LINEAR TECHNOLOGY LT3477 Technical data

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I

SN1

I

SP1

V

IN

I

ADJ1

I

ADJ2

FBN

I

SP2

R1
200k

C2

3.3µF

C1

3.3µF

V

IN

5V

C3
33nF

R4

4.7nF

330mA

DN376 F01

R2
10k

R3
22k

R4

1k

R5

0.3Ω

SW

L1

10µH

FBP SSGND

LT3477

I

SN2

R

T

SHDN

V

C

V

REF

SHDN

D1

C1: TAIYO-YUDEN LMK316BJ335ML
C2: TAIYO-YUDEN TMK325BJ335MN
D1: DIODES INC. DFLS120L
L1: TOKO A915AY-100M

Monolithic Converter Drives High Power LEDs – Design Note 376

Keith Szolusha

Introduction

High power LEDs are quickly expanding their reach as the
lightsource of choice for flat panel computer and TV
monitors, TV projection, signage, portable lights and
automotive interior, trim and brake lighting. The input
voltage and LED voltage combinations across these appli­cations are as diverse as the applications themselves,
precluding the ability for a single topology to satisfy the
needs of them all. Nevertheless, all LED drivers, whether
buck, boost, buck-boost or SEPIC must regulate a con­stant LED current, regardless of input and output volt­ages. Now a single switching regulator with the ability to
be configured in a large variety of topologies for high
power constant LED current is available.

The LT®3477 can drive high power LEDs at constant
current in any of the topologies stated above. It is a
current mode, 3A DC/DC step-up converter that incorpo­rates dual rail-to-rail current sense amplifiers and an
internal 3A, 42V switch. It combines a traditional voltage
feedback loop and two unique current feedback loops to
operate as a constant-current and/or constant-voltage
source. The floating rail-to-rail current sense amplifiers
allow for both ground-referenced and floating LED solu­tions in different topologies, along with the added benefit
of inrush current or short-circuit protection.

Both current sense voltages are 100mV and can be
adjusted independently using the I

ADJ1

and I

ADJ2

pins.
Efficiencies of up to 91% can be achieved in typical
applications. The LT3477 features a programmable soft­start function to limit inductor current during start-up.
Both inputs of the error amplifier are available externally,
allowing positive and negative output voltages. The switch­ing frequency is programmable from 200kHz to 3.5MHz
through an external resistor. It comes in two thermally
enhanced packages: a 20-pin (4mm × 4mm) QFN and a
20-pin TSSOP.

Boost Driver

The LT3477’s internal ground-referenced 3A NPN power
switch is most commonly used for boost applications.
Figure 1 shows a 5V to four 1W LED boost converter with
open LED protection and 330mA constant LED current.
The constant current is regulated using a current sense
amplifier and a 0.3Ω sense resistor. The feedback voltage
amplifier is only used for overvoltage protection in case
the LEDs are removed from the circuit.

The forward voltage of the four LEDs ranges from 12V to
16V. The input voltage range is 2.5V (minimum LT3477
input voltage) up to just below the LEDs’ forward voltage.
The LT3477 can drive more LEDs as long as the peak
switch current remains below 3A.

11/05/376

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

Figure 1. 330mA Boost LED Driver with Open LED Protection

3.3V

PV

IN

32V

R1

0.1Ω

D2

•

LED

1A

•

STRING

•

D5

L1

33µH

I

I

SN1

V

IN

C3

3.3µF

SHDN

R2
1k

C5

4.7nF

C1: NIPPON UNITED CHEMICON NTS40X5R1H225M
C2: TAIYO YUDEN GMK316BJ105ML
C3: TAIYO YUDEN LMK316BJ475
L1: TOKO D1OFA814AY-330M
D1: DIODES INC DFLS140

SP1

V

IN

I

ADJ1

I

ADJ2

SHDN

V

C

V

REF

FBP SSGND

LT3477

C2

1µF

D1

SW

FBN

I

SP2

I

SN2

R

T

C4

R3

33nF

22k

R5
309k

R6
10k

DN376 F02

C1

2.2µF

V

IN

3V TO 16V

LED BRIGHTNESS

CONTROL

0mV TO 650mV

SHDN

L1

4.7µH

C1

3.3µF

C3
10nF

C1: TAIYO YUDEN LMK316BJ335ML
C2: MURATA GRM31CR71E475KA88L
D1: DIODES, INC. B320A
L1: TOKO FDV0630-4R7M

I

SP1

V

IN

I

ADJ1

I

ADJ2

SHDN

V

C

V

REF

FBP SSGND

I

SN1

LT3477

SW1

D3

FBN

I

SP2

I

SN2

R

T

C4
33nF

600mA

D2

R5
200k

R6
10k

R1

0.16Ω

C2

4.7µF

DN376 F03

D1

R3
18k

Figure 2. 1A Buck LED Driver

Buck Driver

LEDs are best driven with a constant current source, but
unlike most system loads, they do not have to be ground
referenced. Therefore, the LT3477 with its floating current
sense amplifiers can be converted into a buck LED driver
with low LED ripple current. Figure 2 shows a 32V input
voltage buck converter driving a string of 1A LEDs. The
LEDs are tied to the input source through a sense resistor
and the typical output Schottky catch diode is tied back to
V

, converting the boost IC to a monolithic buck LED

IN

driver. Once again, the feedback voltage amplifier is only
used for overvoltage protection—this time on the input—
to prevent damage to the 42V switch. In buck mode, the
V

pin need only be tied to a 3.3V or 5V source for

IN

maximum efficiency, and to keep it below its absolute
maximum rating of 25V. The dimming I

pin functions

ADJ

work the same in buck mode as boost mode, reducing the
LED current proportionally.

Data Sheet Download

http://www.linear.com

Figure 3. Buck-Boost LED Driver

Buck-Boost Driver

If the battery range lies both above and below the forward
voltage range of the LEDs, the LT3477 can simply be
converted into a buck-boost converter as floating current
sense amplifiers allow the LED string and its series sense
resistor to be tied anywhere in the circuit. By altering the
typical boost LED driver application so that the LED string
returns to V

as opposed to ground, step-up/step-down

IN

capability is provided.

Conclusion

The LT3477 is a versatile, monolithic boost, buck, and
buck-boost or SEPIC LED driver with a high power 3A,
42V switch. It can also be used for boost or SEPIC voltage
converters requiring inrush or short-circuit protection.
Two floating current sense amplifiers and a ground­referenced voltage feedback amplifier help give the LT3477
its high level of versatility. The externally programmable
switching frequency, a shutdown pin, LED current dim­ming adjustment and a single soft-start capacitor satisfy
the additional requirements of LED drivers and boost/
SEPIC regulators.

For applications help,

call [phone], Ext. 2759

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone] ● FAX: [phone] ● www.linear.com

dn376f LT/TP 1105 409K • PRINTED IN THE USA

© LINEAR TECHNOLOGY CORPORATION 2005