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Topic: Ultra low power switch comparator (Read 304 times) previous topic - next topic

Taiwanguy

I'm building a resistor multiplex circuit that will use a couple of low power Op amp comparators. The comparator outputs will be connected to the IO pins of the MCU. This will be a coin cell powered device with ultra low power requirements. Needs a standby time measured in years, not hours. So even the lowest power devices are not low enough. It can only be active when the button is pushed. The MCU is ultra low power and has sleep mode, so that isn't an issue. I just can't have the comparator circuit active.

So I need to turn on the comparator IC when the buttons are pressed. My thought is to use an FET to power the comparator IC. Problem is, with a coin cell, the low end is about 2.5V, and leaving enough range for my buttons, I could have a voltage as low as 0.5V available to turn on the circuit.

Anyone have a recommendation for an FET that has very low Vgs and can drive about a 10mA circuit? This will be a mass produced product, so it needs to be a very low cost, common component. Maybe a 2N7000 would be ok?

MarkT

What's an opamp comparator?

Do you mean a comparator?

If you want lowest power choose a comparator with low quiescent current drain and push-pull logic output
(so you don't need a pull-up resistor).

Note you cannot just power down a comparator if its inputs have live voltages on them, this will destroy the device inputs.


There'll be thousands of FETs match your requirements, they will all be SMT though.  Any small signal PNP transistor will be fine too.

Better to find a comparator with ultra low current drain or an enable input.
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gilshultz

Not to complicated, have the switch power the device from the input power then the device can shunt the switch with a fet  or whatever you like, do its thing then shut off. This circuit yields zero standby current other then circuit leakages.

Without all the information I cannot give you a circuit but this should get you there rapidly.

Good Luck and Have Fun,
Gil
This response is to help you get started in solving your problem, not solve it for you.
Good Luck & Have Fun!
Gil

MarkT

Yes, don't confuse opamps with comparators - using an opamp as a comparator is dodgy at best (I've just done
it myself solely because I had several unused sections of a quad opamp available, but I feel a bit shabby doing it!!)

Comparators are usually much lower quiescent power than opamps anyway, but you'll need to select a push-pull output comparator to avoid the current consumption of an output pull-up resistor.

I don't understand why you say only 0.5V is available if the battery is 2.5V.  A push button on the base resistor of a PNP signal transistor would be able to power up your circuit nicely, drawing only a few nA when not pressed due to transistor leakage.

Perhaps you could post a circuit diagram?
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Smajdalf

Maybe you can use ADC instead of comparators?
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