High-output LEDs are being used in an ever-widening range of applications due to their advantages over conventional lighting sources. Among those advantages are higher efficiency, longer lifetime, and improved reliability. One manufacturer of high-output LED modules, Lumileds Lighting, (www.lumileds.com) produces several modules under the product name Luxeon.
The lifetime of any semiconductor device is reduced when it operates above the maximum operating temperature range, and irreparable damage occurs when the junction temperature exceeds a certain point. Lumileds devices are fabricated on a thermally-enhanced substrate that improves thermal performance and allows continuous operation at the high current levels necessary for high-brightness lighting. In applications such as camera flashes, an additional thermal protection mechanism is required to prevent damage due to power dissipation during continuous operation.
The circuit of Figure 1 details a charge-pump regulator (IC1) designed to supply regulated current for up to eight white LEDs in a camera-flash application.
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Figure 1. This circuit drives a high-output white LED, and shuts off the LED current in response to excessive temperature and to open- or short-circuit thermistor faults.
By paralleling all eight drivers, you can supply up to 480mA for a single 1W Luxeon Star high-output white LED module. IC1 enters shutdown mode when the open-drain input, EN, is pulled to ground.
A compact and cost-effective thermal shutdown circuit can be implemented with a thermistor (Murata NCPZ series) 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. That occurs when the thermistor (R2) is hot. Comparator B is configured as an open-wire failsafe that drives EN low when the thermistor connection is broken. In that 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.
About the Author
Randall White works at Maxim Integrated Products, Sunnyvale, CA, www.maxim-ic.com.