Abstract: The circuit drives a high-power white LED (WLED). When the temperature is too high and the thermistor has an open or short circuit fault, the LED will be turned off.

Summary of white LED driver with high temperature protection function: The circuit drives a high-power white LED (WLED). When the temperature is too high and the thermistor has an open or short circuit fault, the LED will be turned off. Compared with traditional light sources, high-power LEDs have many advantages, so they are more and more widely used. The advantages of LEDs include higher efficiency, longer life, and more reliability. However, high-power LEDs also generate more heat than traditional LEDs.

If working outside the limit operating temperature range, the life of any IC will be shortened. When the junction temperature of the chip exceeds a certain value, it will be completely damaged. Philips Lumileds LUXEON® high-power LED modules are manufactured on a heat-enhanced substrate, so they generate less heat. This base material improves thermal performance and allows continuous operation at high currents, thus meeting the requirements of high-brightness lighting. However, for applications such as camera flashes, additional thermal protection is required to avoid damage to the device due to continued power consumption.

The circuit in Figure 1 includes a charge pump regulator (IC1) suitable for camera flash applications. The device can provide regulation current for up to eight white LEDs (WLEDs). Paralleling all 8-channel drivers can provide up to 480mA for a single 1W, LUXEON Star high-power WLED module. When the open-drain input EN is pulled to ground, IC1 enters shutdown mode.

Figure 1. The circuit drives a high-power WLED. When the temperature is too high and the thermistor has an open or short circuit fault, the LED will be turned off.
Figure 1. The circuit drives a high-power WLED. When the temperature is too high and the thermistor has an open or short circuit fault, the LED will be turned off.

A thermistor and a dual open-drain comparator (IC2) with an internal reference can be used to build a compact and cost-effective thermal shutdown circuit. When VTHERM drops below the 1.2V internal reference voltage, comparator A pulls EN low to ground. This operation is performed when the thermistor (R2) temperature is high. Comparator B is used to provide open circuit failure protection, that is, when the thermistor contact is opened, EN will be pulled low. When the thermistor fails open, VTHERM is pulled high by R1, which causes comparator B to pull EN low. The resistor divider R3-R4 sets the threshold voltage for the open circuit fault, and the resistor R1 and the thermistor R2 set the thermal shutdown threshold.

A similar article was published on the EE TImes website in February 2006.


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