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BQ35100: Current sense resistor formula.

Adrian
Prodigy 70 points
Part Number: BQ35100
Other Parts Discussed in Thread: TIDA-01236

Hello,

What are the units to be entered in 8.2.2.5 (current sense resistor selection)? There are millivolts, milliohms, and milliamps, are they entered as integers in those sub multiples? The results make no sense, for instance, for a current of 250mA, and using the indicated formula would be:

125mV / 250 mA = 0.5 mΩ or 500 micro ohms?

Also I could not guide myself from the reference design TIDA-01236, because there it uses a 1 ohm resistor, such a high drop.

There is also the issue that in that reference design the thermistor is replaced by a 10k resistor, I see nothing about that if using the internal temperature sensor there needs to be a "dummy" 10k value on the thermistor input.

And finally there's the issue I don't understand in 7.3.4.5 about "The device assumes that when GE is asserted the cell is at rest and uses the initialization voltage reading to determine the Open Circuit Voltage (OCV). If the cell were not fully relaxed at that point, then the voltage after the pulse could rise above the OCV. This causes an incorrect impedance to be calculated." So, there is the load from the power regulator, the microcontroller, etc. they can be reduced to a minimum, but there will reasonably be a system load, how does that work regarding that requirement?

Thanks.

  • 0
    TI__Guru* 76230 points
    In 8.2.25, what is being discussed there is sense resistor selection not a value to be entered into the data flash.

    1ohm is used in the reference design because it is anticipated that the current in the application is small so it is important to use a larger sense resistor to be able to get reasonable measurements above the noise floor.
    -Yes if using tge internal sense, use a dummy 10k, don't keep the pin floating.
    - As long as the current is sufficiently low, it will be seen as an OCV measurement as the actual OCV will not differ significantly under very light loads relative to the battery capacity.
    thanks
    Onyx
  • 0 in reply to Onyx Ahiakwo
    Prodigy 70 points
    Thanks for the response Onyx,
    Regarding 8.2.2.5 I still need to know what are the units expected in order to make the selection of the sense resistor.
    Regarding the 1 Ohm resistor I don't see anything about the noise floor, it even recommends a typical 1 milliohm sense resistor.
    Thanks.
  • 0 in reply to Adrian
    TI__Guru* 76230 points
    The unit for the sense is milliohm. The gauge measures current by measuring the voltage drop accoss the srn and srp pins. The max voltage across those pins is 125mV. So you should divide that by the max current your application is expected to see and keep your sense resistor under that value. Also, the smallest current resolution which the gauge reports is 1mA.