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 efficiency. 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 VIN for the LED 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 wart 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, flyback and other topologies. Its high power capability provides potentially hundreds of watts of LED power over a wide input voltage range. Its 100V floating 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 VOUT to protect the converter in the 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 40VIN , 75VOUT 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. VIN 10V TO 80V L1A, B 22μH 2× VIN 40V TO 60V 2.2μF 100V ×2 499k SHDN/UVLO FB VREF 23.2k OVP= 95V VIN 24.3k LT3756 0.068Ω CTRL 100k PWM SS RT VC 83V LED STRING 1N4448HWT Si7322DN 10k 4.7μF L1 = 2× SERIES SLF12575T-220M4R0 Q1 = 2× PARALLEL Si7322DN 69.8k 150kHz 147k ILED 1A Q1 10μF 16V 4.7k 12.4k M2 CTRL 1 OR 2 LEDs 3.5V EACH 0A–1A 51k 1k GND INTVCC Figure 1. A 125W, 83V at 1.5A, 97% Efficient Boost LED Driver for Stadium Lighting 03/09/461 0V–12V 9.1k FOR 0A–1A ILED 0.1μF 10k 0.1Ω 6.2V ISN FB LT3756 M3 PWMOUT PWMOUT 4700pF 120k VREF PWM 0.018Ω ISP 30.9k SENSE OPENLED 10k VIN SHDN/UVLO Q1A, B GATE 42.2k 250kHz ILED 1.5A 196k ISN 30.9k 0.1μF 2.2μF 100V ×5 1.8M ISP 1M INTVCC 2.2μF 100V ×2 PDS5100 PWM OPTIONAL L1 33μH D1 OPENLED M1 SS GATE RT VC SENSE GND INTVCC 0.01μF 0.05Ω 4.7μF 100k D1: DIODES INC B2100 L1: SUMIDA CDRH8D38-330 M1: VISHAY SILICONIX Si4484EY M2: VISHAY SILICONIX Si2307BDS M3: VISHAY SILICONIX Si2328DS Q1: MMBT5401 Figure 2. An 80VIN Buck Mode LED Driver With 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. 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 configuration trades some efficiency, component cost, and board space for design simplicity, and time-to-market. 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% efficiency, 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 VIN is higher than VLED , the LT3756 can serve equally 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 VIN and lower LED 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-VIN-to-low-VLED (80VIN to one 3.5V LED) and low-VIN-dropout requirements (10VIN to 7VLED). 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 VIN . Although the buck mode components will survive this scenario, the LEDs may not survive being plugged into a potential equal to VIN . Data Sheet Download www.linear.com 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 6VIN . COUT, DI and MI can see 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. 10V–50V LED– L1 22μH VIN 9V TO 36V M1 110k VIN 499k GATE SENSE 130k FB 4.7μF COUT 2.2μF 100V s2 VIN 4.7k 2.49k LT3756 CTRL 0.25Ω 0.025Ω 3906 SHDN/UVLO PWMOUT PWM INTVCC VREF ISP ISN 100k OPENLED 140k ILED 400mA D1 2.2μF 50V s2 1M LED+ SS VC 0.1μF 10k GND RT 5.1k 4700pF M1: VISHAY SILICONIX Si7454DP D1: DIODES INC. PDS3100 L1: SUMIDA CDRH127-220 28.7k 400kHz Figure 3. A Buck-Boost Mode LED Driver With Wide-Ranging VIN and VLED For applications help, call (408) 432-1900, Ext. 3801 Linear Technology Corporation dn461 LT/TP 0309 155K • PRINTED IN THE USA FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2009 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ●