Pulse-Width Modulator Features Versatile Operating Parameters

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).

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 V_CC/2. A virtual connection between the inverting and noninverting inputs allows a constant current through R equal to I = V_CC/2R, which charges capacitor C. Thus, the U1A integrator output increases linearly with time. When it reaches 0.75 V_CC, the comparator output (U1b) changes to its maximum output voltage (V_CC). At that point, the integrator begins to de-integrate, causing the output voltage to decrease linearly. When it reaches 0.25 V_CC, 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 V_CC and 0.75 V_CC.

U1c compares the triangular wave against the dc level V_IN. Its output is a square wave, with a duty cycle that varies from 0 to 100% as V_IN varies from 0.25 V_CC to 0.75 V_CC (Fig. 2). Frequency is determined by R, C, R1, and R2:

f = R2/(4RCR1)

where R2 > R1.

The ratio of R2 and R1 affects the operating frequency and the amplitude of the triangular wave. Given that V_TH is the triangular wave's maximum voltage and V_TL is its minimum voltage, the amplitude swing is:

V_TH = V_CC(R1 + R2)/2R2, and

V_TL = V_CC(R2 − R1)/2R2,

where R2 > R1. Therefore:

V_TH − V_TL = (R1/R2)V_CC (R2 > R1).

The triangular wave's peak-to-peak voltage is centered at the V_CC/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. (The quad op amp shown comes in a single package.)