Design Note 462 - LINEAR: Simplify & Save on Medium LCD LED Backlights with a Single Inductor LED Driver for 60 LEDs

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 Typical Application

One inductor, one IC, one string of LEDs. This is the Figure 1 shows the LT3598’s six channels driving 60 LEDs, conventional way to build a boost LED driver for LCD with each string programmed at 20m A. The CTRL pin and display backlights. Although this is a perfectly accept- PWM pin provide analog and digital dimming, respectively. able solution for small LCD displays that only require a True Color PWM™ dimming delivers constant LED color few strings, in larger displays the number of control ICs with a 3000:1 dimming ratio. Figure 2 shows the typical and inductors multiplies quickly, as do the expenses and ±0.5% current matching between strings, which yields PCB real estate requirements. This is a major hurdle in the uniform light distribution that is so important in large the race to replace CCFLs with robust, spectrally superior backlight applications.

LEDs in medium sized, bright displays.

Need More Current?

A better driver is needed to bring the cost and complex- For applications that demand more than 30m A per string, ity of LED backlights in line with CCFLs. The LT3598 multiple channels of the LT3598 can be easily combined

answers the call by driving six strings of ten LEDs at up for higher LED current. Figure 3 shows a confi guration that to 30m A per string. It also has a built-in power switch drives two strings at up to 90m A per string. The 1000:1 to save space and design time. Effi ciency is optimized PWM dimming waveform at 125°C junction temperature via an adaptive feedback loop that monitors all LED pin (worst case) is shown in Figure 4. voltages to provide an output voltage just high enough

TSET Pin for Thermal Protection

to light all LED strings. The LED current is regulated even

when VIN is greater than VOUT. The LED current can be The TSET pin voltage can be programmed to limit the derated based on programmed LED temperature through internal junction temperature of the LT3598. Once this

an NTC resistor divider or by programming die junction 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

temperature.

are the property of their respective owners.

PVIN, 8V TO 40V 10μH

VOUTSW

VIN 1.5

VIN VO_SW

5V ALL SIX LED STRINGS

FBOPENLED

SHDN SHDN

PWM PWM

LT3598

SYNC SYNC

LED1 –1.0

CTRL LED2 LED3 20m A

LED4 –1.5

100k LED5 –50 50–25

60.4k dn4DC F01 TEMPERATURE (°C)

ISET GNDTSET VCSS

dn4DC F02

15n F Figure 2. Current Matching for Figure 1

Figure 1. LED Driver for 60 × 20m A LEDs