White LED driver with high temperature protection

Abstract: The circuit drives a high-power white LED (WLED) and turns off the LED when the temperature is too high and the thermistor fails open or short.

White LED Driver with High-Temperature Protection Abstract: The circuit drives a high-power white LED (WLED) and turns off the LED when the temperature is too high and the thermistor fails open or short. High-power LEDs are increasingly used because of their many advantages over traditional light sources. The advantages of LEDs include higher efficiency, longer life, and greater reliability. However, high-power LEDs also generate more heat than traditional LEDs.

The life of any IC will be shortened if it is operated outside the operating temperature range. When the junction temperature of the chip exceeds a certain value, it will be completely destroyed. Philips Lumileds LUXEON® high-power LED modules generate less heat because they are manufactured on a thermally enhanced substrate. This substrate material improves thermal performance and allows continuous operation at high currents to meet the requirements of high-brightness lighting. However, for applications such as camera flash, additional thermal protection is required to prevent the power consumption during continuous operation from damaging the device.

The circuit in Figure 1 includes a charge-pump regulator (IC1) for camera flash applications that provides regulated current for up to eight white LEDs (WLEDs). Connecting all eight drivers in parallel can deliver up to 480mA for a single 1W, LUXEON Star high-power WLED module. IC1 enters shutdown mode when the open-drain input, EN, is pulled to ground.


Figure 1. The circuit drives a high-power WLED and turns off the LED when the temperature is too high and the thermistor fails in an open or short circuit.

A compact and cost-effective thermal shutdown circuit can be constructed using a thermistor and a dual open-drain comparator (IC2) with an internal reference. Comparator A pulls EN low to ground when VTHERM drops below the internal 1.2V reference voltage. This occurs when the thermistor (R2) is hot. Comparator B provides open-circuit fail-safe protection, where EN is pulled low when the thermistor contacts are open. When the thermistor fails open, VTHERM is pulled high by R1, causing comparator B to pull EN low. Resistor divider R3-R4 sets the threshold voltage for open-circuit faults, and resistor R1 and thermistor R2 set the thermal shutdown threshold.