Tool/software:
Hi,
I am designing a PWM generator that its frequency and duty cycle are adjustable. I am trying to produce low frequency PWM signals as low as 10Hz and the lowest duty cycle as possible. In my application I observe oscillations as can be seen in the picture. Is this an expected behavior in this frequency range or is my circuit wrong?
Thanks.
CT pin:
RT pin:
DTC pin:
1IN+, 2IN+ pins are 0V and 1IN-, 2IN- are 5V as expected. Feedback pin is 0V. Ref output is 5.1V. Collector pins get 12V properly.
CT vs DTC signals in the picture. The green one is DTC signal. It seems they affect each other somehow. I tested the IC at 160kHz frequency, there is no oscillations at PWM output.
Hi,
I wanted to see this below with CT pin and with DTC pin waveforms to find if the noise caused PWM oscillations.
CT pin and DTC pin at low frequency look quite noisy.
Hi,
It looks CT and DTC both have noise when PWM glitches. What about FEEDBACK waveform?
Can you add a high frequency capacitor (ceramic) with 0.1uF in parallel with capacitor you used on CT pin to filter out the noise? Also add a small capacitor on DTC, such as 1nF to filter out the noise. Also, can you vary DTC to see if at any other voltages, the glitches get different to help find other possible reasons.
Please provide FEEDBACK waveform along with the PWM.
After adding 0.1uF parallel to CT pin capacitor and 10nF to DTC pin, oscillation problem is solved at 1kHz.
1kHz PWM (lowest duty cycle) with feedback pin waveform:
10Hz PWM (lowest duty cycle) with feedback pin waveform:
10Hz PWM (highest duty cycle) with feedback pin waveform:
DTC pin highest duty cycle (10Hz):
DTC pin lowest duty cycle (10Hz):
I did not spot glitches on other pins with varying DTC pin voltage. I can see that at higher frequency or duty cycle, DTC pin ripple decreases.
Adding a potentiometer to feedback pin to adjust voltage between 0V-5V mostly solved the problem. I can now get lowest duty cycle stable at 20Hz. A better layout probably improve the result. Thanks for the help.
20Hz PWM signal:
