Triple LED Driver in 4mm × 5mm QFN Supports LCD Backlights in Buck, Boost or Buck-Boost Modes and Delivers 3000:1 PWM Dimming Ratio Design Note 449 Hua (Walker) Bai Introduction By integrating three independent LED drivers, the LT®3496 offers a highly efficient, compact and cost-effective solution to drive multiple LED strings. All three drivers have independent on/off and PWM dimming control, and can drive different numbers or types of LEDs. High side current sensing and built-in gate drivers for PMOS LED disconnect allow the LT3496 to operate in buck, boost, SEPIC or buck-boost modes with up to 3000:1 True Color PWM™ dimming ratio. The LT3496 is offered in a single 4mm × 5mm QFN or FE28 package. The efficiency of each driver can exceed 95%. Integrated PMOS Drivers Improve PWM Dimming Ratio to 3000:1 A high PWM dimming ratio is critical in many display applications, especially in high end LCD panels. Beware, though, the definition of dimming ratio varies among suppliers. When comparing dimming ratios, pay close attention to the PWM dimming frequency and linearity of the LED average current at different PWM duty cycles. For instance, the LT3496’s high 3000:1 PWM dimming ratio can be achieved at a 100Hz PWM frequency—high enough to keep the display flicker-free over the entire dimming range. L, LT, LTC and LTM are registered trademarks and True Color PWM is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. PVIN 12V L1 15μH PVIN 42V MAX TG1 CAP1 CAP2 CAP3 200mΩ 200mΩ 200mΩ LED1 LED2 LED3 TG2 M1 M2 C1 TO C3 1μF s3 L2 15μH D1 C2 1μF 50V TG3 M3 C1 1μF 50V D2 CAP1 0.5Ω C3 1μF 50V 0.5A C4 0.47μF 0.5A 0.5A 0.5Ω VIN 3.3V OR 5V PWM1-3 SHDN C7, 1μF D1 SW1 CAP1-3 LED1-3 VIN PWM1-3 SHDN L2 L3 10μH 10μH D2 SW2 LT3496 GND SW3 D3 LED3 TG3 M2 TG1-3 OVP1-3 VC1-3 VREF FADJ CTRL1-3 C1-C3, C7: MURATA GRM31MR71H105KA88 C4-C6: MURATA GRM188R71C474KA88 D1-D3: DIODES DFLS160 7.15M 200mA OVP1 7 LEDs 7.15M 200mA OVP2 191k 22k 470pF DN449 F01 L1-L3: TAIYO YUDEN NP04SZB 100M M1-M3: ZETEX ZXMP6A13F Figure 1. Triple Buck Mode Can Drive 3x 500mA LED Strings 09/08/449 0.5Ω M3 C6 0.47μF C5 0.47μF 7 LEDs L1 10μH CAP3 C4 1μF 50V LED2 TG2 M1 D3 CAP2 LED1 TG1 L3 15μH VIN 3.3V OR 5V SW1 CAP1-3 LED1-3 VIN PWM1-3 SHDN 7 LEDs 7.15M 200mA OVP3 191k SW2 191k SW3 LT3496 TG1-3 OVP1-3 VC1-3 VREF FADJ CTRL1-3 PWM1-3 SHDN GND C5 DN449 F02 1μF 6.3V C1: TAIYO YUDEN UMK325BJ105MH L1-L3: SUMIDA CDRH3D14/HP-150 C2-C4: MURATA GRM21BR71H105K M1-M3: ZETEX ZXMP6A13F D1-D3: ROHM RB160M-60 Figure 2. Triple Boost Mode Can Drive 200mA LEDs 18.2k 1nF Buck Mode Circuit Drives Three 500mA LED Strings Figure 1 shows a triple buck mode LED driver. Each channel drives 500mA of current to its LEDs. Each string can have from eight to twelve LEDs, depending on type. The 2.1MHz switching frequency minimizes the solution size by allowing the use of low profile inductors and capacitors. The overall size of the circuit is less than 16mm × 16mm, with a maximum height of 1.5mm. Efficiency can be above 95% for a LT3496 buck mode driver. A further reduction in the parts count is possible by removing M1, M2 and M3. However, the dimming ratio drops without those MOSFETs. To improve the efficiency, the VIN pin should be biased from a 3.3V or 5V supply. Energy to the LEDs is supplied by PVIN. OVP protection is omitted in Figure 1. Boost Mode Circuit Drives Three 200mA LED Strings Figure 2 shows a triple boost mode driver that delivers 200mA to each LED string from a regulated 12V. Figure 3 shows the superior PWM dimming performance of the circuit. The LED current reaches a programmed 200mA in less than 500ns. The efficiency of this circuit is 90% at a 2.1MHz switching frequency. Unlike the buck mode driver, the boost mode and buck-boost mode drivers always require an OVP circuit at the output for open LED protection. Buck-Boost Mode Circuit Survives Load Dump Events In automotive applications, load dump is a condition under which an IC is expected to experience 40V transient. In such applications, the LED string voltage often falls in the middle of the 8V to 40V input supply range, thus requiring buck-boost mode. In a buck-boost circuit, the switch voltage is the sum of the input voltage and the LED voltage. Therefore, it is necessary to turn off the internal power switch before the input voltage gets too high. The LT3496 circuit in Figure 4 drives four LEDs, at 200mA per channel. The circuit monitors the Schottky diodes’ cathode voltage (VSC). The OVP logic turns off the main switch when VSC is above 38V, preventing the switch voltage from rising further. Since no IC pin experiences absolute maximum voltage, the circuit survives the load dump event. Conclusion Multiple output LED drivers, such as the LT3496, offer excellent current matching, efficiency and space savings. The flexibility to operate in buck, boost or buckboost mode makes the LT3496 feasible in many rugged applications. PVIN, 8V TO 16V 40V TRANSIENT C1 1μF 50V TG1 L1 15μH LED1 PWM 5V/DIV (100Hz) M1 7.15M OVP1 TG2 L2 15μH LED2 191k 0.5Ω D1 0.5μs/DIV 4 LEDs D2 VIN 3.3V OR 5V C8 1μF 6.3V PWM SHDN SW1 CAP1-3 LED1-3 VIN PWM1-3 SHDN 7.15M OVP3 191k CAP3 4 LEDs 4 LEDs 200mA D3 VSC C3 2.2μF PVIN 25V M3 0.5Ω 191k 200mA VSC DN449 F03 Figure 3. Achieving Greater Than 3000:1 PWM Dimming Ratio with a PMOS Disconnect OVP2 TG3 L3 15μH LED3 CAP2 200mA ILED 200mA/DIV 7.15M 0.5Ω CAP1 IL 500mA/DIV M2 VSC C5 2.2μF PVIN 25V SW2 SW3 LT3496 GND C7 2.2μF PVIN 25V TG1-3 OVP1-3 VC1-3 VREF FADJ CTRL1-3 18.2Ω 1nF L1-L3: SUMIDA CDRH3D14/HP-150 C1: TAIYO YUDEN UMK325BJ105MH C3, C5, C7: TAIYO YUDEN TMK212BJ225MG M1-M3: ZETEX ZXMP6A13F DN449 F04 D1-D3: ROHM RB160M-60TR Figure 4. Triple Buck-Boost Mode Can Drive 200mA LEDs While Surviving Load Dump Data Sheet Download For applications help, call (408) 432-1900, Ext. 2759 www.linear.com Linear Technology Corporation dn449 LT/TP 0908 246K • PRINTED IN THE USA FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2008 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ●