LINEAR TECHNOLOGY LT3756 Technical data

100V Controller Drives High Power LED Strings from Just about
Any Input – Design Note 461

by Keith Szolusha

Introduction

Strings of high power solid-state LEDs are replacing
traditional lighting technologies in large area and high
lumens light sources because of their high quality light
output, unmatched durability, relatively low lifetime cost,
constant-color dimming and energy effi ciency. The list of
applications grows daily, including LCD backlights and
projection, industrial and architectural lighting, automo­tive lights, streetlights, billboards and stadium lights.

As the list expands, so does the range of V

for the LED

IN

drivers. LED drivers must be able to handle wide ranging
inputs, including transient voltages of automotive batter­ies, a wide range of other batteries and wall war t voltages.
For LED lighting manufacturers, applying a different LED
driver for each application means stocking, testing and
designing with a number of controllers. It would be better
to use just one that can be applied to many solutions.

The LT3756 high voltage LED driver features a unique
topological versatility that allows it to be used in boost,
buck-boost mode, buck mode, SEPIC, fl yback and other
to po lo gi es . I ts h ig h p ow er cap ab il it y p ro vi des po te nti al ly
hundreds of watts of LED power over a wide input voltage

L1A, B

PDS5100

V

IN

LT3756

GND

22μH 2×

OVP=
95V

ISP

0.068Ω

ISN

GATE

SENSE

PWMOUT

INTV

Q1A, B

0.018Ω

CC

4.7μF
L1 = 2× SERIES SLF12575T-220M4R0

Q1 = 2× PARALLEL Si7322DN

1.8M

I

LED

1.5A

83V
LED
STRING

Si7322DN

24.3k

2.2μF
100V
×5

40V TO 60V

V

IN

0.1μF

2.2μF
100V
×2

10k

INTV

499k

23.2k

CC

SHDN/UVLO FB

V

REF

1M

CTRL

30.9k

OPENLED

100k

PWM
SS
R

T

V

C

42.2k
250kHz

10k

4700pF

range. Its 100V fl oating LED current sense inputs provide
accurate LED current sensing. Excellent PWM dimming
architecture produces high dimming ratios.

A number of features protect the LEDs and surround­ing components. Shutdown and undervoltage lockout,
when combined with analog dimming derived from the
input, provide the standard ON/OFF feature as well as
a reduced LED current should the battery voltage drop
to unacceptably low levels. Analog dimming is accurate
and can be combined with PWM dimming for a wide
range of brightness control. Soft-start prevents spiking
inrush currents. The OPENLED pin informs of open or
missing LEDs and the SYNC (LT3756-1) pin can be used
to sync switching to an external clock. The FB voltage
loop limits the max V

to protect the converter in the

OUT

case of open LEDs.

The 16-pin IC is available in a tiny QFN (3mm

×

3mm)
and an MSE package, both thermally enhanced. For lower
input voltage requirements, the 40V

IN

, 75V

OUT

LT3755

LED controller is a similar option.

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

10V TO 80V

V

IN

0V–12V

FOR

0A–1A I

2.2μF
100V
×2

9.1k

LED

PWM

1N4448HWT

0.1μF

10k

196k

30.9k

1k

69.8k
150kHz

SHDN/UVLO

V

REF

CTRL

LT3756

PWM

OPENLED

SS

R

T
C

0.01μF

V

IN

GNDV

PWMOUT

GATE

SENSE
INTV

ISP

ISN

120k

0.1Ω

6.2V

FB

1 OR 2

51k

CC

4.7μF

LEDs

3.5V

EACH

M3

0A–1A

M1

D1: DIODES INC B2100
L1: SUMIDA CDRH8D38-330
M1: VISHAY SILICONIX Si4484EY

0.05Ω
M2: VISHAY SILICONIX Si2307BDS
M3: VISHAY SILICONIX Si2328DS
Q1: MMBT5401

100k

M2

L1
33μH

I
1A

147k

LED

10μF

Q1

12.4k

OPTIONAL

D1

16V

4.7k

Figure 1. A 125W, 83V at 1.5A, 97% Effi cient Boost LED
Driver for Stadium Lighting

03/09/461

Figure 2. An 80V

Buck Mode LED Driver With

IN

PWM Dimming for Single or Double LEDs

Boost

Buck-Boost Mode

Lighting systems for stadiums, spotlights and billboards
require huge strings of LEDs running at high power. The
LT3756 controller drives up to 100V LED strings. The
125W LED driver in Figure 1 has a 40V–60V input.

The high power gate driver switches two 100V MOSFETs
at 250kHz. This switching frequency minimizes the size
of the discrete components while maintaining high
97% effi ciency, producing a less-than-50ºC discrete
component temperature rise—more manageable than
the heat produced by the 125W LEDs.

Even if PWM dimming is not required, the PWMOUT
MOSFET is useful for LED disconnect during shutdown. It
prevents current from running through the string of LEDs.

If the LED string is removed, the FB constant-voltage
loop takes over and regulates the output at 95V. Without
overvoltage protection, the LED sense resistor would
see zero current and the output cap voltage would go
over 100V, exceeding several max ratings. While in OVP
OPENLED goes low.

Buck Mode

When V

is higher than V

IN

, the LT3756 can serve

LED

equ ally well as a buck mode LED driver. The buck mode
LED driver in Figure 2 operates with a wide 10V-to-80V
input range to drive one or two LEDs at 1A.

PWM dimming requires a level-shift from the PWMOUT
pin to the high-side LED string. The max PWM dimming
ratio increases with higher switching frequency, lower
PWM dimming frequency, higher V

and lower LED

IN

power. In this case, a 100:1 dimming ratio is possible
with a 100Hz dimming frequency and a 48V input.
Although higher switching frequency is possible, the
duty cycle has its limits. Generous minimum on-time
and minimum off-time restrictions require a frequency
on the lower end of its range (150kHz) to meet both the
harsh high-V
and low-V

-to-low-V

IN

-dropout requirements (10VIN to 7V

IN

(80VIN to one 3.5V LED)

LED

LED

).

OVP of the buck mode LED driver has a level shift
as well. Without the level-shifted OVP network tied
to FB, an open LED string would result in the output
capacitor charging up to V

. Although the buck mode

IN

components will survive this scenario, the LEDs may not
survive being plugged into a potential equal to V

Data Sheet Download

www.linear.com

.

IN

A common LED driver requirement is that the ranges
of both the LED string voltage and the input voltage are
wide and overlapping. In fact, some designers prefer
to use the same LED driver circuit for several differ­ent battery sources and several different LED strings.
Such a versatile confi guration trades some effi ciency,
component cost, and board space for design simplicity,
and time-to-market.

The buck-boost mode driver in Figure 3 uses a single
inductor. It accepts inputs from 9V to 36V to drive
10V–50V LED strings at 400mA.

The inductor current is the sum of the input current
and the LED string current; the peak inductor current
is equal to the peak switching current. Below 9V input,
CTRL analog dimming scales back the LED current to
keep the inductor current under control. UVLO turns
off the LEDs below 6V

. C

IN

, DI and MI can see

OUT

voltages as high as 95V here.

Conclusion

The LT3756 controller is a versatile high power LED
driver. It has all the features required for large (and small)
strings of high power LEDs. Its high voltage rating,
optimized LED driver architecture, high performance
PWM dimming, host of protection features and accurate
high side current sensing make the LT3756 a single-IC
choice for a variety of lighting systems.

V

L1

22μH

C

5.1k

4700pF

SENSE

GND

10V–50V

PWMOUT

V

ISP
ISN

R

LED

D1

M1

0.025Ω

FB

REF

T

28.7k
400kHz

3906

M1: VISHAY SILICONIX Si7454DP
D1: DIODES INC. PDS3100
L1: SUMIDA CDRH127-220

400mA

110k

2.49k

+

LED

I

LED

0.25Ω

C

OUT

2.2μF
100V
s2

V

IN

4.7k

–

LED

9V TO 36V

V

IN

2.2μF
50V
s2

GATE

V

IN

SHDN/UVLO

CTRL
PWM
INTV

CC

OPENLED

SS

0.1μF

10k

LT3756

1M

140k

499k

130k

4.7μF

100k

Figure 3. A Buck-Boost Mode LED Driver With
Wide-Ranging VIN and V

For applications help,

call [phone], Ext. 3801

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone]

●

FAX: [phone] ● www.linear.com

dn461 LT/TP 0309 155K • PRINTED IN THE USA

© LINEAR TECHNOLOGY CORPORATION 2009