I'm getting unexpected simulation behavior using FCM (forced continuous mode) without SSM (spread spectrum modulation) on the quad buck LT8692S. Attached is my simulation and a screenshot.
The HV converter is outputting 5V which powers channels 2, 3, and 4. Channel 2 is 3V3 at 30 mA, channel 3 is 1V at 300 mA, and channel 4 is 1V8 at 60 mA. Sequencing is configured like so: 1 -> 3 -> 4 -> 2.
The issue I'm seeing is voltage overshoot on 3V3 and 1V8 (see screenshot).
The following solves the issue:
- Use Burst mode by grounding the SYNC/MODE pin
- Increasing the load current on 3V3 to 300 mA instead of 30 mA
The following does not solve the issue:
- Changing to lower inductance values (1.5 uH)
- Burst mode with SSM (tying the pin to INTVCC)
- Changing output capacitance to a lower value
- Decreasing or increasing feedback resistor values by 10x
What am I missing here? This seems related to light load operation. Is the converter going unstable at light loads, and if so, how does BURST mode (without SSM) solve it?
Here is the simulation if I use BURST mode (grounding SYNC/MODE pin)
Edit Notes
Added screenshot for working sim[edited by: johnfu at 9:44 PM (GMT -5) on 21 Dec 2023]