Pulse-width modulation (PWM) generators are integrated in nearly every switching power device.
There are two ways to implement a single-device PWM waveform generator. One method uses an ICM7555 timer, while the other uses a MAX998 low-power comparator. We will look at each.
Method 1: Use a Low-Power Timer as a PWM Generator
In this method an ICM7555 timer is configured as in Figure 1.
Figure 1: A PWM generator and timer for a single device.
In Figure 1 the pulse width of the output at Pin 3 is modulated by the control voltage (V CONTROL) applied at Pin 5. Lab tests were done on the design with the power supply set at 5V. Figures 2 through 5 show the PWM output at three different control voltages, 1V, 2V, and 4V. C1 is charged to V CONTROL by the supply voltage (V SUPPLY) and discharged from V CONTROL/2 to ground. When no external control voltage is applied, V CONTROL is at 2/3 of V SUPPLY.
Figure 2: PWM output with control voltage = 1V
Figure 3: PWM output with control voltage = 2V
Figure 4: PWM output with no control voltage
Figure 5: PWM output with control voltage = 4V
The data illustrate how the control voltage applied at Pin 5 changes the threshold voltage of the two internal comparators. Without the applied control voltage (Figure 4), the device sets the charging and discharging of C1 at 1/3 and 2/3 of the supply voltage. This is equidistant from the supply voltage and ground, thus effecting a 50% duty cycle. The different control voltages change the charging time for C1 to reach V CONTROL and the discharging time for C1 to discharge to V CONTROL/2. This process alters the pulse width of the output waveform.
The charging time is expressed as:
-t/RC = ln [1 – (V CONTROL/(2VSUPPLY - V CONTROL))]
The discharging time