Sync Buck SMPS Current Regulator for LED App for GEC Apr 04.doc

A Synchronous Buck Switch Mode Power Supply for Driving an LED Array
Jack Wagner
Sr. Field Applications Engineer
ON Semiconductor
Abstract – LED devices are ing more and more popular. As higher power, higher luminous output e to market, a simple linear approach to driving these devices is found to be wasteful and worse, thermally too challenging. This is especially true in the automotive environment where the typical temperature operating extremes force a more efficient approach to the driving electronics.
One interesting use for white LEDs is in an automotive headlamp. These LEDs would need to be high luminous flux output devices implying high power requirements. Since the desirable output parameter is luminous flux and this parameter is dependent on the forward current through the LED, a current regulator circuit is ultimately needed. Further, since the luminous flux output of an LED can vary from manufacturing lot to lot even for a fixed forward current, a scheme to provide programmable output current is desirable to match the circuit for a particular manufacturing lot of LED devices. Also, the programmability allows for such features as dimming. This paper will describe just such a circuit.
Background - Automotive lighting assembly suppliers are considering LED devices pete with High Intensity Discharge (HID) lighting. For one, LED devices require less sophisticated d