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By pieterc
#133258
Hi Sparkfunions!

Does anyone have a nifty circuit suggestion for the following problem? You MUST have had this requirement this before :)

I have a 2 x AA cell powered circuit that I want to stay off (draw < 10 uA) until the user keeps a button pressed for, say 1s or more. The button must then enable the BOOST power supply that outputs +3V3 to the micro controller. The micro controller must then keep itself powered. Thereafter the button becomes a user interface, so it must be wired to an micro input. After a while, the micro controller must be able to power down by releasing the enable line.

Attached is a SCH of my best solution so far (but not perfect!). Feel free to shoot it down and point down all the pitfalls of doing it that way. I do not want any parasitic current to flow from the 2 x AA cells to the +3V3 circuit and drain the batteries, therefor I indicated the ESD diodes inside the micro controller.

I am looking for the simplest / cheapest / most robust solution. Maybe the best solution should be captured for posterity...

thanks in advance,
Pieter

P.S. It is not an option for me to keep the micro controller in a deep sleep mode and wake up on interrupt. The BOOST supply MUST be switched off.
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#133260
Here is an updated design using Schottky diodes and a PNP transistor. Red arrows indicate direction of current flow.

Diode D4 is required to stop current from flowing through Q1 (when switch S1 is pressed) to VBAT when the micro pulls PSU_HOLD_O high.

R5 and C3 is used to give a sufficient RC turn on delay. R6 is used to discharge C3 and swith off the boost supply.

Comments?
Pieter
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