PWM circuit uses one op amp

Ferran Bayes, Electronica Digital, Barcelona, Spain -- EDN, 7/6/2000

A previous design idea is reminiscent of a similar but somewhat simpler circuit (see "Low-power PWM circuit is simple, inexpensive," EDN, Jan 6, 2000, pg 119). This circuit delivers a rectangular signal with duty cycle varying between 0 and 100% in response to an input signal varying from 0 to 5V dc (Figure 1). As with the above-mentioned circuit, the frequency is not constant (Figure 2), but the circuit is so simple that it can be useful in certain applications. In response to the hysteresis R₂ provides and the time constant R₃C₁, the comparator delivers the rectangular wave (Figure 3). The voltage V- at the inverting input swings between the two threshold levels, V_TH and V_TL. If you assume that R₂>>R₁, then V+ is always very close to V_IN. R₃C₁ averages the signal at V_OUT, and the dc voltage at V- is proportional to the duty cycle of V_OUT. The closed feedback loop tries to make V- equal to V+; therefore, the duty cycle at V_OUT is proportional to V_IN.

The voltage at V_OH determines both the output signal's high level and the full-scale range of V_IN. It can have any value, insofar as it does not surpass the common-mode input range of the comparator. The mathematical analysis of the circuit is easy if we assume that, because V_TH-V_TL is small, we can approximate the exponential charge and discharge of C₁ to assume the characteristic stemming from a constant-current source/sink. During the charging phase, the current is approximately (V_OH-V_IN)/R₃, so:

Similarly, during the discharge phase, we can assume the current is V_IN/3, and

Matching the two equations yields

and the duty cycle is

You can see that the duty cycle is directly proportional to V_IN: 0% for V_IN=0V and 100% for V_IN=V_OH. Moreover, the duty cycle is essentially independent of the component values, with the constraint that R₂>>R₁ to keep hysteresis small. An inverse relationship between duty cycle and V_OH can be useful in some applications, so consider V_OH as an additional input. The output frequency follows the relationship

reaching its maximum at V_IN=V_OH/2.

Tests with a TLC393 CMOS comparator and a bipolar LM393 reveal that the TLC393 performs better at low values of V_IN, because of its lower V_OL. Avoid loading the comparator's output; buffer it if necessary, because the loading can degrade the switching levels. (DI #2552)