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edited Jan 30, 2023 at 16:18

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Switching regulator becoming unstable as voltage increases

I have a circuit which includes two back to back switching regulators. The first is used to take the input of 14V to 5V and the second charges a 2 cell lithium battery. I have verified the problem to be with the first regulator. I cut the connection between the Vout of the 14V to 5V regulator and supplied the second regulator with a constant 5V and it worked as intended.

The problem with the 14V to 5V regulator seems to be instability and a changing nominal output voltage. At 6V Vin the regulator is outputting something close to 5V but as the input voltage increases (even to just 14V), so does instability and the nominal output voltage increases above 6V which is too high for my application. The ICs on the output side are outputting over voltage protection at about 6V. On the instability, I see the output voltage on the oscope having +/- 0.7V on the nominal output voltage. I have fried my IC during testing, but can solder a new one on here shortly.

Why would the nominal output voltage of this regulator potentially rise with input voltage? If I add more capacitance on the output voltage will it smooth the output signal and prevent these voltage fluctuations?