I have several power sources that are able to power the 3.3v rail in my circuit. Only the U2 needs to ever be disabled. The problem however is when PWR1In is brought up slowly and the other two sources are off there is a point where U5 oscillates on and off. Looks like the input voltage is dropping which is tied to the enable pin. This turns the LDO off. Once the LDO goes off the input voltage comes back up and the LDO comes on again. etc. See the scope shot.

  • Green = Input to U5
  • Red = Output of U5
  • Blue = Input to U2

What is the standard way to setup a circuit like this so that it A. takes power from multiple sources B. does not cycle on and off for any input voltage range.

Note: If I change the micro startup voltage the oscillation point changes but it still has an oscillation range. VConn is a 5V source, PWR2In is a 5V source, PWR1In is a 5V to 20V source.

I've added a second scope shot. In this one blue is still PWR1In, Red is still 3.3v rail, but green is the EN of U2. (still getting used to formatting on stackexchange)

In this one blue is still PWR1In, Red is still 3.3v rail, but green is the EN of U2.

Scope of Circuit Performance

Power Circuit

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    \$\begingroup\$ What specific LDO are you using? \$\endgroup\$ Mar 26, 2020 at 15:34
  • 1
    \$\begingroup\$ What voltage is your 5V bus under this condition? If the 5V LDO isn't getting enough voltage to actually regulate, stuff like this is to be expected. \$\endgroup\$
    – Hearth
    Mar 26, 2020 at 15:35
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    \$\begingroup\$ Have you tried putting a cap on the enable line, like you did for U5? \$\endgroup\$
    – Aaron
    Mar 26, 2020 at 15:41
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    \$\begingroup\$ Have you measured the currents? The drop across the diode and the enable line of U2 would be interesting to see as well. \$\endgroup\$ Mar 26, 2020 at 16:09
  • \$\begingroup\$ 5V LDO = NCP718BSN500T1G, 3.3V LDO = NCP718BSN330T1G. The 5V Bus is the red line excepting the diode drop. No I did not cap the enable line of U2 since it has direct control. I didn't suspect U2 of contributing to the issue. But I can check its enable voltage and see what it is doing. \$\endgroup\$ Mar 26, 2020 at 16:16


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