Auto off circuit

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I am using the auto off circuit as shown on the attachment. I came up with this, and it seems to work fine, but I am wondering what others use to achieve auto off, and whether there are problems with this one that I haven't considered. The circuit gives an ON, an AUTO OFF (uC controlled), and an OFF, depending upon the position of the switch.

 

A few points about it:

-The switch is a 3 position DPDT type that connects pins 1-2 or 3-2, if the switch is centered there are no connections.

-The auto off position is when the switch is centered. When the uC takes POWER GATE low, the voltage on the gate Q1 goes low, causing Q2 to turn off.

-The off page POWER GATE comes from an output on the uC. As soon as power is applied, the uC pulls it high.

-D7 makes sure that the uC is isolated from the input voltage. The pin on the uC is pulled high, so when it goes low, the gate of Q1 goes low.

-When the switch is in the 1-2 position, the voltage on the gate of Q1 is not zero, but its lower than the Vgs of Q1 and Q1 turns off.

-The off page VSENSE goes to an ADC pin for low voltage sensing.

-Q2 is a PFET with an Rds(ON) of 30 milli-ohms.

-The purpose of R9 is to pull the input rail down faster when the PFET turns off.

-The peak current through Q2 is less than 200 mA. It averages about 120mA. 

-The diode D8 is to protect against the user inadvertently reversing the polarity of the battery connection. I am not sure if it necessary considering the internal diode in Q2.

 

Appreciate any comments.

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D7 is probably better just being a normal silicon diode. Schottkys are leaky. You might want to consider your resistor values to lower the off current.

I did a similar circuit last week - I used a Torex low drop out regulator that has an enable pin. A pushbutton applies battery to the enable and when the micro wakes up, the port pin keeps the voltage on the enable. You can do similar things with certain switched mode regs that have enable pins.

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Thanks for the suggestions.

 

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jimlake wrote:
... but I am wondering what others use to achieve auto off, ...

All apps are different, but for the low-power AVR apps our outfit has done we don't do any real "off", as a deep-sleeping AVR draws only a few uA and less with sleeping BOD.

 

What are the tradeoffs?  Well, if you just go to sleep then you don't need the described circuitry so you save cost and board space.  OTOH perhaps you have part of the circuit that is very difficult to get to near-zero uA and actually cutting power saves.  YMMV.

You can put lipstick on a pig, but it is still a pig.

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