LED driver provides oscillator for microcontroller

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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