555 timer as a switch-mode power supply

Most switch-mode power supplies rely on a PWM (pulse-width-modulated) output that is controlled via voltage feedback. A 555-timer IC can inexpensively perform PWM. The circuit in Figure 1 shows how to turn a 555 PWM circuit into an switch-mode power supply with only one simple equation. The design uses two 555s. IC1, in astable mode, triggers IC2 in PWM mode. IC1 is set to oscillate at approximately 60 kHz at a high duty cycle.

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