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BQ76952: Temperature Jump issue & Auto balancing check & Sch review

Rannie Zhou
Expert 4220 points
Part Number: BQ76952

Tool/software:

Hi team,

I have one issue and three questions around BQ76952 that need your support.

Issue summary:

When using BQ76952 to read the cell temperature through TS1/2/3/4/5/6, the temperature will jump from 30 to 168, which is abnormal as the temperature should be a parameter change continuously. Could you please help to check the root cause or give some debug direction? The sch and PCB is as below. (Pin 31,32,25,21,22,23 are connected to thermistor to sense the temperature of each cell) The filter capacitor (470pF) of TS4/5/6 is set on another layer in PCB board. There are 2 assumptions: (1) The internal register is abnormal, but we don’t know the potential reason, customer implement CRC check in software; (2) There are noise on input signal. But there are 470pF capacitor series in the input signal path.

Support need:

Help to check the root cause and provide the debug direction.

 

 

Questions:

1.Do we have the common issue summary or user guide of BQ76952 to help customer minimize their potential design risk in application? Except help to review the sch, what can we support?

2.Do we have the example of software register set-up recommendation for customer’s reference?

3.For the cell balancing timing: When the cell is measured, the balance for this cell will be disabled. Does this only work for auto cell balance by BQ76952 itself or also work for MCU control cell balance?

  • If it only works for Auto cell balancing, customer should turn off the cell that is being measured by MCU. Right? If the sample time is 100ms, how to ensure the time delay between MCU send command and cell disabled? Customer said they found this time delay is about 1s, which is too long.

Thanks!

BRs,

Rannie

  • 0
    TI__Mastermind 25980 points

    Hi Rannie,

    Rannie Zhou said:
    When using BQ76952 to read the cell temperature through TS1/2/3/4/5/6, the temperature will jump from 30 to 168, which is abnormal as the temperature should be a parameter change continuously. Could you please help to check the root cause or give some debug direction? The sch and PCB is as below. (Pin 31,32,25,21,22,23 are connected to thermistor to sense the temperature of each cell) The filter capacitor (470pF) of TS4/5/6 is set on another layer in PCB board. There are 2 assumptions: (1) The internal register is abnormal, but we don’t know the potential reason, customer implement CRC check in software; (2) There are noise on input signal. But there are 470pF capacitor series in the input signal path.

    You may want to check that the raw temperature value is being converted correctly from ADC counts to Celsius. You might also want to check if there are any unexpected shorts on the thermistor pins.

    You could try observing the pin voltages on a scope to see what the peak-to-peak voltage is. The thermistor bias voltage is only enabled during measurements, so it will be necessary to use a scope for this.

    Also, what pullup resistor is selected in the AFE? The 18k or the 180k? The default is the 18k.

    Rannie Zhou said:
    Do we have the common issue summary or user guide of BQ76952 to help customer minimize their potential design risk in application? Except help to review the sch, what can we support?

    We have a lot of application notes with tried and tested design recommendations in them. We also have several FAQs, both in an app note and posted on E2E.

    Rannie Zhou said:
    Do we have the example of software register set-up recommendation for customer’s reference?

    We don't have a specific recommended configuration for the device settings. This tends to be whatever the customer needs for their application.

    Rannie Zhou said:
    For the cell balancing timing: When the cell is measured, the balance for this cell will be disabled. Does this only work for auto cell balance by BQ76952 itself or also work for MCU control cell balance?

    The cell balancing will pause for the measurements in both autonomous and host-controlled balancing.

    Regards,

    Max Verboncoeur

  • 0 in reply to Max Verboncoeur
    TI__Expert 4220 points
    Max Verboncoeur said:

    You may want to check that the raw temperature value is being converted correctly from ADC counts to Celsius. You might also want to check if there are any unexpected shorts on the thermistor pins.

    You could try observing the pin voltages on a scope to see what the peak-to-peak voltage is. The thermistor bias voltage is only enabled during measurements, so it will be necessary to use a scope for this.

    Also, what pullup resistor is selected in the AFE? The 18k or the 180k? The default is the 18k.

    18k. The temperature jump is random, which means it's difficult to scope.

  • 0 in reply to Rannie Zhou
    TI__Expert 4220 points

    This temperature jump is random happened. Do you think whether it might caused by the EMI or something distort the communication?

  • 0 in reply to Rannie Zhou
    TI__Mastermind 25980 points

    Hi Rannie,

    The most common source of these types of errors in the thermistor measurement is from too much capacitance on the pin. The bias voltage is only applied during the measurement itself, so too much capacitance can cause the voltage to not fully rise in time for the measurement.

    The customer's capacitors are well within the recommended limit for the 18k pullup, so I'm wondering where the problem is: outside the part, or inside the part. Seeing the peak-to-peak voltage and what raw value the AFE is reporting would help to narrow down where the root cause is.

    Regards,

    Max Verboncoeur