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Original question:

BQ77908A: Current flows

BQ77908A: Current flows

Yoshinobu Kagawa
Expert 2000 points
Part Number: BQ77908A

This is a comment from the customer.

If you think about it correctly, the content you are inferring is correct, but I think that it is not the current flow from the measured value of the current consumption.
It's an old product, so it's hard to find out (someone who knows it hasn't left the company already)?
Since the purpose is to screen abnormal products by inspection, if it is difficult to confirm the details, we think that there is no choice but to consider judging from the actual measurement value.

Best regards,
Kagawa

  • 0
    TI__Guru 54715 points

    Hi Kagawa,

    I'm not certain I understand the question. With protector and monitor devices the IC is typically grounded at the battery- so that the signal at SENSE+ and SENSE- is positive and in the larger range of the IC.  The range below VSS is typically more limited. This is the connection shown in the BQ77908A data sheet figure 2 and others. The load current of the IC (ICC in the data sheet) is not sensed.  The ICC current is 75uA max as shown in the data sheet.  Load current for the FETs will flow through the sense resistor, but this is typically small.  Load current for the regulator could be returned to VSS where it would not be sensed, or to the load side of the sense resistor where the voltage would be modulated by the load current.  Since the regulator output current is limited (VREG short circuit current may be as low as 20 mA), the sense resistor values are typically small, and the FETs would not offer protection with this connection this connection method is not normally used. 

    Again I may not understand the question, please clarify if needed.

  • 0 in reply to WM5295
    Expert 2000 points

    Is the current normally measured at the VCx pin correct in the following directions?
    I don't think the current value can be stated because there is no specification, but if the direction is fixed, I think it would be better for the customer to see the measured value and refer to it.

    Best regards,
    Kagawa

  • 0 in reply to Yoshinobu Kagawa
    TI__Guru 54715 points

    Hi Kagawa,

    Correct, there is not a specification for the cell input current.

    Anecdotally the currents were typically into VC1 through VC7 and out VC8 and VC9.  VC1 current was around 5 uA, VC2-VC7 input was less than 1 uA,  Current out VC8 and VC9 was about 2.5 uA.  You should be able to measure a similar current.  Open wire check will momentarily disrupt a measurement.