555_RAMP_GENERATOR

Published:2009/7/14 23:17:00 Author:Jessie | From:SeekIC

When the timing capacitor of an ordinary 555 circuit charges, an exponential curve across the capacitor results. Obviously, a fast-rising square wave is seen at the output. An accurate ramp voltage can be generated by the 555, though, by ignoring the square-wave output and sneakily using a constant-current source instead of the resistor charging network. The figure shows how a simple J505 constant-current diode can be used to create a reasonable ramp, which will be observed across the capacitor C1. D1 is specified at 1 mA nominal. Assuming that C1 is discharged initially, after power-up the 555 triggers (Vtring, <1/3 Vcc), and C1 starts to charge at a fixed rate through the constant-current source D1 until the threshold voltage of 2/3 Vcc is reached. Then the 555 (IC1) will rapidly discharge the capacitor into pin 7 down to1/3Vcc, when the chip will trigger again. With a 12-V rail, the result is a sawtooth waveform of 4 V p-p. A fast series of spikes is seen at pin 3.



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