help w/ low voltage cutout

[Derekc]

I need help figuring what mosfet resistor and zenor to use. I am running a dc dc step down converter off two 3.7 v lion batteries. The controle pin is normally on. It's off when a 20 k ohm pull up resistor is connected from it to positive. I would like to add a depletion transistor , a zener diode in for low voltage cutoff around 7 v but am confused on what transister to use and if I would need a resistor before the zener.

 

[(*steve*)]

That's probably not going to work.

I'd look for a 7 volt voltage detector

Here is a chip that would work (and look, it's designed for protecting batteries). Note that the voltage limits are adjustable and that the maximum Vcc is 7V. You would need to use a resistor and zener to ensure that this device is powered with less than 7V. It looks like it would be able to directly drive a mosfet or the on/off control for the regulator.

You would need to be careful that the power supply for this chip didn't further flatten the battery.

 

[Derekc]

Thank you I am determined to make this work but I'm defiantly in over my head.

 

[(*steve*)]

Perhaps something simpler then.

Here is a simple TL431 circuit that lights a couple of LEDs when the input voltage drops below a certain point.

edit: if you connect the output of this circuit to the control input of your regulator, it might be all you need.

In your case you'll want to remove the LEDs

 

[Derekc]

That I can do. Im ordering the parts now thanks man

 

[Derekc]

Got the parts hooked it up and got a huge voltage drop on the output of my regulator. The tl431 circuit has a constant 2 volt output even while battery is fully charged guess it's not enough to turn it all the way off but cause major voltage drops under load.