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What could cause the output of an LD1117 Voltage Regulator to increase above its specification?

SCamp.1041
Associate III

‎2022-05-30 02:06 PM

My design is using an LD1117S33CTR voltage regulator. I am experience some rather odd behaviour where the regulators output is actually increasing from the expected 3.3V all the way up to 4V.

What could cause such behaviour? I am stumped.

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5 REPLIES 5
MasterT
Lead

‎2022-05-30 04:39 PM

Oscillation comes to mind. Have you properly bypassed IC with caps at the input / output? Check out data sheet of the product for manufacturers recommendation regarding capacitor sizing, ESR, etc.

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SCamp.1041
Associate III
In response to MasterT

‎2022-05-30 06:27 PM

So if you take a look at the schematic below, I think this is a pretty standard design no? The LD117 gets supplied by a 5v switching reg. Could a capacitor cause oscillation?

0693W00000NqtIQQAZ.png 

Regarding the oscillation, is there a way to use a scope probe to detect this?

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MasterT
Lead

‎2022-05-30 07:05 PM

It's a good idea to use a scope, especially if it has SA mode. I read

https://www.st.com/resource/en/datasheet/ld1117.pdf

unfortunately ST doesn't specify type of the capacitor at the output just says 10uF . But based on my experience with LDO, most of them work better with aluminium caps due to intrinsic 0.1-0.5 Ohm ESR. You may try different caps or if ceramic caps in use, than set a small 0.1 resistor in series with cap.

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SCamp.1041
Associate III
In response to MasterT

‎2022-05-30 07:15 PM

Ok, I will have to give this a read and test this out tomorrow. So oscillation can increase the voltage?

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