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Backfeeding an LM7805

Mike Baron1
Prodigy 50 points

I'm using an LM7805 for a very low power circuit application (consuming uA's). I found that if I have a voltage on the output of the 7805 it is backfeeding to the input of the 7805 and to my low power circuit. So with 5V on the output I see 4.3V on the input. On the 7805 datasheet there appears to be diode path back to the input, so this makes sense.

My circuit on the input side is now working (without another source), and seems to be working fine!

If my circuit on the input side is consuming less than 1 mA, is this safe for the LM7805?

If so, what are the current limits to backfeed?

I know this is not the intended use for this 5V regulator, but I find it interesting.

Also, I found this related post but if there is already an internal diode on the 7805, I'm not sure why another was suggested;

http://e2e.ti.com/support/power_management/linear_regulators/f/321/p/300783/1048976.aspx#1048976

  • 0
    TI__Guru**** 164796 points

    Mike,

    The internal diode paths are not rated for high current flow. Therefore an external diode will pass the majority of the current. 1mA will not cause damage. If the output comes up quickly then any capacitance on the input could result in a high surge current which may cause damage.