I'm working on a project that measures battery voltage.
The device, I designed, works on coin cell.
I have to extend battery life, that's why I used the following design
But it requires two pins
I don't want to use 2 pins. Can we redesign by using a single pin which will act like analogue as well as digital output, and we can measure voltage when we require, with very low power?
May I offer a different approach?
Instead of turning off the voltage divider, reduce its current drastically and buffer the voltage.
simulate this circuit – Schematic created using CircuitLab
I'll assume that the VDD is not really intended and that this switch only operates when battery voltage sampling is required. If Q2 also supplies Vdd to the system a different design is needed.
If you drive Qlower gate via a diode D1 from BATADC with a digital high, and add a capacitor C1 to Qlower gate to create a 'stay turned on' time constant with R19, then you can.
Assert BATADC digital high - turns on sampling
Convert BATADC to analog input
Measure battery voltage before falling voltage on C1 starts to affect the reading.
D1 must be silicon, not Schottky to avoid potentially high reverse bias currents in Schottky.
AO3401A is an "entirely OK" part to use - Rdson will probably be around 60 milliOhm. This is insignificant given the value of the resistor divider. BUT that divider MAY be loaded somewhat by the ADC depending on circuit details. Using a sub-10-milliOhm Rdson FET will probably cost no more.
The effect of the 2 x 47k values on precision should be checked (and may have been already).
The AO3400 hs an Vgsth max of about 1.5V which allows lots of headroom when designing the RC turnoff time.
Can we redesign by using a single pin which will act like analogue as well as digital output, and we can measure voltage when we require, with very low power?
No.
Any convoluted solution we come up with to multiplex two functions into one GPIO would affect the accuracy of the reading and make that solution pointless.
You can try this procedure:
Switch pin as digital.
Set output High. This charges the C1 to hold mosfets ON for a while.
Switch pin to analog.
Measure the battery level.
The C1 discharges itself through R1 after some time. You can use a schottky instead of 1n4148 to increase measurement range. Theres no need to R3 and R4 to be so high since the batt. measurement is in minor periods only (1k,1k should be ok).
simulate this circuit – Schematic created using CircuitLab
How about something like this?
simulate this circuit – Schematic created using CircuitLab
When idle, let the pin float and quiescent current should be approach. Prior to measurement, set pin low for awhile, then float it and take two measurements with a known amount of time between them. If both measurements are taken while the cap voltage is sufficiently below VDD, VBAT may be estimated as a multiple of the difference between the two measurements. Alternatively, if all one has is an analog comparator with e.g. a 1 volt fixed threshold, one could start by grounding the pin long enough to pretty well discharge the cap, then float the pin and see how long it takes for it to reach one volt.
