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I'm using an LP38691 3.3V fixed regulator which is overshooting. This wasn't an issue for years, but now a component is being replaced and the current on the 3.3V rail has gone up from 300mA to 500mA. The datasheet claims an output voltage is within 2% of the nameplate, and 4% at worst. It jumps to 3.8V for 40ms, and then steps down to 3.3V.

Is this normal behaviour for LDO regulators? The input voltage is 5V and the thing is surrounded by bypass capacitors and a low ESR 100u capacitor.

Edit: Here's the schematic. I can't post the entire schematic because there are a few dozen pages. The component that changed was a microprocessor. I also scoped the 5V rail, and it it ramps up and stabilizes 20ms before the regulator starts to ramp up. The odd thing is that we use the same exact circuit on other boards, and those boards don't have that issue with the new processor. enter image description here

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    \$\begingroup\$ Post the schematic. What was the component that was changed? \$\endgroup\$
    – Leon Heller
    Oct 31, 2014 at 13:06
  • \$\begingroup\$ From the datasheet pages 11 & 12, it seems like its pretty tetchy about layout as well as input/output capacitors. As Leon says, schematic and tell us what exactly changed. \$\endgroup\$
    – Spehro Pefhany
    Oct 31, 2014 at 13:33
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    \$\begingroup\$ it if sits at a higher voltage for 40ms, which is a LONG time in electrical land, there must be something terribly wrong somewhere.. \$\endgroup\$
    – KyranF
    Oct 31, 2014 at 13:56
  • \$\begingroup\$ I would be suspicious of the 5V supply. \$\endgroup\$
    – Matt Young
    Oct 31, 2014 at 14:20
  • \$\begingroup\$ If something after the LDO is sourcing current onto the 3.3V rail, the LDO can't regulate it down. Are any of the components on the 3.3V rail connected to higher voltages that may be feeding through? \$\endgroup\$
    – Justin
    Oct 31, 2014 at 19:38

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I figured out the cause. The inrush current to the new processor board was much greater than the old processor board. It exceeded maximum current of the LDO regulator, causing the odd startup behavior.

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