White LED Driver with High Temperature Protection

The circuit drives a high-power white LED (WLED) and turns off the LED when the temperature is too high and the thermistor fails in an open or short circuit. The life of any IC will be shortened if it operates outside the extreme operating temperature range. When the junction temperature of the chip exceeds a certain value, it will be completely damaged. 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 avoid damage to the device due to power consumption during continuous operation.

The circuit in Figure 1 includes a charge pump regulator (IC1) suitable for camera flash applications that can provide regulated current for up to eight white LEDs (WLEDs). Connecting all eight drivers in parallel can provide up to 480mA of current 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 open or short.

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 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 junction is disconnected. When the thermistor fails open-circuit, VTHERM is pulled high by R1, causing comparator B to pull EN low. Resistor divider R3-R4 sets the threshold voltage for an open-circuit fault, and resistor R1 and thermistor R2 set the thermal shutdown threshold.