White-LED Driver Includes Overtemperature Protection
The lifetime of any IC is reduced when it operates above its maximum operating temperature range. Irreparable damage occurs when the junction temperature exceeds a certain point. Philips Lumileds LUXEON® high-output LED modules reduce some of this heat because they are fabricated on a thermally-enhanced substrate. This material improves thermal performance and allows continuous operation at the high-current levels necessary for high-brightness lighting. However, in applications such as camera flashes, additional thermal protection is required to prevent damage from power dissipation during continuous operation.
The Figure 1 circuit details a charge-pump regulator (IC1) designed to supply regulated current for up to eight white LEDs (WLEDs) in a camera-flash application. By paralleling all eight drivers, you can supply up to 480mA for a single 1W, LUXEON Star high-output WLED module. IC1 enters shutdown mode when the open-drain input, EN, is pulled to ground.
Figure 1. This circuit drives a high-output WLED. It shuts off the LED current in response to excessive temperature and open- or short-circuit thermistor faults.
A compact, cost-effective thermal shutdown circuit can be implemented with a thermistor and a dual open-drain comparator with internal reference (IC2). Comparator A is configured to pull EN to ground when VTHERM falls below the internal 1.2V reference voltage. This occurs when the thermistor (R2) is hot. Comparator B is configured as an open-wire fail-safe that drives EN low when the thermistor connection is broken. In that latter case, VTHERM is pulled high by R1, causing comparator B to pull EN to ground. Resistive divider R3-R4 sets the open-wire threshold voltage, and resistor R1 with thermistor R2 sets the thermal-shutdown threshold.
A similar design idea appeared in February 2006 on the EE Times website.