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I'm currently working on a project where I need the MCU to be able to keep running for at least a few seconds after the power have been interrupted (in case of power outage for example).

So I choosed a 470uF cap for the output of the regulator (AP7333-33), but unfortunatly when the power is interrupted, the cap discharge quickly in the regulator (and through the regulator, in the wall adapter).

I'd like to prevent this phenomenon, but I can't find a satisfying solution...

I tried to put a schottky diode between the regulator and the cap, but this is far from an ideal solution, as voltage drop and reverse leakage current lower the time my MCU have before shuting down.

I also tried various mosfet and BJT configuration, but could not find anything good.

I feel a bit stupid, ok I'm a beginner, but I'm sure such a simple problem have a simple solution !

schematic

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  • \$\begingroup\$ Schematic? And why are you worried about the wall adapter receiving this charge? Is it poorly designed? Details about it? If you really want to discharge locally use a bleeder resistor, but it will draw power all the time. The typical regulator protection schemes do assume that you can discharge back into the upsream supply. \$\endgroup\$ Oct 19, 2015 at 19:02
  • \$\begingroup\$ By typical schemes I mean electronics.stackexchange.com/questions/148340/… \$\endgroup\$ Oct 19, 2015 at 19:09
  • \$\begingroup\$ You should perhaps firstly look into lowering the power consumption of your MCU by deactivating peripherals (c.f. datasheet), if it is possible. \$\endgroup\$ Oct 19, 2015 at 19:13
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    \$\begingroup\$ I'd move the diode and capacitor to the input side of the regulator. This will allow the regulator to drain down the capacitor until it drops out, allowing the circuit to power your MCU with a stable voltage for longer. \$\endgroup\$
    – Jon
    Oct 19, 2015 at 19:16
  • \$\begingroup\$ @Respawned I'm not worried about the wall adapter. I'm worried about any element eating up the charge in my output cap, should they be before the regulator, or the regulator itself. Actually if I unplug the wall adapter from the wall, the cap discharge in 500ms, but if I disconnect the wall adapter from the circuit, the cap discharge more slowly. This is why I use a Schottky just before the cap, so that it will not discharge trough or in the regulator. \$\endgroup\$
    – Chenard
    Oct 19, 2015 at 19:48

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Ok thanks for your comments. The tank cap before the LDO seems to be the right way. I'm gonna test this solution.

By the way I also found this question, and the answers interesting : Help with power-loss protection using capacitor

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  • \$\begingroup\$ What about the idle current of the reg .Ye olde 78 series are hungry.Deal to thev reg as well as what you are doing already and your cap value and hence size and cost should be OK.Remember 1 millifarad =1 milliampsecond per volt. \$\endgroup\$
    – Autistic
    Oct 20, 2015 at 8:54

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