LED driver provides oscillator for microcontroller
Edited by Bill Travis
Wallace Ly, National Semiconductor, Santa Clara, CA -- EDN, September 16, 2004
The major building blocks for a white-LED driver are an oscillator, a charge pump, and a regulated current source. National Semiconductor (www.national.com) produces a device that contains all these building blocks in the highly integrated LM2791/2 IC. You usually use white-LED drivers in tandem with cellular baseband controllers or microcontrollers. You can easily adapt the LM2791/2 to provide a clock source. You can realize a simple yet useful circuit by accounting for the fact that a pseudo square wave is present across the flying capacitor's (C1) pins. You can take this pseudo square wave from these pins and clean it up.
To accomplish this task, you inject the signal, via a 330Ω resistor, R1, into a simple inverter gate, such as a DM7404 hex inverter (Figure 1). The net signal is a clean, 2-MHz clock source. The oscilloscope graph depicts the pseudo square wave and the resultant square wave at the output of the inverters (Figure 2). You can use this signal as a simple clock source for a baseband controller or microcontroller to perform simple tasks such as keypad decoding or battery-identification detection.
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