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Original question:

BQ76925: Drift in current measurement and fluctuation up to 400mA

BQ76925: SENSEN reading strategy

Raam Kasinathan1
Prodigy 30 points
Part Number: BQ76925

Hello,

Thank you for the reply. I think I should refine my question and also explain it a bit.

At 0A,I get an offset of 2V when I use (REFSEL=1) 3V reference. As per the data sheet, when there is no current is drawn, both SRP and SRN should be equal(both should give comparably equal voltage).

In my test, after drawing high current of 50A, when I remove the load, I see a difference between SENSEN and SENSEP, I read 1.7A even though I consume in micro amps lesser than the sensitivity of the sensor. But this difference between SENSEN and SENSEP gets corrected when I update the SENSEN again.

According to the data sheet, p.no: 16 of BQ76925 data sheet, section 8.3.2.3. I read the SENSEN w.r.t. VSS at the beginning and read the SENSEP w.r.t. to VSS regularly. When there is an environmental change(word used in data sheet), I assumed that  it is the temperature change. I update the SENSEN whenever there is a 5 degree increase/decrease in temperature.

In addition, I get a fluctuating reading, the distortion is up to 400mA which is pretty large for the current integration. 

My question is

1. How often should I update the SENSEN value?

2. Should it also be calibrated with respect to the current applied, as it could cause the temperature increase? if possible, kindly tell me the interval.

3. How can I get a stable reading without fluctuation, (I read the value after 6tau of the input filter)

Thank you in advance. 

  • 0
    TI__Guru 54715 points

    Hi Raam,

    1. The part works by comparing the SENSEP with SENSEN  I would update SENSEN as often as practical.  If you are measuring at 1 s, update at 1 s if 500 ms,update at 500 ms.  If the update eats into your timeline, update less frequently.

    2. The sense resistor and traces will heat with current flow, how much depends on their design and rating.  In general yes I would recommend updating with current, as often as practical in your timeline.

    3. If the current fluctuation seems abnormal, and abnormal may be difficult to determine since there is not a specification, compare to another part.  If VIOUT has a slow drift, it may be normal, if it is rapid movement it seems abnormal.  If there is no concern that the part has had abnormal signals applied, you might send a part back through the supply chain for analysis.