LINEAR TECHNOLOGY LTC3453 Technical data

FEATURES

LTC3453

Synchronous Buck-Boost

High Power White LED Driver

U

DESCRIPTIO

■

High Efficiency: 90% Typical Over Entire
Li-Ion Battery Range

■

Wide VIN Range: 2.7V to 5.5V

■

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 Package

U

APPLICATIO S

■

Cell Phones

■

Digital Cameras

■

PDAs

■

Portable Devices

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

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 the
largest 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).

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.

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

TYPICAL APPLICATIO

High Efficiency Torch/Flash LED Driver

V

IN

1-CELL

2.7V to 4.2V

0.1µF

Li-Ion

8.25k

2.2µF

EN1 (TORCH)

EN2 (FLASH)

1%

3.48k
1%

I

I

SET1

SET2

EN1

EN2

V

PV

IN

IN

V

C

U

L1

4.7µH

SW1 SW2 V

1MHz

BUCK-BOOST

GND PGNDGND

OUT

LTC3453

3453 TA01a

150mA/500mA

LED1

LED2

LED3

LED4

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

EN1 EN2

0

0

1

0

0

1

1

1

D1

I

LED

0 (SHUTDOWN)
150mA
350mA
500mA

4.7µF

Torch Mode Efficiency vs V

100

(%)

IN

90

/P

LED

80

70

60

LED POWER EFFICIENCY P

I

= 150mA

LED

= 25°C

T

A

50

3.1 3.5 3.9 5.54.3 5.1

2.7
VIN (V)

EFFICIENCY

I

IN

4.7

IN

3453 TA01b

180

160

INPUT CURRENT (mA)

140

120

100

80

3453fa

1

LTC3453

16 15 14 13

5 6 7 8

UF PACKAGE

16-LEAD (4mm × 4mm) PLASTIC QFN

9

10

11

17

12

4

3

2

1

TOP VIEW

V

IN

EN1

I

SET1

LED1

V

C

EN2

I

SET2

LED4

PVINSW1

SW2

V

OUT

GND

LED2

LED3

GND

WW

W

U

ABSOLUTE AXI U RATI GS

(Note 1)

VIN, PVIN, SW1, SW2, V

LED1 to LED4 Voltage ...... –0.3V to (V

, EN1, EN2,

V

C

I

, I

SET1

Voltage .......... –0.3V to (VIN + 0.3V) or 6V

SET2

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

Voltage ............ –0.3V to 6V

OUT

+ 0.3V) or 6V

OUT

UUW

PACKAGE/ORDER I FOR ATIO

T

= 125°C, θJA = 40°C/W, θJC = 2.6°C/W

JMAX

EXPOSED PAD (PIN 17) IS PGND, MUST BE SOLDERED TO PCB

ORDER PART NUMBER

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.

UF PART MARKING

3453LTC3453EUF

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at T

PARAMETER CONDITIONS MIN TYP MAX UNITS

Input Supply Voltage

Input DC Supply Current

Normal Operation 2.7V ≤ V
Shutdown 2.7V ≤ V
UVLO V

Undervoltage Lockout Threshold VIN Rising

EN1,2 DC Threshold for Normal Operation 2.7V ≤ VIN ≤ 5.5V, V

EN1,2 DC Threshold for Shutdown (I

EN1,2 Input Current V

I

Servo Voltage R

SET1,2

LED Output Current Ratio I

LED Output Current Matching (MAX – MIN)/[(MAX + MIN)/2] • 100%, I

LED Pin Drain Voltage I

Regulated Maximum V

PMOS Switch R

NMOS Switch R

Forward Current Limit Switch A 1125 1612 2100 mA

Reverse Current Limit Switch D 200 mA

ON

ON

OUT

LEDx

IN

V

IN

= 0) 2.7V ≤ VIN ≤ 5.5V, V

EN1,2

ISET1,2

R

ISET1,2

LED

2.7V ≤ V

V

LEDx

LEDx

V

LEDx

Switches A and D, @ 100mA 0.3 Ω

Switches B and C, @ 100mA 0.25 Ω

≤ 5.5V, R

IN

≤ 5.5V; V

IN

< UVLO Threshold 3 5 µA

Falling 1.6 1.9 V

= 3.6V

= 4.12k, 0°C ≤ TA ≤ 85°C 788 800 812 mV
= 4.12k, –40°C ≤ TA ≤ 85°C

/(I

+ I

SET1

≤ 5.5V

IN

= 300mV

= 75mA 130 mV

= 0V

The ● denotes the specifications which apply over the full operating

= 25°C, VIN = V

A

||R

ISET1

= V

EN1

Rising

EN1,2

Falling

EN1,2

), I

SET2

= 75mA, V

LEDx

= 3.6V unless otherwise noted. (Note 2)

OUT

●

2.7 5.5 V

= 51.1k, I

ISET2

= 0V 6 18 µA

EN2

LEDx

= 0 (Note 4) 0.6 1 mA

LEDx

●

●

●

0.2 0.63 V

●

–1 1 µA

●

780 800 812 mV

= 300mV, 365 384 403 mA/mA

●

357 384 403 mA/mA

= 75mA 2 6 %

LEDx

●

4.4 4.5 4.6 V

2 2.3 V

0.65 1 V

3453fa

2

LTC3453

TEMPERATURE (°C)

–55

FREQUENCY (kHz)

1050

1040

1030

1020

950

1010

1000

990

980

970

960

45 65 85 10525

3453 G07

–35 –15 5 125

VIN = 5.5V

V

OUT

= 3V

VIN = 4.2V

VIN = 3.6V

VIN = 2.7V

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at T

PARAMETER CONDITIONS MIN TYP MAX UNITS

PMOS Switch Leakage Switches A and D

NMOS Switch Leakage Switches B and C

Oscillator Frequency 0.9 1 1.1 MHz

Soft-Start Time 0.65 ms

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 statistical
process controls.

TYPICAL PERFOR A CE CHARACTERISTICS

Input DC Supply Current in
Shutdown vs Temperature

20

FRONT PAGE APPLICATION

18

16

14

12

10

(µA)

IN

I

8

6

4

2

0

–35 –15 5 125

–55

I

SET1,2

812

TA = 25°C
R

ISET1,2

808

804

(mV)

800

ISET1,2

V

796

792

788

2.7

Servo Voltage vs V

3.1 5.5

VIN = 5.5V

TEMPERATURE (°C)

= 8.25k

3.9 4.3 4.7 5.13.5
VIN (V)

VIN = 4.2V

45 65 85 10525

VIN = 3.6V

VIN = 2.7V

IN

UW

3453 G01

3453 G05

The ● denotes the specifications which apply over the full operating

= 25°C, VIN = V

A

= 3.6V unless otherwise noted. (Note 2)

OUT

Note 3: T

dissipation P
T

= TA + (PD • θJA °C/W).

J

Note 4: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.

Undervoltage Lockout Threshold
vs Temperature

2.2

2.1

VIN RISING

2.0

1.9

UVLO THRESHOLD (V)

1.8

1.7
–55

VIN FALLING

–35 –15 5 125

TEMPERATURE (°C)

45 65 85 10525

Regulated Maximum V
Temperature

4.55
VIN = 3.6V

ALL LED PINS OPEN

4.54

4.53

4.52

4.51

(V)

4.50

OUT

V

4.49

4.48

4.47

4.46

4.45

–35 –15 5 125

–55

TEMPERATURE (°C)

45 65 85 10525

●

●

is calculated from the ambient temperature TA and power

J

according to the following formula:

D

I

Servo Voltage vs

SET1,2

Temperature

812

VIN = 3.6V

= 8.25k

R

ISET1,2

808

804

(mV)

800

ISET1,2

V

796

792

OUT

vs

3453 G02

788

–35 –15 5 125

–55

TEMPERATURE (°C)

Oscillator Frequency vs

45 65 85 10525

Temperature

3453 G06

1 µA

1 µA

3453 G04

3453fa

3

LTC3453

UW

TYPICAL PERFOR A CE CHARACTERISTICS

Efficiency vs LED Current Output Voltage Ripple Startup Transient

100

90

FRONT PAGE APPLICATION

, VIN = 3.6V, TA = 25°C

P

LED/PIN

CH1, V

2V/DIV

OUT

80

70

EFFICIENCY (%)

60

50

150 200 250 500

100

LED CURRENT (mA)

350 400 450300

3453 G07

20mV/DIV

AC COUPLED

FRONT PAGE APPLICATION

= 3.6V

V

IN

= 150mA

I

LED

UUU

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 PV

EN1 (Pin 2): Enable Input Pin for I

I

(Pin 3): White Led Current Programming Pin. A

SET1

resistor to ground programs each current source output to
I

= 384(0.8V/R

LED

any amount set by EN2/I

). This amount of current adds to

ISET1

SET2

LED1 to LED4 (Pins 4, 6, 7, 9): Individual Low Dropout
Current Source Outputs for White LED Current Biasing.
Connect each white LED between V
LEDx pin. Unused LEDx outputs should be connected to

.

V

OUT

GND (Pins 5 and 8): Signal Ground Pin. Connect to PGND
(Exposed Pad).

I

(Pin 10): White Led Current Programming Pin. A

SET2

resistor to ground programs each current source output to
I

= 384(0.8V/R

LED

any amount set by EN1/I

). This amount of current adds to

ISET2

SET1

(Pin 16).

IN

Current.

SET1

if also used.

and an individual

OUT

if also used.

0V

CH2, EN1

1V/DIV

0V

5µs/DIV

3453 G08

FRONT PAGE APPLICATION

= 3.6V

V

IN

= 150mA

I

LED

EN2 (Pin 11): Enable Input Pin for I

1ms/DIV

SET2

Current.

3453 G09

VC (Pin 12): Compensation Point for the Internal Error
Amplifier Output. Recommended compensation capacitor
to GND is 0.1µF ceramic or larger.

V

(Pin 13): Buck-Boost Output Pin. Recommended

OUT

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.

(Pin 16): Power Voltage Input Supply Pin. Connect to

PV

IN

V

(Pin 1).

IN

Exposed Pad (Pin 17): Power Ground Pin. Connect to
GND (Pin 8) and solder to PCB ground for optimum
thermal performance.

4

3453fa

BLOCK DIAGRA

LTC3453

W

V

2.7V TO 5.5V

OPTIONAL

15 14

LIMIT

SW1

DRIVERS

ANTICROSS-

CONDUCTION

PV

IN

IN

16

V

IN

1

UNDERVOLTAGE

LOCKOUT

OVERTEMP

PROTECTION

SWITCH A

UV

OT

SWITCH B SWITCH C

FORWARD

CURRENT

+

BANDGAP

REFERENCE

1.23V

1612mA 200mA

–

+

LOGIC

–

AB PWM

COMPARATOR

UV

MAIN

ERROR AMP

OSCILLATOR

V

C

12

V

LED CURRENT
SETTING AMP 1

BIAS

V

–

FB

+

1.23V

GATE

AND

1MHz

SW2

COMPARATOR

SAFETY

ERROR AMP

START

CLAMP

REVERSE

CURRENT

LIMIT

CD PWM

OT

SOFT

SWITCH D

+

–

+

–

–

+

1.23V

OPTIONAL

V

OUT

LED1

LED

DETECT

LED2

LED

DETECT

LED3

LED

DETECT

V

OUT

327k

123k

LED4

LED

DETECT

V

13

4

6

7

9

OUT

OR

4

R

R

ISET1

ISET2

800mV

+

–

I

SET1

3

LED CURRENT

800mV

+

SETTING AMP 2

–

I

SET2

10

EN1

2

EN2

11

I
384

I
384

LED

LED

∑

SHUTDOWN

GND

5

6

7

9

GND

8

EXPOSED PAD (PGND)

17

3453 BD

3453fa

5

LTC3453

OPERATIO

U

Buck-Boost DC-DC Converter

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 four
internal power switches. The error amp output voltage on
the V

pin determines the duty cycle of the switches. Since

C

the V

pin is a filtered signal, it provides rejection of

C

frequencies well below the factory trimmed switching
frequency of 1MHz. The low R

, low gate charge

DS(ON)

synchronous switches provide high frequency pulse width
modulation control at high efficiency. Schottky diodes
across synchronous rectifier switch B and synchronous
rectifier 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.

Figure 1 shows a simplified diagram of how the four
internal power switches are connected to the inductor,
VIN, V

and GND. Figure 2 shows the regions of opera-

OUT

tion of the buck-boost as a function of the control voltage
V

. The output switches are properly phased so transi-

C

tions between regions of operation are continuous, fil­tered and transparent to the user. When V
V

, the buck-boost region is reached where the conduc-

OUT

approaches

IN

tion time of the four switch region is typically 150ns.
Referring to Figures 1 and 2, the various regions of
operation encountered as VC increases will now be
described.

Buck Mode (V

IN

> V

OUT

)

In buck mode, switch D is always on and switch C is always
off. Referring to Figure 2, when the control voltage V

is

C

above voltage V1, switch A begins to turn on each cycle.
During the off time of switch A, synchronous rectifier
switch B turns on for the remainder of the cycle. Switches
A and B will alternate conducting similar to a typical
synchronous 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 DC

DC

BUCK

where DC

|max given by:

BUCK

|max = 100% – DC

equals the duty cycle in % of the “four

4SW

4SW

switch” range.

DC

= (150ns • f) • 100%

4SW

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 (V

IN

≈ V

Referring to Figure 2, when the control voltage V

OUT

)

is above

C

voltage V2, switch pair AD continue to operate for duty
cycle DC

|max, and the switch pair AC begins to phase

BUCK

in. As switch pair AC phases in, switch pair BD phases out
accordingly. When the V

voltage reaches the edge of the

C

buck-boost range at voltage V3, switch pair AC completely
phases out switch pair BD and the boost region begins at

75%

D

MAX

BOOST

PV

IN

16

PMOS A

SW1

15

NMOS B

Figure 1. Simplified Diagram of Internal Power Switches Figure 2. Switch Control vs Control Voltage, V

SW2

14

V

OUT

13

PMOS D

NMOS C

3453 F01

D

MIN

BOOST

D

MAX

BUCK

DUTY

CYCLE

A ON, B OFF

PWM CD SWITCHES

FOUR SWITCH PWM

D ON, C OFF

PWM AB SWITCHES

0%

BOOST REGION

BUCK/BOOST REGION

BUCK REGION

6

3453 F02

V4 (≈2.1V)

V3 (≈1.65V)

V2 (≈1.55V)

V1 (≈0.9V)

CONTROL
VOLTAGE, V

C

C

3453fa

OPERATIO

LTC3453

U

duty cycle DC

. The input voltage VIN where the four

4SW

switch region begins is given by:

V

= V

IN

and the input voltage V

/[1 – (150ns • f)]

OUT

where the four switch region

IN

ends is given by

V

= V

IN

Boost Mode (V

• (1 – DC

OUT

IN

< V

OUT

4SW

)

) = V

• [1 – (150ns • f)]

OUT

In boost mode, switch A is always on and switch B is
always off. Referring to Figure 2, when the control voltage
V

is above voltage V3, switches C and D will alternate

C

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.

Forward Current Limit

If the current delivered from V

through PMOS switch A

IN

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

Soft-Start

The LTC3453 includes an internally fixed soft-start which
is active when powering up or coming out of shutdown.
The soft-start works by clamping the voltage on the V

C

node and gradually releasing it such that it requires

0.65ms to linearly slew from 0.9V to 2.1V. This has the
effect of limiting the rate of duty cycle change as V

C

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 an
undervoltage or overtemperature condition.

Main Error Amp

The main error amplifier is a transconductance amplifier
with source and sink capability. The output of the main
error 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 the
capacitor 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

If the current delivered from V

backwards through

OUT

PMOS switch D exceeds 200mA (typical), switch D is shut
off immediately. Switches A and C are turned on for the
remainder of the cycle in order to safely discharge the
reverse inductor current at the maximum rate possible.

Undervoltage Lockout

To prevent operation of the power switches at high R

DS(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 are
turned 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.

Safety Error Amp

The safety error amplifier is a transconductance amplifier
with sink only capability. In normal operation, it has no
effect on the loop regulation. However, if any of the LED
pins open-circuits, the output voltage will keep rising, and
safety error amp will eventually take over control of the
regulation loop to prevent V

runaway. The V

OUT

OUT

thresh-

old at which this occurs is approximately 4.5V.

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

3453fa

7

LTC3453

OPERATIO

U

at the current programmed by resistors R
R

. Individually enabled, each LED current setting

ISET2

ISET1

and/or

amplifier programs the output LED current to

= 384 (0.8V/R

I

LED

ISET1,2

)

If both enable inputs are logic high, the setting currents are
summed internally and the output LED current will be
given by

I

= 384 [0.8V/(R

LED

ISET1

|| R

ISET2

) ]

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 specifically
designed for low dropout. The LTC3453 employs a propri-

U

WUU

APPLICATIO S I FOR ATIO

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 for
regulating 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 of
current.

LED Detect Circuit

If fewer than four LED outputs are required, unused ones
should be connected to V

. Each LED pin has an internal

OUT

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.

Component Selection

Inductor Selection

The high frequency operation of the LTC3453 allows the
use 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:

VVV

L

>

VV V

L

>

f I Ripple V

2

•– •%

IN MIN OUT IN MIN

() ()

f I Ripple V

••%•

OUT IN MAX OUT

••%•

OUT MAX IN MAX

()

OUT MAX OUT

()

•–•%

()

()

() ()

100

100

,

2

where f = operating frequency, Hz

%Ripple = allowable inductor current ripple, %

V

V

= minimum input voltage, V

IN(MIN)

IN(MAX)

= maximum input voltage, V

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 to
handle 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

Coilcraft www.coilcraft.com

Cooper/Coiltronics www.cooperet.com

Murata www.murata.com

Sumida www.japanlink.com/sumida

Vishay-Dale www.vishay.com

V

= output voltage, V

OUT

I

OUT(MAX)

= maximum output load current

8

3453fa

LTC3453

U

WUU

APPLICATIO S I FOR ATIO

Input Capacitor Selection

Since the V
recommended to place at least a 2.2µF, low ESR bypass
capacitor to ground. See Table 2 for a list of component
suppliers.

Table 2. Capacitor Vendor Information

SUPPLIER WEB SITE

AVX www.avxcorp.com

Sanyo www.sanyovideo.com

Taiyo Yuden www.t-yuden.com

TDK www.component.tdk.com

Output Capacitor Selection

The bulk value of the capacitor is set to reduce the ripple
due to charge into the capacitor each cycle. The steady
state ripple due to charge is given by:

%_

Ripple Boost

Ripple Buck

%_

pin is the supply voltage for the IC it is

IN

IVV

OUT MAX OUT IN MIN

=

VV

()

IN MAX OUT

=

8

••••

VfLC

IN MAX OUT

•– •

() ()

()

()

CCV f

OUT OUT

–•

()

2

2

••

1100

%

100

%

Optional Schottky Diodes

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.

Closing the Feedback Loop

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:

f

FILTER POLE

where C

=

_

is the output filter capacitor.

OUT

2

1

•• •

π

LC

Hz

OUT

The output filter zero is given by:

f

FILTER ZERO

_

=

•• •

2

1

RC

π

ESR OUT

Hz

where C

= output filter capacitor, F

OUT

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

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.

where R

is the capacitor equivalent series resistance.

ESR

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

2

V

f

RHPZ

=

2• • • •

IN

ILV

π

OUT OUT

Hz

The loop gain is typically rolled off before the RHP zero
frequency.

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.

3453fa

9

LTC3453

U

WUU

APPLICATIO S I FOR ATIO

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

g

f

=

UG

where gm is the error amp transconductance (typically
1/5.2k) and C

pin. For the white LED application, a 0.1µF or greater

V

C

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, all
four 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

I

= 384 [0.8V/(R

LED

m

C

π2• •

VC

is the external capacitor to GND at the

VC

ISET1

|| R

ISET2

)]

Since the maximum continuous output current is limited
to 500mA, this sets a minimum limit on the parallel
combination of R

= (R

R

MIN

ISET1

ISET1

|| R

and R

ISET2)|MIN

equal to

ISET2

= 4(384[0.8V/500mA])

= 2458Ω

Although the LTC3453 can safely provide this current
continuously, the external LED(s) may not be rated for this
high a level of continuous current. Higher current levels
are generally reserved for pulsed applications, such as
LED camera flash. This is accomplished by programming
a high current with one of the R

resistors and pulsing

ISET

the appropriate enable pin.

Varying LED Brightness

Continuously variable LED brightness control can be
achieved by interfacing directly to one or both of the I

SET

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.

VOLTAGE

DAC

V

IN

ENx

LED1

LTC3453

I

LED4

SETx

R

≥ R

SET

MIN

V

DAC

(a)

V

IN

ENx

LED1

LTC3453

I

LED4

SETx

R

MIN

R

POT

(c)

V

OUT

0.8V – V

I

= 384

LED

I

= 384

LED

DAC

R

SET

CURRENT

V

OUT

0.8V

R

+ R

MIN

POT

DAC

R
100

V

IN

ENx

LED1

LTC3453

I

LED4

SETx

0.8V

IDAC ≤

R

MIN

V

IN

ENx

LED1

LTC3453

I

LED4

SETx

SET

R

≥ R

SET

MIN

V

PWM

1µF

f

≥ 5kHz

PWM

(b)

(d)

I

= 384 • IDAC

LED

I

= 384

LED

= 384

DV

V

OUT

V

OUT

0.8V – V

0.8V – (DC% • V

CC

PWM

R

SET

R

3453 F03

)

DVCC

SET

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

3453fa

10

LTC3453

U

WUU

APPLICATIO S I FOR ATIO

Unused Outputs

If fewer than 4 LED pins are to be used, unused LEDx pins
should be connected to V

. The LTC3453 senses which

OUT

current 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 to
detect 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

U

PACKAGE DESCRIPTIO

UF Package

16-Lead Plastic QFN (4mm × 4mm)

(Reference LTC DWG # 05-08-1692)

operation as described previously (if the output were
initially designated unused at power-up by connecting its
LEDx pin to V

). Efficiency is not materially affected.

OUT

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 V
LED will never accept its programmed current, V

higher. As the open circuited

OUT

OUT

must
be voltage-limited by means of a secondary control loop.
The LTC3453 limits V

to 4.5V in this failure mode. The

OUT

other LEDs will still remain biased at the correct pro­grammed current but the overall circuit efficiency will
decrease.

4.35 ± 0.05

2.90 ± 0.05

0.72 ±0.05

2.15 ± 0.05
(4 SIDES)

PACKAGE OUTLINE

0.30 ±0.05

0.65 BSC

RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS

4.00 ± 0.10
(4 SIDES)

PIN 1
TOP MARK
(NOTE 6)

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 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE

0.75 ± 0.05

2.15 ± 0.10
(4-SIDES)

0.200 REF

0.00 – 0.05

BOTTOM VIEW—EXPOSED PAD

R = 0.115

TYP

1615

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

0.55 ± 0.20

1

2

(UF16) QFN 1004

0.30 ± 0.05

0.65 BSC

3453fa

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

11

LTC3453

TYPICAL APPLICATIO

U

High Efficiency 4 White LED Driver

PV

IN

4.7µH

SW1

1MHz

BUCK-BOOST

GND

SW2 V

PGNDGND

OUT

LTC3453

3453 TA02

LED1

LED2

LED3

LED4

4.7µF 30mA2.2µF

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

30mA30mA30mA

V

1-CELL

Li-Ion

0.1µF

IN

10.2k

V

IN

V

C

EN1

EN

EN2

I

SET1

I

SET2

RELATED PARTS

PART NUMBER DESCRIPTION COMMENTS

LT1618 Constant Current, Constant Voltage 1.4MHz, High Efficiency VIN: 1.6V to 18V, V

Boost Regulator MS10 Package/EDD Package

LT1930/LT1930A 1A (ISW), 1.2MHz/2.2MHz, High Efficiency Step-Up VIN: 2.6V to 16V, V

DC/DC Converter ThinSOT Package

LT1932 Constant Current, 1.2MHz, High Efficiency White LED VIN: 1V to 10V, V

OUT(MAX)

Boost Regulator ThinSOT Package

LT1937 Constant Current, 1.2MHz, High Efficiency White LED VIN: 2.5V to 10V, V

Boost Regulator ThinSOT Package/SC70 Package

LTC3205 High Efficiency, Multi-Display LED Controller VIN: 2.8V to 4.5V, V

QFN-24 Package

LTC3216 1A Low Noise High Current LED Charge Pump with VIN: 2.9V to 4.4V, V

Independent Flash/Torch Current DFN Package

LTC3440/ 600mA/1.2A I

, 2MHz/1MHz, Synchronous Buck-Boost VIN: 2.4V to 5.5V, V

OUT

LTC3441 DC/DC Converter MS-10 Package/DFN Package

LTC3443 600mA/1.2A I

, 600kHz, Synchronous Buck-Boost VIN: 2.4V to 5.5V, V

OUT

DC/DC Converter DFN Package

LTC3454 1A Synchronous Buck-Boost High Power LED Driver VIN: 2.7V to 5.5V, 1MHz, ISD < 6µA, DFN Package

LT3465/LT3465A Constant Current, 1.2MHz/2.7MHz, High Efficiency White LED VIN: 2.7V to 16V, V

Boost Regulator with Integrated Schottky Diode ThinSOT Package

LT3466 Dual Constant Current, 2MHz, High Efficiency White LED VIN: 2.7V to 24V, V

Boost Regulator with Integrated Schottky Diode DFN Package

LT3479 3A, Full Featured DC/DC Converter with Soft-Start and VIN: 2.5V to 24V, V

Inrush Current Protection DFN Package/TSOPP Package

= 34V, IQ = 1.8mA, ISD = <1µA,

OUT(MAX)

= 34V, IQ = 4.2mA/5.5mA, ISD = <1µA,

OUT(MAX)

= 34V, IQ = 1.2mA, ISD = <1µA,

= 34V, IQ = 1.9mA, ISD = <1µA,

OUT(MAX)

= 6V, IQ = 50µA, ISD = <1µA,

OUT(MAX)

= 5.5V, IQ = 300µA, ISD = <2.5µA,

OUT(MAX)

= 5.25V, IQ = 25µA/50µA, ISD = <1µA,

OUT(MAX)

= 5.25V, IQ = 28µA, ISD = <1µA,

OUT(MAX)

= 34V, IQ = 1.9mA, ISD = <1µA,

OUT(MAX)

= 40V, IQ = 5mA, ISD = <16µA,

OUT(MAX)

= 40V, IQ = 6.5mA, ISD = <1µA,

OUT(MAX)

D4D3D2D1

12

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone] ● FAX: [phone]

●

www.linear.com

3453fa

LT 0206 REV A • PRINTED IN USA

© LINEAR TECHNOLOGY CORPORATION 2005