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BQ2970: Achieving low OCC and OCD thresholds for an ultra-low-power wearable application

Part Number: BQ2970
Other Parts Discussed in Thread: CSD83325L, CSD85302L


We are working on an application for a wearable device that utilizes an extremely small capacity lithium polymer cell (30mAh) and completes the battery protection circuitry on the device PCB.  For this protection circuitry, we would like to use the bq297xx protection IC coupled with a dual N-Channel Common Drain power MOSFET, such as the CSD85302L or  the CSD83325L.

With such a small li-po cell, we need the OCD cut-off threshold for our application to be around 100 milliamps and the OCC cut-off threshold to be much less than this (ideally less than 50 milliamps).  We are struggling to find an off-the-shelf combination of bq297xx configuration + MOSFET source-to-source-on-resistance that would enable this behavior.

First Question:  If we were able to qualify for a TI factory-customized bq297xx variant, what would be the minimum possible OCD and OCC threshold voltages that the IC hardware can support?

Second Question:  Would there be any issues (other than the waste of energy) of adding an additional series resistance (beyond the MOSFET's source-to-source-on-resistance) in between the bq297xx's VSS and PACK_NEGATIVE?  As an example, in the circuit schematic snippet below, you'll see an arrangement of the bq29700 (OCC of -0.100V and OCD of 0.100) and a 1.0 Ohm resistor that seems to yield "tuned" OCC and OCD thresholds at around 100 milliamps.

Note:  we got the idea of adding this additional series resistance in between VSS and PACK_NEGATIVE from TI's "Smartwatch Battery Management Solution Reference Design" which you can find here:

Thanks in advance for your help!

-- Michael