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Measuring Silver Oxide voltage with nRF52

rugarcia

Hi,

I have read "Measuring Lithium battery voltage with nRF52" posted on 5-23-16 just as a point of reference.

System setup: In my system the nRF52 is powered (VDD) from an IC that doubles the voltage from a single silver oxide battery (VBAT) which can range from 1.2V to 1.6V. The charge pump that used to double the voltage will nominal provide 3.1V (no load) when VBAT cell is 1.55V. Unfortunately, the VDD voltage as seen by the nRF52 will vary depending on the load of the nRF52.

In our system there is a mode where our device is essentially "shut down" but not completely. What this means that we have one ASIC that drives the charge pump output to the nRF52 VDD. During this "shut down" time VDD to the nRF52 is ~= 0V. VBAT is essentially still present and connected to either GPIO's and an SAADC input. We needed to isolate VBAT so that we won't back feed the ESD protection diodes on the nRF52 when in "shut down". We found a high side FET switch that we can use.

VBAT battery measure:

  1. We are going to connect VBAT (via the high side FET switch) using a resistive divider into one of the SAADC positive input pins.

  2. The plan is to set the gain to 1/2 and use the internal reference. This would give us an ADC input range of 1.2V. With this setting, when VBAT is 1.6V the ADC input will see 1.14V and when VBAT is 1.2V the ADC input will see 856.16mV.

  3. We have a requirement to measure VBAT under heavy load. There are a couple of ways of doing this and I wanted the minimum number of components in the solutions.

  • Option 1: Provide an n-FET where the gate is connected to a GPIO. We want to load it down by approximately 10mA. The drain will be connected to the isolated VBAT via a fairly low value resistor (120 to 160 ohms). The source is tied to ground. Under GPIO control when the gate is enabled a 10mA load will be applied to VBAT and we then measure using SAADC.
  • Option 2: Connect isolated VBAT directly to GPIO. Set GPIO to sink in high drive mode. Is this possible? Looking at the GPIO spec's I think we can do it but I'm not sure. I want to make sure that the load current is completely controlled by VBAT and the external resistor.

Can you comment on this and let me know what issues you see?

Thanks, R.G.

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