LINEAR LTC3756 User's Manual

DESIGN IDEAS L

100V Controller in 3mm × 3mm QFN or MSE Drives High Power LED Strings

from Just About Any Input by Keith Szolusha

Introduction

Strings of high power solid-state L1A, B

22µH 2× PDS5100

LEDs are replacing traditional lighting

VIN = PVIN

40V TO 60V 2.2µF 2.2µF

technologies in large area and high 499k 1.8M

(LED CURRENT 100V VIN 100V

OVP = 95V

lumens light sources because of their REDUCED WHEN ×2 ×5

SHDN/UVLO FB

VIN < 40V)

high quality light output, unmatched VREF ISP 24.3k 1M

23.2k LT3756 ILED

durability, relatively low lifetime cost, 0.068Ω CTRL 1.5A

constant-color dimming and energy ef- ISN

ficiency. The list of applications grows GATE Q1A, B

daily, including LCD television back- SENSE

OPENLED

lights and projection system bulbs, 83V

100k LED

industrial and architectural lighting STRING

INTVCC PWM

systems, automotive headlamps, tail-

PWMOUT

lights and indicator lights, computer RT

0.1µF GNDVC INTVCC

monitors, street lights, billboards and

42.2k 250k Hz Si7322DN

even stadium lights.

As the number of applications ex-

pands, so does the complexity of input

L1 = 2× SERIES SLF12575T-220M4R0 requirements for the LED drivers. Q1 = 2× PARALLEL Si7322DN

LED drivers must be able to handle

Figure 1. A 125W, 83V at 1.5A, 97% efficient boost LED driver for stadium lighting

wide ranging inputs, including the harsh transient voltage environment A number of features protect the voltage if the constant current sense presented by automotive batteries, LEDs and surrounding components. voltage is not used. This is a side benefit the wide voltage range of the Li-ion Shutdown and undervoltage lockout, of the LT3756’s overvoltage protection cells and wallwart voltages. For LED when combined with analog dimming feature, in which the current control lighting manufacturers and design- derived from the input, provide the loop is superceded by the FB voltage ers, applying a different LED driver standard ON/OFF feature as well as loop in the case of an open LED string, for each application means stocking, a reduced LED current should the thus preventing the controller from a testing and designing with a wide battery voltage drop to unacceptably running up the voltage in an effort to variety of LED controllers. This can low levels. Analog dimming is accu- maintain current. be an expensive and time-consuming rate and can be combined with PWM

proposition. It would be far better to dimming for an extremely wide range 125W Boost LED Driver for use a controller that can be applied of brightness control. The soft-start Stadium Lights or Billboards

to many solutions. feature prevents spiking inrush cur- Lighting systems for stadiums, spot- The LT3756 high voltage LED driver rents during start-up. The OPENLED lights and billboards require huge features a unique topological versatil- pin informs of open or missing LEDs strings of LEDs running at high power. ity that allows it to be used in boost, and the SYNC (LT3756-1) pin can be The LT3756 controller can drive up buck-boost mode, buck mode, SEPIC, used to sync switching to an external to 100V LED strings with its floating flyback and other topologies. Its high clock. sense resistor inputs ISP and ISN. power capability provides potentially The 16-pin IC is available in a The 125W LED driver in Figure 1 hundreds of watts of steady-state LED tiny QFN (3mm × 3mm) and an MSE accepts a wide-range 40V–60V input power over a very wide input voltage package, both thermally enhanced. taken from the output of a high power range. Its 100V floating LED current For applications with lower input volt- transformer. sense inputs allow the LED string to age requirements, the 40VIN, 75VOUT The LT3756’s high power GATE float above ground, as shown in the LT3755 LED controller is a similar driver switches two 100V MOSFETs buck mode and buck-boost mode to- option to the LT3756. at 250k Hz. This switching frequency pologies in this article. Excellent PWM Although it is typically used as an minimizes the size of the discrete com- dimming architecture produces high LED driver, the LT3756’s voltage FB ponents while maintaining high 97% dimming ratios, up to 3000:1. pin provides a well-regulated output efficiency, thus producing a less-than-

Linear Technology Magazine • January 2009