60V Buck-Boost Controller Drives High Power LEDs, Charges Batteries and Regulates Voltage with Up to 98.5% Efficiency at 100W and Higher – Design Note 501 Keith Szolusha Buck-Boost Controller Drives 100W LED String for Airplane and Truck Lights Airplanes and big trucks with 24V batteries need powerful, efficient and robust headlights and spotlights. Figure 1 shows a 33.3V, 3A (nine Luminus SSR-90 LEDs) VIN 15V TO 58V RIN 0.003Ω VIN CVIN 1μF TG1 INTVCC BG1 LT3791 SHORTLED OPENLED OPENLED PWM DIM PWM IVINMON ISMON CLKOUT VREF 0.1μF DIM D2 4.7μF 10V M2 0.1μF L1 10μH M4 1M M3 34.2k 98 RLED 0.033Ω SNSN PGND BG2 33.3VLED 3A 100W FB ISN RT SYNC VC 2.2k 10nF 86.6k 300kHz D1: BAT46WJ L1: COOPER HC9-100-R M1, M2: RENESAS RJK0651DPB M3-M5: RENESAS RJK0451DPB LUMINUS SST-90 w9 M5 DN501 F01 Figure 1. 15–58VIN to 33.3V 3A LED Driver with Up to 98.5% Efficiency 04/12/501 BUCK 96 BUCK-BOOST 95 94 92 TG2 PWMOUT 10nF BOOST 97 93 SW2 ISP CTRL SGND 100 99 RSW 0.004Ω 10k SS COUT 4.7μF 50V w5 SNSP 200k SHORTLED M1 SWI EN/UVLO OVLO CIN 2.2μF 100V w4 0.1μF BST1 IVINP 499k 200k L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. D1 470nF 43.2k The solution in Figure 1 features up to 100:1 PWM dimming at 100Hz for accurate brightness adjustment without color shift and analog LED dimming when a PWM oscillator is not present. BST2 IVINN 27.4k The 4-switch synchronous topology drives high power LEDs with minimal switch power loss (and minimal temperature rise). Unlike other topologies, the LT3791 buck-boost can be shorted from LED+ to both LED– and GND and, as a feature, can be programmed to latch off or keep trying to turn back on if the short is removed. Diagnostic output flags report both short-circuit and open-LED conditions. INTVCC INTVCC 50Ω 499k buck-boost LED driver that runs from 15V to 58V input with up to 98.5% efficiency. EFFICIENCY (%) Introduction The LT®3791 is a 4-switch synchronous buck-boost DC/DC converter that regulates both constant-current and constant-voltage at up to 98.5% efficiency with a single inductor. It can deliver hundreds of watts and features a 60V input and output rating, making it ideal for driving high power LED strings and charging high voltage batteries when both step-up and step-down conversion is needed. It can also be used as a constantvoltage buck-boost regulator with current limiting and monitoring for both input and output. 91 90 33.3V 3A LEDs 10 15 20 25 30 35 40 45 50 55 60 DN501 F02 INPUT VOLTAGE (V) Figure 2. Efficiency of Figure 1 36V, 2.5A SLA Battery Charger The buck-boost converter shown in Figure 3 charges a 36V 12Ah SLA battery at 44V with 2.5A DC from a 9V to 58V input. Figure 1 can be easily turned into a 100W voltage regulator with VOUT between 6V and 55V by placing a 100k resistor between the SS and VREF pins to defeat soft-start reset during a fault. Specially integrated C/10 current sensing and battery voltage detection drops the battery voltage from its charging voltage (44V) to its float voltage (41V) when the battery is near full charge. The OPENLED flag is used to change the state of the charger from charge to float. When the battery voltage drops far enough, voltage feedback returns the charger to its charge state. The voltage regulator is short-circuit proof. The SHORTLED flag reports when there is a short circuit on the output. The input current monitor can still be used in constant-voltage regulation to protect any system that is input-current limited. RLED sets the output current limit for short circuit, but can be removed to provide a PGOOD flag using OPENLED by shorting ISP to ISN. The LT3791 can be tailored to charge a range of battery chemistries and capacities from a variety of input sources regardless of the voltage relationship between them. An external microcontroller can be programmed and used to create a maximum power point tracking device to charge the battery from a solar panel. The output diagnostics and dimming input pins make this a simple interface for high power solar panel applications design. Conclusion The LT3791 synchronous buck-boost controller delivers 100W and higher power at up to 98.5% efficiency to a number of different loads. The wide, 4.7V to 60V input and 0V to 60V output voltage range make it both powerful and versatile, and its short-circuit capability makes this a robust choice for many applications in potentially hazardous environments. This seemingly limitless IC can be used in applications where a typical buck or boost converter cannot because of the crossover of input and output voltage ranges. 120W, 6V to 55V Voltage Regulator The LT3791 can also be used in high power constantvoltage buck-boost applications. The FB pin doubles as both the main voltage feedback for the buck-boost, and the overvoltage protection detection and regulation when used as an LED driver. PVIN 9V TO 58V RIN 0.003Ω VIN 1μF INTVCC D1 4.7μF D2 BST2 50Ω IVINN TG1 IVINP 499k BG1 OVLO INTVCC 200k 30.9k M1 LT3791 M4 0.1μF SWI EN/UVLO COUT 4.7μF 50V w2 0.1μF BST1 470nF 57.6k CIN 2.2μF 100V w4 L1 10μH M2 M3 SNSP RS1 0.04Ω RSW 0.004Ω SHORTLED PWMOUT SNSN + PGND IVINMON ISMON CLKOUT CTRL SGND CHARGE CURRENT CONTROL BG2 SW2 TG2 FB ISP 1μF C8 0.1μF SS RT SYNC VC 10nF D1, D2: BAT46WJ L1: WÜRTH 74435571100 10μH M1-M5: RENESAS RJK0651DPB 1k 36V SLA BATTERY AGM TYPE 41V FLOAT 44V CHARGE AT 25°C 50Ω ISN OPENLED VREF PWM 2.5A CHARGE 1.00M INTVCC 86.6k 300kHz 10k 402k 30.1k M5 4.7nF DN501 F03 Figure 3. 36V, 2.5A SLA Battery Charger with C/10 Charge Termination Data Sheet Download www.linear.com For applications help, call (408) 432-1900, Ext. 3801 Linear Technology Corporation dn501f LT/AP 0412 196K • PRINTED IN THE USA FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2012 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ●