65ms to linearly slew from 0.9V to 2.1V. This has theeffect of limiting the rate of duty cycle change as VC
transitions from the buck region through the buck-boost
region into the boost region. Once the soft-start times out,
it can only be reset by entering shutdown, or by anundervoltage or overtemperature condition.
Main Error Amp

If the current delivered from VIN through PMOS switch A
exceeds 1612mA (typical), switch A is shut off immediately. Switches B and D are turned on for the remainder ofthe cycle in order to safely discharge the forward inductor
current at the maximum rate possible.

The main error amplifier is a transconductance amplifier
with source and sink capability. The output of the mainerror amplifier drives a capacitor to GND at the VC pin. This
capacitor sets the dominant pole for the regulation loop.
(See the Applications Information section for selecting thecapacitor value.) The error amp gets its feedback signal
from a proprietary circuit which monitors all 4 LED current
sources to determine which LED to close the regulation
loop on.

Reverse Current Limit

Safety Error Amp

If the current delivered from VOUT backwards through
PMOS switch D exceeds 200mA (typical), switch D is shut
off immediately. Switches A and C are turned on for theremainder of the cycle in order to safely discharge thereverse inductor current at the maximum rate possible.

The safety error amplifier is a transconductance amplifier
with sink only capability. In normal operation, it has noeffect on the loop regulation. However, if any of the LED
pins open-circuits, the output voltage will keep rising, andsafety error amp will eventually take over control of theregulation loop to prevent VOUT runaway. The VOUT threshold at which this occurs is approximately 4.5V.

Forward Current Limit

Undervoltage Lockout
To prevent operation of the power switches at high RDS(ON),an undervoltage lockout is incorporated on the LTC3453.
When the input supply voltage drops below approximately
1.9V, the four power switches and all control circuitry areturned off except for the undervoltage block, which draws
only several microamperes.
Overtemperature Protection
If the junction temperature of the LTC3453 exceeds 130°C
for any reason, all four switches are shut off immediately.
The overtemperature protection circuit has a typical hysteresis of 11°C.

LED Current Setting Amplifiers and Enable Circuit
The LTC3453 includes two LED current setting amplifiers
that work in conjunction with dual external current setting
resistors and dual enable pins to program LED current to
one of four levels (including shutdown). All four LED
current source outputs are programmed to the same level.
When both enable inputs are logic low, the LTC3453 is in
shutdown, the buck-boost is disabled and all LED currents
are zero. In shutdown, the input supply current is typically
6µA. If either enable input is logic high, the buck-boost will
regulate the output voltage such that the LEDs are biased
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LTC3453U
OPERATIO
at the current programmed by resistors RISET1 and/or
RISET2. Individually enabled, each LED current setting
amplifier programs the output LED current to
ILED = 384 (0.8V/RISET1,2)
If both enable inputs are logic high, the setting currents are
summed internally and the output LED current will begiven by
ILED = 384 [0.8V/(RISET1 || RISET2) ]Thus three different (nonzero) current levels are programmable, optimal for low current LED torch and high current
LED camera flash applications.
LED Current Sources
Each LED pin is driven by a current source specificallydesigned for low dropout. The LTC3453 employs a propri-

etary architecture that determines which of the four LEDs
requires the largest forward voltage drop at its programmed current, and then generates a feedback voltage
based on this one for closing the buck-boost regulation
loop. This results in the lowest output voltage required forregulating all of the LEDs and thus the highest LED power
efficiency. The voltage present at the LED pin of the“controlling LED” will be typically 130mV at 75mA ofcurrent.
LED Detect Circuit
If fewer than four LED outputs are required, unused ones
should be connected to VOUT. Each LED pin has an internal
LED detect circuit that disables the output current source
to save power if an output is not needed. A small 30µA
current is employed to detect the presence of an LED at
startup.

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APPLICATIO S I FOR ATIO
Component Selection
Inductor Selection

The high frequency operation of the LTC3453 allows theuse of small surface mount inductors. The inductor current ripple is typically set to 20% to 40% of the maximum
inductor current. For a given ripple the inductance terms
are given as follows:

L>
L>

(

)

VIN(MIN)2 • VOUT – VIN(MIN) • 100%f • IOUT(MAX) • %Ripple • VOUT2

(

,

)

VOUT • VIN(MAX) – VOUT • 100%f • IOUT(MAX) • %Ripple • VIN(MAX)where f = operating frequency, Hz%Ripple = allowable inductor current ripple, %For high efficiency, choose an inductor with a high frequency core material, such as ferrite, to reduce core loses.
The inductor should have low ESR (equivalent series
resistance) to reduce the I2R losses, and must be able tohandle the peak inductor current without saturating. Molded
chokes or chip inductors usually do not have enough core
to support peak inductor currents >1A. To minimize radiated noise, use a toroid, pot core or shielded bobbin
inductor. For the white LED application, a 4.7µH inductor
value is recommended. See Table 1 for a list of component
suppliers.
Table 1. Inductor Vendor Information
SUPPLIER

WEB SITE

Coilcraftwww.coilcraft.com

Cooper/Coiltronics

www.cooperet.com

Muratawww.murata.com

Sumidawww.japanlink.com/sumida

Vishay-Dale

www.vishay.com

VIN(MIN) = minimum input voltage, V
VIN(MAX) = maximum input voltage, V
VOUT = output voltage, V
IOUT(MAX) = maximum output load current
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LTC3453

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APPLICATIO S I FOR ATIO
Input Capacitor Selection

Optional Schottky Diodes

Since the VIN pin is the supply voltage for the IC it isrecommended to place at least a 2.2µF, low ESR bypass
capacitor to ground. See Table 2 for a list of component
suppliers.

Schottky diodes across the synchronous switches B and
D are not required, but provide a lower drop during the
break-before-make time (typically 20ns) of the NMOS to
PMOS transition, improving efficiency. Use a Schottky
diode such as an MBRM120T3 or equivalent. Do not use
ordinary rectifier diodes, since the slow recovery times
will compromise efficiency.

Table 2. Capacitor Vendor Information
SUPPLIER

WEB SITE

AVX

www.avxcorp.com

Sanyowww.sanyovideo.com

Taiyo Yuden

www.t-yuden.com

TDK

www.component.tdk.com

Closing the Feedback Loop

Output Capacitor Selection
The bulk value of the capacitor is set to reduce the ripple
due to charge into the capacitor each cycle. The steadystate ripple due to charge is given by:%Ripple _ Boost =%Ripple _ Buck =()

IOUT(MAX ) • VOUT – VIN(MIN) • 100
2

COUT • VOUT • f%( VIN(MAX) – VOUT ) • 100 %8 • VIN(MAX ) • f 2 • L • COUT

The LTC3453 incorporates voltage mode PWM control.
The control to output gain varies with operation region(Buck, Boost, Buck/Boost), but is usually no greater than
15. The output filter exhibits a double pole response
given by:fFILTER _ POLE =1Hz
2 • π • L • COUT

where COUT is the output filter capacitor.
The output filter zero is given by:
fFILTER _ ZERO =1
2 • π • RESR • COUT

Hz

where COUT = output filter capacitor, F

where RESR is the capacitor equivalent series resistance.

The output capacitance is usually many times larger inorder to handle the transient response of the converter.
For a rule of thumb, the ratio of the operating frequency tothe unity-gain bandwidth of the converter is the amountthe output capacitance will have to increase from theabove calculations in order to maintain the desired transient response.

A troublesome feature in Boost mode is the right-halfplane zero (RHP), and is given by:

The other component of ripple is due to the ESR (equivalent series resistance) of the output capacitor. Low ESR
capacitors should be used to minimize output voltage
ripple. For surface mount applications, Taiyo Yuden, TDK,
AVX ceramic capacitors, AVX TPS series tantalum capacitors or Sanyo POSCAP are recommended. For the white
LED application, a 4.7µF capacitor value is recommended.
See Table 2 for a list of component suppliers.

2

VIN
fRHPZ =
Hz
2 • π • IOUT • L • VOUT
The loop gain is typically rolled off before the RHP zerofrequency.
A simple Type I compensation network can be incorporated to stabilize the loop but at a cost 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.

3453fa9

LTC3453

U

U

W

U

APPLICATIO S I FOR ATIOThe unity-gain frequency of the error amplifier with theType I compensation is given by:

fUG =

gm
2 • π • CVC

Since the maximum continuous output current is limited
to 500mA, this sets a minimum limit on the parallel
combination of RISET1 and RISET2 equal to
RMIN = (RISET1 || RISET2)|MIN = 4(384[0.8V/500mA])= 2458Ω

where gm is the error amp transconductance (typically
1/5.2k) and CVC is the external capacitor to GND at theVC pin. For the white LED application, a 0.1µF or greater
capacitor value is recommended.
Paralleling LED Outputs for Higher Current
Two or more LED output pins can be connected together
in parallel to achieve higher output current in fewer than 4
LEDs. For a very high power LED such as a LumiLED, allfour outputs can be connected in parallel for maximum
total output current, as shown in the cover page application of this datasheet.
Maximum LED Current
As described in the Operation section, the output LED
current with both enable pins logic high is equal to
ILED = 384 [0.8V/(RISET1 || RISET2)]
VIN

Although the LTC3453 can safely provide this current
continuously, the external LED(s) may not be rated for thishigh a level of continuous current. Higher current levels
are generally reserved for pulsed applications, such as
LED camera flash. This is accomplished by programminga high current with one of the RISET resistors and pulsing
the appropriate enable pin.
Varying LED Brightness
Continuously variable LED brightness control can beachieved by interfacing directly to one or both of the ISET
pins. Figure 3 shows four such methods employing a
voltage DAC, a current DAC, a simple potentiometer or a
PWM input. It is not recommended to control brightness
by PWMing the enable pins directly as this will toggle the
LTC3453 in and out of shutdown and result in erratic
operation.
VIN

VOUT

ENx

VOUT

ENx

LED1
LTC3453

ISETx

VOLTAGE
DAC

ISETx

LED4
ILED = 384

RSET ≥ RMIN

LED1
LTC3453

0.8V – VDACRSETILED = 384 • IDAC

0.8V
RMIN

IDAC ≤CURRENT
DAC

VDAC

LED4

(a)
VIN

(b)
VIN

VOUT

ENx

VOUT

ENx

LED1
LTC3453

ISETx

ISETx

LED4

RMIN

LED1
LTC3453

ILED = 384

0.8V
RMIN + RPOT

RSET
100

RSET ≥ RMIN

LED4ILED = 384

VPWM

RPOT

= 384

1µF

0.8V – VPWM
RSET
0.8V – (DC% • VDVCC)
RSET

DVCC
fPWM ≥ 5kHz

(c)

(d)

3453 F03

Figure 3. Brightness Control Methods: (a) Using Voltage DAC, (b) Using Current DAC, (c) Using Potentiometer, (d) Using PWM Input
3453fa10

LTC3453

U

W

U

U

APPLICATIO S I FOR ATIO
Unused Outputs

If fewer than 4 LED pins are to be used, unused LEDx pins
should be connected to VOUT. The LTC3453 senses whichcurrent source outputs are not being used and shuts off
the corresponding output currents to save power. A small
trickle current (~30µA) is still applied to unused outputs todetect if a white LED is later switched in and also to
distinguish unused outputs from used outputs during
startup.
LED Failure Modes
If an individual LED fails as a short circuit, the current
source biasing it is shut off to save power. This is the same

operation as described previously (if the output were
initially designated unused at power-up by connecting its
LEDx pin to VOUT). Efficiency is not materially affected.
If an individual LED fails as an open circuit, the control loop
will initially attempt to regulate off of its current source
feedback signal, since it will appear to be the one requiring
the largest forward voltage drop to run at its programmed
current. This will drive VOUT higher. As the open circuited
LED will never accept its programmed current, VOUT must
be voltage-limited by means of a secondary control loop.
The LTC3453 limits VOUT to 4.5V in this failure mode. The
other LEDs will still remain biased at the correct programmed current but the overall circuit efficiency will
decrease.

U

PACKAGE DESCRIPTIOUF Package
16-Lead Plastic QFN (4mm × 4mm)
(Reference LTC DWG # 05-08-1692)

0.72 ±0.05

4.35 ± 0.05
2.15 ± 0.05
2.90 ± 0.05 (4 SIDES)

PACKAGE OUTLINE
0.30 ±0.05
0.65 BSC
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONSBOTTOM VIEW—EXPOSED PAD
4.00 ± 0.10
(4 SIDES)

0.75 ± 0.05

R = 0.115
TYP

15

PIN 1 NOTCH R = 0.20 TYP
OR 0.35 × 45° CHAMFER

16
0.55 ± 0.20

PIN 1
TOP MARK(NOTE 6)1
2.15 ± 0.10
(4-SIDES)

2(UF16) QFN 1004

0.200 REF

0.30 ± 0.05

0.00 – 0.05

0.65 BSC

NOTE:
1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGC)
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

3453faInformation 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.

11

LTC3453U

TYPICAL APPLICATIOHigh Efficiency 4 White LED Driver
4.7µH
VIN
1-CELL
Li-Ion

VIN

2.2µF

SW1

PVIN

SW2

VOUT

4.7µF

30mA

30mA

30mA

30mA

D1

D2

D3

D4

LED1
LED2
VC
0.1µF

EN

1MHz
BUCK-BOOST

LED3
LED4

EN1
EN2

D1 TO D4: NICHIA NSCW100
L1: VISHAY DALE IDCS-2512

ISET1
10.2k

ISET2

LTC3453
GND

GND

PGND

3453 TA02

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3453fa12

Linear Technology Corporation

LT 0206 REV A • PRINTED IN USA

1630 McCarthy Blvd., Milpitas, CA 95035-7417(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com© LINEAR TECHNOLOGY CORPORATION 2005