BQ27Z561-R2: Recommended Rsense for Low Current Application

Hey Antonio,

Hope you have been doing well!

For this use case you essentially have two main options you could go with.

1. You could rely on the OCV table, looking up OCV values to correlate with a SOC. However, this approach would not be utilizing the impedance track functionality of the gauge.

2. You can scale your current so that the resolution of the ADC does not severely impact your reported current value when <1mA. The deadband current is defaulted to 3mA so at <1mA the gauge will report 0mA most of the time (this would cause substantial SOC error over time). To utilize this approach, you should calibrate the gauge to some value (say 100mA) and adjust the CC gain parameter such that the gauge thinks it is getting 1A. This would achieve a 10x scale which should be sufficient to get out of the deadband range when only drawing <1mA. The unfortunate drawback here is that of course now the gauge thinks it is seeing 10x the current it actually is so you must then scale back any of the Data Flash settings that use mA or mAh.

I would refer to https://www.ti.com/lit/an/slua792/slua792.pdf?ts=1635260410839&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FBQ34Z100-G1%253FkeyMatch%253DBQ34Z100-G1%2526tisearch%253Dsearch-everything%2526usecase%253DGPN where this process is outlined in much greater detail.

One important disclaimer is that this of course is a use case that we do not specifically run tests for so the customer should definitely test it with their own setup to verify it works properly for their application.

Also, make sure that the max current doesn't exceed the max rating for the CC ADC if they increase the RSENSE resistor.

Thanks and let me know if any other info is needed,

Jackson