L DESIGN IDEAS Triple Output LED Driver Works with Inputs to 60V and Delivers 3000:1 PWM Dimming by Hua (Walker) Bai Introduction The LT3492 is a 60V triple output LED driver for high input and/or high output voltage backlighting or direct lighting applications. A single 4mm × 5mm IC can drive a large number of LEDs, reducing overall solution cost when compared to less capable drivers. A built-in gate driver for a disconnect PMOS in series with the LED string, along with other techniques, enables a 3000:1 PWM dimming ratio. When coupled with the part’s analog dimming functions, the overall dimming ratio can be as high as 30,000:1. The LT3492 can be configured into buckmode, boost-mode or buck-boost mode, depending on the available input voltage source and the number and configuration of LEDs to be driven. PVIN 43V to 58V TG1 ISP2 ISP3 330mΩ 330mΩ 330mΩ ISN1 ISN2 ISN3 TG2 M1 M2 68.1k 0.3A 8 LEDs C4 0.47µF 5.6k M4 2k L1 33µH C5 0.47µF D1 VIN 5V 1µF 6.3V PWM1-3 SHDN 0.3A 8 LEDs 5.6k M5 M6 2k C6 0.47µF 5.6k OVP2 OVP3 L2 33µH D2 SW1 68.1k 0.3A 8 LEDs TG3 M3 68.1k OVP1 2k L3 33µH SW2 D3 SW3 ISP1-3 ISN1-3 VIN PWM1-3 SHDN LT3492 TG1-3 OVP1-3 VC1-3 VREF CTRL1-3 1nF 430k C1: MURATA GRM31CR72A105KA01L C4–C6: MURATA GRM21BR72A474KA73 D1–D3: DIODES B1100 Many “regulated” supplies actually have fairly loose tolerances. For ex- 0Ω (1MHz) FADJ GND High Input Voltage Triple Buck Mode LED Driver C1 1µF s3 ISP1 100k L1–L3: WE 7447789133 M1–M3: ZETEX ZXMP6A13F M4–M6: PHILIPS BC858B Figure 1. Triple buck mode LED driver with open LED protection PVIN 9V TO 40V C1 1µF s3 L1 22µH DESIGN IDEAS Triple Output LED Driver Works with Inputs to 60V and Delivers 3000:1 PWM Dimming....................................26 L2 22µH D1 Hua (Walker) Bai C2 1µF 100V Bidirectional Power Manager Provides Efficient Charging and Automatic USB On-The-Go with a Single Inductor ..........................................................28 L3 22µH D2 ISP1 C3 1µF 100V 1.65Ω TG1 Supercapacitor Charger with Adjustable Output Voltage and Adjustable Charging Current Limit ..........................................................30 TG2 ISN3 TG3 M2 1020k 60mA 1.65Ω ISN2 M1 14 LEDs ISP3 C4 1µF 100V 1.65Ω ISN1 Sauparna Das D3 ISP2 M3 1020k OVP1 20k 14 LEDs 60mA 1020k OVP2 14 LEDs 20k OVP3 60mA 20k Jim Drew Monolithic Triple Output Converter for Li-Ion Powered Handheld Devices.......32 SW1 Chuen Ming Tan Ultralow Power Boost Converters Require Only 8.5µA of Standby Quiescent Current..............................34 Xiaohua Su 15VIN, 4MHz Monolithic Synchronous Buck Regulator Delivers 5A in 4mm × 4mm QFN................................35 VIN 5V 1µF 6.3V PWM1-3 SHDN SW2 ISP1-3 ISN1-3 VIN PWM1-3 SHDN SW3 LT3492 TG1-3 OVP1-3 VC1-3 VREF CTRL1-3 18.2k 2.2nF 215k FADJ GND C1–C4: MURATA GRM31CR72A105KA01L D1–D3: DIODES B1100 L1–L3: COILCRAFT MSS6132 M1–M3: ZETEX ZXMP6A13F (1MHz) 49.9k Tom Gross Figure 2. Triple boost mode 60mA × 14 LED driver 26 Linear Technology Magazine • June 2009 DESIGN IDEAS L 350 ample, a 48V supply can range between 43V and 58V, well above most LED drivers’ safe operating voltage ratings. The LT3492’s 60V input voltage rating makes it an easy fit in such volatile voltage environments. Figure 1 shows a triple buck-mode LED driver for high voltage inputs. Each channel can drive up to eight 300mA white LEDs in series, a limit set by assuming 4V maximum forward voltage and a 43V minimum input voltage. Red LEDs or infrared LEDs have much lower forward voltage, therefore each output can drive as many as 20 infrared LEDs. The VIN pin in Figure 1 is tied to a 5V supply, as opposed to PVIN, to improve circuit efficiency. OUTPUT CURRENT(mA) 200 150 100 50 10 20 30 60 is a function of the ratio of output voltage to minimum input voltage. Figure 3 shows the maximum output current vs output voltage for a 9V minimum input (assuming 85% efficiency at 1MHz). For applications that require less than 40V output, the LT3496 should be considered instead. Conclusion The LT3492 is a high voltage triple output LED driver with 60V rated switches, allowing high input voltage and/or high output voltage operations with accurate LED current. It can run in buck mode, boost mode or buckboost mode with 3000:1 PWM dimming capability. L Triple Buck-Boost Mode LED Driver Regulates During Load Dump Events PWM 5V/DIV 0V Buck-boost mode is used when the LED string voltage falls within the input voltage range. Figure 4 shows a buck-boost application that uses one inductor per driver. The LED string is returned to the input—returning all LED strings to the same potential allows easy heat sinking. To prevent body diode conduction, the drain of the disconnect PMOS is tied to the 4 LEDs 150mA M1 ISN1 0.68Ω ISP1 D1 1150k 0.68Ω 20k ISP2 D2 C3 2.2µF 25V ISP1-3 ISN1-3 VIN PWM1-3 SHDN 1µs/DIV Figure 5. High dimming ratio (>3000:1) improves quality of an LCD display 1150k ISN3 OVP2 0.68Ω 20k PVIN SW2 M3 TG3 ISP3 D3 C5 2.2µF 25V 1150k OVP3 20k PVIN SW3 LT3492 C7 2.2µF 25V TG1-3 OVP1-3 VC1-3 VREF CTRL1-3 18.2k 215k FADJ GND C1: MURATA GRM31CR71H225KA88L C3, C5, C7: MURATA GRM21BR71E225K 4 LEDs 150mA M2 ISN2 OVP1 PVIN 0V L3 27µH TG2 SW1 ILED 50mA/DIV 4 LEDs 150mA L2 27µH TG1 PWM SHDN 50 Figure 3. Maximum output current capability of an LT3492 boost circuit L1 27µH VIN 5V 1µF 6.3V 40 OUTPUT VOLTAGE (V) Figure 2 shows a triple boost mode LED driver that delivers 60mA to each LED string. Due to the LT3492’s 60V switch rating, each output can support up to 14 LEDs. The 9V-to-40V input range covers a diverse range of applications, including regulated 12V, 24V, 32V to 36V, etc. Unlike in a buck mode regulator, where the output current capability is determined by the switch current limit, the current driving capability of a boost regulator C1 2.2µF 50V 250 0 Triple Boost Mode Driver Supports 14 LEDs per Output from a 9V–40V Input PVIN 9V TO 40V anode of the LED string. The high input voltage of the circuits in Figure 2 and Figure 4 is a real benefit in automotive applications, where the ability to ride through 40V load dump events while maintaining LED current regulation is required. Figure 5 shows the greater than 3000:1 PWM dimming ratio achievable with the LT3492. This high PWM dimming ratio helps improve the picture quality of an LCD display under various dynamic conditions. PVIN = 9V, 85% EFFICIENCY 300 D1–D3: DIODES B1100 L1–L3: COILCRAFT MSS6132 M1–M3: ZETEX ZXMP6A13F 2.2nF (1MHz) 49.9k Figure 4. Triple buck-boost mode 150mA × 4 LED driver Linear Technology Magazine • June 2009 27