I'm implementing a simple manually controlled low frequency (120 Hz) PWM which drives MOSFET switching a rather heavy load (battery is 2S Li-poly 7.4V high drain, load current is up to 80A). Simplified schematic of that part of device flollows (I omitted a sawtooth generator and the other parts of device which are not involved here and are physically disconnected from this curcuit during testing anyway):
AMS1117 here is a 3.3V fixed LDO.
I found out that LM393 (actually LM2903) output is oscillating during MOSFET turn-off period:
(Yellow channel is MOSFET drain and cyan one is the LM393 output, note that channel scales are different).
After making research I found that LDO output, providing reference voltage to the comparator through R3-R5-R6 divider is guilty:
(Here yellow is LM393 output and cyan is LDO output)
Although input and output of LDO and reference voltage input of comparator are debounced with rather large capacitors, LDO output still bounces along with LDO input during transition of MOSFET which significantly affects comparator reference voltage:
I'm mostly newbie in questions of power, my primary skill is MCs and what I usually do to power rails is selecting debouncing capacitor according to IC datasheet. So I had to read some application note from Murata on debouncing and constructed a pi-section filter at LDO input as per their recommendations:
… which didn't change the situation. I mean there's no eye visible change of waveforms on the LDO input oscillorgam after adding a pi-section filter. At this point I realize that either physics works wrong or I don't understand something. I believe the latter is more probable.
Any suggestions are appreciated.
P.S. Adding a hysteresis to comparator didn't help either as the oscillation amplitude is over 2 V and hysteresis would be unacceptably huge.
P.P.S. All the measurements are taken regarding to the negative pole of battery.