SA555: Pulse-Width Modulation

Xiaoming Kong

Part Number: SA555
Other Parts Discussed in Thread: LMC555, TLC555

Hello Sir

I am trying to use the pulse-width modulation function of SA555 as an A/D converter.

In the application, I hope SA555 outputs a PWM signal with its duty cycle varying linearly with the voltage amplitude of the external analog signal applied to pin 5 (CONT).

However I got a PWM waveform with variable duty cycle together with variable frequency, and the duty can only vary from 50% to 100% (VCC= 5V or 12V).

I did not apply a continuous pulse train as clock to pin 2. Is it possible to accomplish the function without using a continuous pulse train ?

Best regards

Xiaoming

over 6 years ago

0 Ron Michallick over 6 years ago

TI__Guru**** 167576 points

Xiaoming,

Getting PWM directly for 555 timer results in poor linearity. Here is a circuit that has much better linearity and only requires the addition of a comparator.

This gives 1kHz PWM output varying from near 0% to 100% as the input varies from 0V to supply voltage. I chose TLC555 or LMC555 timer because the output voltage varies all the way from 0V to VDD to get 50% duty cycle pulse train for best accuracy. It still works fairly well even with duty cycles other than 50% timer clock. In case you want to stay with SA555. For 5V operation chose a different 5V comparator as input can be greater than VDD-2V.