Included among the many applications for pulse-width modulation (PWM) are voltage regulation, power-level control, and fan-speed control. A PWM circuit for such systems can be implemented with three op amps on a single quad-op-amp chip (Fig. 1).
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| Figure 1. | This three-op-amp circuit produces both a triangular wave and a variable-pulse-width output. |
Because op amps are used for the modulator, it's suited for a wide variety of applications. For example, low-power op amps can be used in a low-power system, and high-frequency op amps can be used for a high-frequency PWM. The circuit in Figure 1 also generates a triangular wave.
The circuit consists of a triangular-wave generator (U1a and U1b) and a comparator (U1c). U1a is configured as an integrator (or de-integrator) and U1b as a comparator with hysteresis. At power-up, the comparator's output voltage is assumed to be zero.
U1a's noninverting input is biased at VCC/2. A virtual connection between the inverting and noninverting inputs allows a constant current through R equal to
which charges capacitor C. Thus, the U1A integrator output increases linearly with time. When it reaches 0.75 VCC, the comparator output (U1b) changes to its maximum output voltage (VCC). At that point, the integrator begins to de-integrate, causing the output voltage to decrease linearly. When it reaches 0.25 VCC, the comparator output voltage changes to zero, and the cycle repeats. Thus, the integrator output is a triangular wave that swings between the levels of 0.25 VCC and 0.75 VCC.
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| Figure 2. | These waveforms show the circuit's PWM and triangular-wave outputs. |
U1c compares the triangular wave against the dc level VIN. Its output is a square wave, with a duty cycle that varies from 0 to 100% as VIN varies from 0.25 VCC to 0.75 VCC (Fig. 2). Frequency is determined by R, C, R1, and R2:
where R2 > R1.
he ratio of R2 and R1 affects the operating frequency and the amplitude of the triangular wave. Given that VTH is the triangular wave's maximum voltage and VTL is its minimum voltage, the amplitude swing is:
and
where R2 > R1. Therefore:
The triangular wave's peak-to-peak voltage is centered at the VCC/2 bias voltage generated by R3 and R4. With the circuit configuration shown, the PWM operates on a single supply. Use micropower op amps and larger resistors (R and R1 to R4) for low-power applications and high-frequency op amps for higher-frequency applications.
