Reduce the Cost and Complexity of Medium LCD LED Backlights with a Single Inductor LED Driver for 60 LEDs Design Note 462 Daniel Chen Introduction One inductor, one IC, one string of LEDs. This is the conventional way to build a boost LED driver for LCD display backlights. Although this is a perfectly acceptable solution for small LCD displays that only require a few strings, in larger displays the number of control ICs and inductors multiplies quickly, as do the expenses and PCB real estate requirements. This is a major hurdle in the race to replace CCFLs with robust, spectrally superior LEDs in medium sized, bright displays. A better driver is needed to bring the cost and complexity of LED backlights in line with CCFLs. The LT3598 answers the call by driving six strings of ten LEDs at up to 30mA per string. It also has a built-in power switch to save space and design time. Efficiency is optimized via an adaptive feedback loop that monitors all LED pin voltages to provide an output voltage just high enough to light all LED strings. The LED current is regulated even when VIN is greater than VOUT . The LED current can be derated based on programmed LED temperature through an NTC resistor divider or by programming die junction temperature. Typical Application Figure 1 shows the LT3598’s six channels driving 60 LEDs, with each string programmed at 20mA. The CTRL pin and PWM pin provide analog and digital dimming, respectively. True Color PWM™ dimming delivers constant LED color with a 3000:1 dimming ratio. Figure 2 shows the typical ±0.5% current matching between strings, which yields the uniform light distribution that is so important in large backlight applications. Need More Current? For applications that demand more than 30mA per string, multiple channels of the LT3598 can be easily combined for higher LED current. Figure 3 shows a configuration that drives two strings at up to 90mA per string. The 1000:1 PWM dimming waveform at 125°C junction temperature (worst case) is shown in Figure 4. TSET Pin for Thermal Protection The TSET pin voltage can be programmed to limit the internal junction temperature of the LT3598. Once this L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. True Color PWM is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. 10μH PVIN, 8V TO 40V 2.2μF SW 1.5 1.0 OPENLED SHDN SHDN PWM PWM SYNC FB 30.9k LT3598 RT 51.1k VREF 10k CTRL 100k 60.4k TSET ISET SS 100k 47pF 14.7k 10nF 0.5 0 –0.5 –1.0 LED1 LED2 LED3 LED4 LED5 LED6 GND VC 20mA dn4DC F01 –1.5 –50 –25 0 25 50 75 100 125 TEMPERATURE (°C) dn4DC F02 2.61k 15nF Figure 1. LED Driver for 60 × 20mA LEDs 04/09/462 ALL SIX LED STRINGS 1.00M 100k SYNC 4.7μF VOUT VO_SW VIN MATCHING (%) VIN 5V 2.2μF Figure 2. Current Matching for Figure 1 temperature is reached, the LED current will linerarly decrease if the junction temperature keeps increasing, as shown in Figure 5. This thermal regulation feature provides important protection at high ambient temperatures, and allows a given application to be optimized for typical, instead of worst-case, ambient temperatures. Conclusion LT3598 is a versatile LED driver with a built-in power switch for multiple LED strings. High PWM dimming is possible even with its robust fault detection. Furthermore, a voltage loop regulates the output voltage when all LED strings are open. Channel Disable Capability Unused LED pins can be tied to VOUT to disable them, so no current flows into the disabled channels. Fault detection ignores any channels tied to VOUT . Figure 6 shows an application with two disabled channels that yields efficiency as high as 90%. L1, 4.7μH PVIN, 6V TO 40V VIN, 5V VIN C3 2.2μF SW VO_SW SHDN CTRL PWM SYNC LT3598 RT VREF Figure 4. 1000:1 PWM Dimming for Figure 3 at 125°C R4 1.00M FB R5 30.9k 25 20 R10 20k R8 60.4k C5 56pF CTRL TSET ISET LED1 LED2 LED3 LED4 LED5 LED6 R3 9.76k SS C4 0.1μF dn4DC F04 5μs/DIV C2 4.7μF VOUT OPENLED R1 51.1k R7 100k D1 100k SHDN CTRL PWM ILED1 100mA/DIV GND VC LED CURRENT (mA) C1 2.2μF PWM 5V/DIV 90mA RC , 5.11k CC , 6.8nF 15 10 5 dn4DC F03 C1: NIPPON CHEMI-CON KTS500B225M32NOTOO C2: MURATA GRM32ER71H475KA88L C3: TAIYO YUDEN LMK212BJ225MG D1: VISHAY SS3H9 L1: WÜRTH ELEKTRONIK 7447785004 0 Figure 3. LED Driver for Two Strings of 90mA LEDs C1 2.2μF VIN 5V C3 2.2μF R6 100k SHDN PWM SYNC R1 51.1k R8 60.4k C1: TAIYO YUDEN GMK325BJ225ML C2: MURATA GRM32ER71H475K C3: TAIYO YUDEN LMK212BJ225MG D1: DIODES, INC. B240A L1: WÜRTH ELEKTRONIK 744777410 RNTC : MURATA NCP18WF104J03RB 125 75 100 50 JUNCTION TEMPERATURE (°C) dn4DC F05 Figure 5. TSET Function Reduces LED Current at High Temperatures D1 L1, 10μH PVIN 6V TO 40V 25 VIN SW C2 4.7μF VOUT VO_SW OPENLED SHDN PWM SYNC RT VREF RHOT 10k R4 1.00M FB R5 30.9k LT3598 CTRL RNTC 100k LED1 LED2 LED3 LED4 LED5 LED6 TSET ISET R7 100k C5 47pF R3 14.7k C4 0.1μF SS GND VC 20mA RC , 2.61k dn4DC F06 CC , 15nF Figure 6. Four LED Strings with Two Channels Disabled Data Sheet Download www.linear.com For applications help, call (408) 432-1900, Ext. 3513 Linear Technology Corporation dn462 LT/TP 0409 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 ●