Multi-string LED backlight application based on LT3598 driver

An inductor, an IC, and a string of LEDs are the traditional ways to build a step-up LED driver for LCD display backlights. Although this is a very desirable solution for small LCD displays that require only a few strings of LEDs, the number of controller ICs and inductors will increase in multiples over larger displays, resulting in cost Expenditure and PCB area requirements are also climbing. This is a major obstacle in the race to replace the CCFL in medium-sized bright displays with LEDs that are robust and have superior spectral characteristics.

A better driver is needed to make the cost and complexity of LED backlights roughly comparable to CCFLs. The LT3598 meets these requirements by driving six strings of 10 LEDs each with up to 30mA per string. The device also features a built-in power supply > switch to save space and design time. Efficiency is optimized by an adaptive feedback loop that monitors all LED pin voltages to provide an output voltage that is high enough to illuminate all of the LED strings. Even when VIN is above VOUT, the LT3598 will continue to regulate the LED current. The LED current derating can be based on programming the LED temperature (either through an NTC resistor divider or by setting the die junction temperature).

typical application

Figure 1 shows the LT3598's six channels for driving 60 LEDs, each with a current set to 20mA. The CTRL pin and the PWM pin are responsible for providing simulated dimming and digital dimming, respectively. True Color PWMTM dimming provides constant LED color and a 3000:1 dimming ratio. Figure 2 shows ±0.5% typical current matching accuracy between LED strings, which produces a consistent light distribution that is important in large backlight applications.

Figure 1: Drive circuit diagram for driving 60 20mA LEDs

Figure 2: LED current matching accuracy shown in Figure 1.

Need more current?

For applications that require more than 30mA per string of LEDs, it is easy to provide higher LED current through a combination of multiple channels of the LT3598. Figure 3 shows a configuration that can drive two strings of LEDs with up to 90mA per string. The 1000:1 PWM dimming waveform at 125 °C junction temperature (worst case) is shown in Figure 4.

Figure 3: Drive circuit diagram for driving two strings of 90mA LEDs

Figure 4: 1000:1 PWM dimming waveform provided by the circuit shown in Figure 3.

TSET pin for thermal protection

The internal junction temperature of the LT3598 can be limited by setting the TSET pin voltage. When this temperature is reached, if the junction temperature continues to increase, the LED current will decrease linearly (as shown in Figure 5). This thermal adjustment provides significant protection at high ambient temperatures and allows for a given application to be optimized for typical (but not worst case) ambient temperatures.

Figure 5: Current change diagram of the Tset function under high temperature conditions

Disable channel capability

Unused LED pins can be connected to VOUT to disable them so that no current flows into the deactivated channel. Fault detection will ignore any channel connected to VOUT. Figure 6 shows an application circuit with two deactivated channels that produces up to 90% efficiency.

Figure 6: Two disabled channels and four strings of LED circuit diagrams

in conclusion

The LT3598 is a versatile LED driver with a built-in power switch that is suitable for driving multiple strings of LEDs. With its rugged fault detection, it even provides a high PWM dimming ratio. In addition, a voltage loop regulates the output voltage when all LED strings are open. (Edit: Technology)