Zero power voltage limiter for logic suggestions...

[Boscoe]

Hi all,

I'm a little stuck on creating a simple, low cost, very low power voltage limited for the input to a logic gate.

The gate is on a 3.3V rail and doesn't like anything above 3.6V. I have a signal from a battery management system that is VBAT + 6V where VBAT is two lithium ion cells (8.4V maximum) when logic 1 and 0V for logic 0. I need to interface this with the gate however struggling to do it in jus ta couple of parts with current consumption less than uA. Naturally I went for the zener + resistor option but keeping the voltage above the gate threshold throughout the whole VBAT range is tricky while maintain low power. Are there any gates or buffers that can tolerate high input voltages?

Thanks
Boscoe

[ledtester]

How about something like this:



Choose the MOSFET to be able to withstand a V_GS of the 8.4+6 V signal. R will have to be in the megaohms to make the quiescent current in the microamps, but it might work for you.

[langwadt]

Hi all,

I'm a little stuck on creating a simple, low cost, very low power voltage limited for the input to a logic gate.

The gate is on a 3.3V rail and doesn't like anything above 3.6V. I have a signal from a battery management system that is VBAT + 6V where VBAT is two lithium ion cells (8.4V maximum) when logic 1 and 0V for logic 0. I need to interface this with the gate however struggling to do it in jus ta couple of parts with current consumption less than uA. Naturally I went for the zener + resistor option but keeping the voltage above the gate threshold throughout the whole VBAT range is tricky while maintain low power. Are there any gates or buffers that can tolerate high input voltages?

Thanks
Boscoe

if the current is limited to 1uA I doubt  the logic gate has any problems input voltages above 3.6V

[BrianHG]

Most CMOS logic gates have input protection diodes.  If your IC does, you can just use a series resistor, like a 10 or more megaohm, from battery to input.  Though, the logic input needs to have no load, IE, it cannot have an internal pull-up resistor.

The battery will need to drop below 1.5v for the input to be considered a logic low unless you add a pull-down resistor at the logic input.

This will work for sensing low frequency / DC levels.  It may fail if your PCB has a ton of dirty crud/really dirty flux residue all over the place.

Zeners wont work as their internal minimum current will be too high.

If you want an added voltage drop, like a zener, try using a simple old fashioned red or green led (in series with the 10 megaohm resistor (it should never light up...)).  They will provide a ~1.5v(red)-2.0v(green) voltage drop with a few nano-amps whereas a typical zener will provide a 0v drop at 500na, almost like a short when drawing a few nano-amps.  IE, a 10 megaohm pulldown will only be ~600na.

(Please excuse the ultra low current drop of the LEDs if I got them wrong, it's been 10 years since I used LEDs as ultra low Iq voltage drop/regulators.  You can measure this by placing the LED on a battery+ and your volt meter all on it's own from the LED cathode and battery GND.)

[David Hess]

Discrete solutions are possible but the simplest "zero power" implementation involves using a comparator with a push-pull output so not pull-up resistor is required.