Pulse Width Modulation with 555 Timer

[kishore3677]

Hi

I was just looking at the data sheet of LM555 (see the attachment) describing PWM circuit in page number 8(fig. 8 and 9). It is very clear to me that when I change control voltage, I get a PWM. It is unclear how 555 would respond to an ac signal as shown in Fig.9 since control voltage will always be changing. It is not steady as in DC.

Datasheet link: **broken link removed**

The datasheet is widely available, there is no reason to attach it in your message and waste bandwidth and forum space, replaced with link

 

[sijukrishnan]

Are you referring to the frequency spec of the AC signal at Pin 5 of LM555?

 

[kishore3677]

Yes, I was just looking for Frequency spec for pin no.5. Also I would like to know if there is any correlation between width of the trigger signal to the frequency of Ac signal at pin no.5 and resolution of output PWM

 

[FvM]

You're mixing three losely related points in your last post. Consequently there's no reasonable answer to it.

Referring to your original question, the modulation input is varying the threshold of the comparator ending the output pulse. Insofern, the instantaneous value of AC voltage matters, it's like being sampled. Unfortunately, the modulaion input is also affecting the trigger input operation, thus the applicable voltage range is restricted.

In my opinion, a 555 timer is something like a "poor man's" PWM generator, you would want to a more suitable solution in most cases.

Finally referring to the unrelated points: PWM resolution is basically infinite for an analog modulator. The trigger pulse has to be shorter than the minimum output pulse for correct operation.

 

[kishore3677]

I understand. Can you kindly explain the freq requirement for pin no.5. Also I do not understand when you say that " The trigger pulse has to be shorter than the minimum output pulse for correct operation". Can you give practical values. And why do you say that PWM with 555 is poor because you have mentioned that PWM output is infinite for analog modulator. Do you have any alternative PWM modulator which takes sine wave as input? Kindly suggest.

 

[FvM]

Can you kindly explain the freq requirement for pin no.5

There's no particularly requirement for the circuit. It's a matter of your PWM application, which I don't know.

why do you say that PWM with 555 is poor

Because of the pulse width restrictions. But it may be sufficient for some applications.

Do you have any alternative PWM modulator

A classical circuit uses a triangular or sawtooth waveform generator and a comparator. It has been often discussed at edaboard. SMPS controllers like SG3524 have a PWM generator built in.