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Low Power CMOS PWM Circuit With Buffer Output Requires Only 5 Components 2017-02-09

PWM (Pulse Width Modulation) is frequently used for efficient control of DC voltage or current.
Here's a simple, low-power circuit to generate a PWM signal that requires only 1 CD40106 buffer chip, 1 potentiometer, 2 diodes, and 1 capacitor.
The pot provides for adjustment of the duty-cycle.

The microwatt power consumption of this circuit is desirable for any small battery powered circuits that use PWM.
It's similar in operation to some designs using a 555 timer but consumes much lower power than a standard 555, although probably comparable to the circuit with a CMOS 555 chip.
Which circuit is preferable is likely mostly determined by the parts' availability and cost.

The LTspice simulation of the circuit is below.

upload_2018-6-11_21-4-3.png


U1 is a CD40106 CMOS Schmitt buffer gate IC configured as an astable multivibrator.
The hysteresis on the Schmitt input establishes the high and low switch level for the oscillation trigger points.
The feedback for the oscillation is fed back through two diodes, D1 and D2, and pot, U7.

The setting on the pot affects the charge time versus the discharge time for C1 which determines the PWM duty-cycle.
The pot can vary the duty-cycle to within a percent or so of both 0 and 100 percent, depending upon the operating frequency
The simulated PWM duty-cycle for pot settings of 0% and 100% was about 0.8% and 99.4% respectively for a PWM frequency of ≈930Hz.
The plot shows the duty-cycle for pot settings of 1% (Red), 50% (Yellow), and 99%(Green).

The spare gates in the package, U2, U3, U4, U5, and U6 are connected in parallel to provide a poor-mans buffer output for the PWM signal.
This isolates the output load from the multivibrator circuit, and provides a higher current drive for faster switching of high capacitance loads such a MOSFET gate.

Required power and ground connections for the CD40106, as well as decoupling caps, are not shown.
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