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In my current project I implemented two MC34063 switching converters to provide a 3.3V and a 4V rail. Testing the converter circuitry I found some odd behaviour.

Unloaded I get a solid 4.0V, but when applying a 40Ω load the output voltage rises to 4.4V, when applying a 30Ω load it goes up to 4.75V. The needed current will be in the 100 to 300mA range with short bursts up to 2A.

After receiving the boards I realised that I put a completely wrong value and type for the timing cap C25. I've built the 4V regulator with the correct 330pF capacitor, so this mistake is already taken care of.

Any ideas or suggestions what the issue here could be?

schematic PCB layout

Julian F. Weinert
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    Try 1 nF in parallel with R47, this helped for me once. R48 and the upper track may pick up some noise from switching around R49. 330 pF as CT is a very low value for MC34063. Can you provide a scope picture of pin 2? – Jens Mar 03 '23 at 17:01
  • 330 was calculated for a 50kHz target frequency. Unfortunately I don't have a scope here, so can't provide that. Thanks for the idea with the cap, I'll try to bodge one in and will report back – Julian F. Weinert Mar 03 '23 at 17:05
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    Hah! Amazing, that's it! Found an 820pF I strapped across and voila, it regulates well now. Depending on load it still goes up to 4.15V, but with more load it goes down again, so I'll call that a win :) Can you make it an answer, so I can accept it? – Julian F. Weinert Mar 03 '23 at 17:14

1 Answers1

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The voltage feedback path via R48 and its tracks may pick up some noise from switching around the area of R49. Things like that may double trip the hysteretic comparator of this chip.
A small filter capacitor of around 1 nF parallel to R47 may help here to stabilize the regulator loop.
The load current step response of the circuit is affected by this patch. Check if this is still within the requirements of the load, it worked for me.
An alternative approach is to reduce the impedance of the voltage divider R48/R47 by 50 % or even more, but I didn't test that.

Jens
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