L DESIGN FEATURES Triple Output LED Driver Delivers 3000:1 Dimming Ratio in Buck, by Bin Zhang Boost or Buck-Boost Mode Introduction The LT3496 is a triple output DC/DC converter designed for high performance, True Color PWMTM dimming in multichannel LED lighting applications. By integrating three independent driver channels, the LT3496 provides a space-saving and cost-efficient solution to drive multiple LED strings. Figure 1 shows a 50W LT3496 3-channel LED driver that occupies 350mm2 and with a sub-1.5mm profile. SIMPLIFIED TRADITIONAL LED DRIVER BOARD SIX WIRES L1 15µH LED DRIVER ratio in buck, boost, or buck-boost configurations. The 45V capability of the internal power switch, 3V–40V input voltage range, and adjustable frequency result in reliable operation over a wide range of supply and output voltages. Applications for the LT3496 include RGB lighting, billboards and large displays, automotive and avionic lighting, and constant-current sources. Figure 1. A complete LT3496 LED driver fits into 350mm2 The LT3496 features high side current sensing and built-in gate drivers for PMOS high side LED disconnect (patent pending). These two features give the LT3496 its versatility, allowing it to drive LED’s to high PWM dimming M1 LED DRIVER M2 PVIN 42V LED DRIVER High Side LED Disconnect with High Side Current Sensing for System Versatility, Simplicity and Reliability The LT3496’s high side LED disconnect and high side current sensing enable 3000:1 dimming control in buck, boost, or buck-boost configurations. No traditional LED driver can match the simplicity and high PWM CAP1 CAP2 0.28Ω TG1 a. Traditional boost LED driver SIMPLIFIED LT3496 LED DRIVER BOARD L1 15µH THREE WIRES TG2 M2 350mA LED3 M3 TG3 350mA C5 C4 0.47µF 0.47µF D1 0.28Ω LED2 C1-C3 1µF ×3 350mA C6 0.47µF L2 D2 15µH L3 15µH D3 M2 M3 VIN 3.3V TO 24V C7 1µF b. LT3496-based boost LED driver Figure 2. An LT3496-based boost LED driver requires half as many wires as a traditional boost LED driver 10 M1 7 LEDs M1 LT3496 0.28Ω LED1 M3 CAP3 PWM1-3 SHDN SW1 CAP1-3 LED1-3 VIN PWM1-3 SHDN SW2 SW3 LT3496 GND TG1-3 VC1-3 VREF CTRL1-3 FADJ OVP1-3 C8-C10 100nF C1, C2, C3: MURATA GRM31MR71H105KA88 C4, C5, C6: MURATA GRM21BR71H474KA88 C7, GRM188R71C105KA12 L1, L2, L3: TAIYO YUDEN NP04SZB 150M M1, M2, M3: ZETEX ZXMP6A13F D1, D2, D3: DIODES DFLS160 Figure 3. The LT3496 RGB driver for large TFT LCD TVs Linear Technology Magazine • June 2007 DESIGN FEATURES L IL 0.5A/DIV ILED 0.5A/DIV 0.5µs/DIV Figure 4. 5000:1 dimming waveforms for the application circuit of Figure 3 dimming performance of LT3496, especially in buck-boost mode. Implementation of a high side disconnect switch with traditional LED drivers is possible, but uses many additional components, has slow response and burns extra power. Because the LED disconnect and current sensing are on the high side of each LED string, the low sides of the LED strings can be tied together in boost or buck-boost mode to reduce the number of wires returning to the LED driver. In a boost configuration, each of the low side connections can be returned to ground anywhere, allowing a simple 1-wire LED connection VIN 8V TO 30V 90 85 80 75 70 65 60 55 50 0 20 40 60 80 100 PWM DUTY CYCLE (%) Figure 5. Efficiency of the application circuit of Figure 3 here. If the PWM1 pin is pulled low, M1 is turned off, disconnecting the LED string of channel 1 and stopping the current draw from output capacitor C4. The VC1 pin is also disconnected from the compensation capacitor C8. C4 stores the state of the LED voltage and C8 stores the state of the LED current until PWM1 is pulled up again. This leads to a highly linear relationship between pulse width and output light, a large and accurate dimming range, and high efficiency. At 120Hz PWM frequency, the PWM control of the circuit allows 5000:1 dimming as shown in Figure 4. Figure 5 shows the Applications Buck Mode LED Driver The LT3496 can be configured as a buck mode LED driver for applications where the LED voltage is lower than the supply voltage. Figure 3 shows an LT3496 RGB driver for a large TFT LCD TV. The three LT3496 channels operate independently, but function in the same way. For simplicity, the PWM operation of channel 1 is described C1 3.3µF 150mA 150mA L1 22µH 150mA L2 22µH L3 22µH M1 TG1 TG2 LED1 TG3 0.68Ω CAP2 D1 C2 0.1µF C3 1µF R1 3.9M OVP1 R2 100k CAP3 D2 C4 0.1µF VIN C5 1µF R3 3.9M OVP2 R4 100k D3 C6 0.1µF VIN SW2 LT3496 GND M3 LED3 0.68Ω CAP1 SW1 CAP1-3 LED1-3 VIN PWM1-3 SHDN M2 LED2 0.68Ω PWM 1-3 SHDN 95 EFFICIENCY (%) PWM 5V/DIV 100 for each LED string. Traditional LED drivers employ a low side LED disconnect approach, in which both the high side and the low side of each LED string must connect to the LED driver. Figure 2a shows simplified traditional boost LED drivers, where M1–M3 are LED-disconnect NMOS switches. Figure 2b shows a simplified LT3496 triple boost LED driver, where M1–M3 are LED-disconnect PMOS switches. The LT3496 solution removes three wires, increasing system simplicity and reliability. These advantages will become increasingly important as the channels are multiplied in high performance displays. C7 1µF R5 3.9M OVP3 R6 100k VIN SW3 TG1-3 OVP1-3 VC1-3 VREF CTRL1-3 FADJ C1: MURATA GRM55DR71H335KA0193 C3, C5, C7: MURATA GRM31MR71H105KA88 C2, C4, C6: GRM21BR71H104KA01 M1, M2, M3: ZETEX ZXMP6A13F L1, L2, L3: TAIYO YUDEN NP04SZB 220M D1, D2, D3: DIODES DFLS160 0.1µF R7 75k R8 24k Figure 6. Buck-boost mode LED driver for automotive lighting Linear Technology Magazine • June 2007 11 L DESIGN FEATURES efficiency as a function of the PWM duty cycle. Buck-Boost Mode LED Driver In some LED applications, the desired supply voltage range and LED voltage range overlap, thus requiring buckboost mode configuration. Figure 6 shows a LT3496 buck-boost mode LED driver for automotive lighting. The LED voltage is 9V–12V and the automobile battery voltage is 8V–30V. R1–R6 set the overvoltage protection voltage at 40V to guarantee the voltages of SW1–SW3, CAP1–CAP3, LED1–LED3, and TG1–TG3 pins are below the maximum rating voltage. R7–R8 set the switching frequency at 1.3MHz to limit the LT3496 power dissipation and ensure that a junction temperature of 125°C is not exceeded. Figure 7 shows the 3000:1 PWM dimming waveforms at 120Hz PWM frequency. Conclusion IL 0.2A/DIV ILED 0.2A/DIV 0.5µs/DIV Figure 7. 3000:1 dimming waveforms for the application circuit of Figure 6 the supply voltage. Figure 8 shows a LT3496 boost LED driver for automotive lighting. D4, Q1–Q3, and R1–R4 create the battery surge voltage protection circuits to protect the LED string from being damaged by a battery surge voltage. The zener breakdown voltage of D4 is chosen to be lower than the LED voltage. When the VIN surge voltage increases to be close to the LED voltage, D4 breaks down and turns on Q1–Q3. Q1–Q3 pull PWM1–3 low and M1–M3 are turned off immediately to disconnect the LED strings from the LED driver. Boost LED driver The LT3496 can be configured as a boost LED driver for the applications where the LED voltage is higher than VIN 8V TO 16 V Figure 9 shows the 3000:1 PWM dimming waveforms at 120Hz PWM frequency. PWM 5V/DIV C1 3.3µF L1 15µH D4 R1 1k R2 1k R3 1k R4 1k 1Ω TG2 M1 PWM2 0.1A 6 LEDs 20k SW2 PWM1 SHDN PWM3 1k LT3496 PWM2 1k PWM1 Q1 Q2 Figure 9. 3000:1 dimming waveforms for the application circuit of Figure 8 D3 C4 1µF CAP2 1Ω CAP3 1Ω LED3 TG3 M2 M3 825k OVP1 SW1 SHDN 1k 0.5µs/DIV LED2 VIN PWM3 ILED 0.1A/DIV D2 C3 1µF CAP1 825k 6 LEDs IL 0.5A/DIV L3 15µH LED1 TG1 PWM 5V/DIV L2 15µH D1 C2 1µF The LT3496 provides a compact, low cost, high reliability, and high efficiency solution to multichannel LED lighting. With the capability of operating in buck, boost and buck-boost mode, the LT3496 LED driver delivers 3000:1 True Color PWMTM dimming ratio over a wide range of supply and output voltages. L GND 0.1A 825k OVP2 6 LEDs 20k SW3 CAP1-3 LED1-3 TG1-3 OVP1-3 VC1-3 VREF FADJ CTRL1-3 0.1A OVP3 20k 100nF Q3 C1: MURATA GRM55DR71H335KA0193 C2, C3, C4: MURATA GRM31MR71H105KA88 M1, M2, M3: ZETEX ZXMP6A13F L1, L2, L3: TAIYO YUDEN NP04SZB 150M D1, D2, D3: DIODES DFLS160 Figure 8. Boost mode LED driver with battery surge voltage protection for automotive lighting 12 Linear Technology Magazine • June 2007