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Implement single-device PWM waveform generator

Posted: 25 Oct 2013     Print Version  Bookmark and Share

Keywords:Pulse-width modulation  PWM  waveform generator  ICM7555 timer  MAX998 

Pulse-width modulation (PWM) generators are integrated in almost 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.

 

 

 

 

Figure 1: A PWM generator and timer for a single device.

Method 1: Use a low-power timer as a PWM generator
In this method an ICM7555 timer is configured as in figure 1.

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.

In figure 1 the pulse width of the output at Pin 3 is modulated by the control voltage (VCONTROL) 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 VCONTROL by the supply voltage (VSUPPLY) and discharged from VCONTROL/2 to ground. When no external control voltage is applied, VCONTROL is at 2/3 of VSUPPLY.

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 VCONTROL and the discharging time for C1 to discharge to VCONTROL/2. This process alters the pulse width of the output waveform.

The charging time is expressed as:

-t/RC = ln [1 – (VCONTROL/(2VSUPPLY—VCONTROL))]

The discharging time is expressed as:

-t/RC = ln 0.5

Where R = R1 and C =C1.

Method 2: a PWM generator with comparator
In this method a MAX998 comparator is configured as in figure 6.

Figure 6: A PWM generator and comparator.


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