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BQ76925: BQ76925 inaccurate and noisy current measurement

trong
Prodigy 10 points
Part Number: BQ76925

Hi,

I use BQ76925 for 3 cells.

Voltage measurement (VCOUT) are very stable and accurate.

However, current measurement (VIOUT) are not as expected. VREF is 3V, SENSEN is connected to VSS via RC filter (2x1kohm & 0.1uF as recommended in datasheet's table 27).

Rsense is 50mOhm.

At zero current, measured VIOUT is very noisy, the output is 20xx mV (Gain=4) and 22xx mV (Gain=8). When current>0, both VSENSEN and VSENSEP are changed. It's also strange that measured VSENSEP<VSENSEN for discharge current. I inspected code to ensure that I_AMP_CAL=0 to measure VSENSEN and I_AMP_CAL=1 to measure VSENSEP already.

I have no clue to start troubleshooting. Any suggestion?

Thanks,

  • 0
    TI__Guru 54715 points
    Hi Trong,
    Check the connection of VSS to battery- (GND or reference potential) as shown in figure 22 of the data sheet. The simple schematic on the first page of the data sheet or figure 16 do not make that connection clear. If the connection is missing the circuit will not have a reference to GND. The VIOUT circuit is not a differential measurement circuit but a single ended measurement which is switched between the 2 inputs. Note that the definition of the SENSEN and SENSEP connections is shown in the diagrams puts the SENSEP at the pack side of the sense resistor.
    To determine the current the difference of the 2 inputs should be calculated. If the connection between the SENSEN and VSS have much trace length, there can be significant change in the SENSEN voltage with current variation. Since you have a relatively large Rsense, the resistor would be expected to have most of the voltage.
    Be aware of the resolution of the MCU ADC and the granularity it introduces. If you have used the EVM you may have noticed the reported current change by a relatively large step from one count of the ADC.
    Of course check that pins and other components are soldered well. Compare other assembled example boards. Consider if the component may have been damaged during assembly.