Battery Read with VDD at 2.1v without Voltage Divider

[Harsha]

Hello, I need to read by Battery Voltage using ADC. My Battery is 3.3v which is being stepped down to 2.1V using a LDO regulator. So my VDD is 2.1V in DC/DC mode. Now I need to read by battery voltage for battery status indication. Connecting the battery directly to ADC pin causes my VDD to change. Connecting the battery to ADC pin through a voltage divider network causes more current consumption.

Suggest me a way to read the Battery voltage. Thanks. I am using 51822.

[Ole Bauck]

First of all I hope there is a good reason to step Vdd down to 2.1V, you can make your life much easier by just using 3.3V as Vdd directly. Measuring Vdd with the ADC is much simpler.

At 2.1V the DCDC converter efficiency is about the same as LDO, see Product Specification Figure 11 (chapter 8.5.3). If you are using 2.1V as Vdd you should use LDO mode so to save the cost of the extra components needed for the DCDC converter.

The maximum allowed voltage on any pin is Vdd + 0.3V, if you go over this the ESD protection diodes will start to conduct current to Vdd which is the reason your Vdd will rise.

To measure voltages above Vdd you have to use an external voltage divider. There is a blog about this here (here the battery voltage is 3-4.2V while Vdd is 3.3V, but the principle will be the same for you).