[ low voltage detection circuit ]

[texane]

Hi,

I am implementing a circuit relying on a 3v lithium coin battery.
A microcontroller is supplied by the coin unregulated, and  runs
the logic. I would like an external, very low current consumption
(ie. 100uA max) circuit, that detects if the coin voltage drops
below a static voltage (let say 2.4v). This way, the microcontroller
can alert the user to change the battery. Note that the circuit is
also power by the battery, so its input voltage is not fixed.

Also, note that the MCU does not have a brown out detection logic
usable for my purpose.

Also, do you have more general advices regarding low voltage detection
coin provided circuits ? In my case, the user may leave the circuit
for one month. When the user leave, the battery may still be full.
2 weeks later, it may not ...

Any idea is welcome !

Thanks for helping,

Cheers,

[Rerouter]

if you have an output and input free you could use a resistor divider, comparator and diode reference, you would just need to choose a comparator with a low enough current consumption and choose your resistor divider accordingly,

when you want to test the voltage you pull the output to ground, giving the little test circuit a ground, check the state after a few milliseconds to be sure and then switch it back to high impedance until the next measurement, even if the comparator takes 200uA while running it only needs to be on for such a short time that it doesn't matter, but it would not be hard to find ones lower than that,

[Jay_Diddy_B]

Hi,

Have a look at the LTC1540. This is a nanopower reference and comparator.

Link: http://www.linear.com/product/LTC1540

Application circuit



This circuit draws under 1uA when active.
You can adjust the resistor divider for different thresholds.


Jay_Diddy_B

[poorchava]

I would use a voltage divider and switch it with high-side low threshold P-MOS. This way you can enable the divider only when you need to measure the voltage. You'd need to pick an MCU that has and internal bandgap reference.

[Psi]

Check if your MCU has a built-in bandgap reference on one of the ADC channels.
Many AVRs have a 1.1V or 1.3V bandgap option and i imagine other micros do as well.

No need for an external voltage divider and pfet

If you select the ADC MUX to bandgap channel and read the value you can work out the ADC reference voltage (which most people have as VCC)
For example, if VCC&ADCRef is 3V, and the bandgap voltage of your mcu is 1V, then the ADC value for bandgap must be 341.

Using this you can work out VCC at any time with no need for any extra components.

ADCVref =  (MCU_bandgapVoltage /  (ADCvalue / 1024))

Obviously this requires the MCU has an ADC with bandgap input option.
Also, if you're powering the ADC using a reference voltage instead of VCC for some reason then you have nothing relative to VCC to compare against and can't use this technique. (well.. i guess you could if you had a VCC voltage divider as your ADC Vref)

[dannyf]

Doesn't the mcu have bod? Or adv? Or a Schmitt input? ...

[texane]

Thanks for your answers. I will use the the internal bandgap reference and
internal analog comparator and the associated interrupt to wake up from
sleep.

t.