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Original question:

BQ76952EVM: BQ76952 PRECHARGE TEST/PREDISCHARGE TEST VOLTAGE IN BATTERY MANAGMENT STUDIO

BQ76952EVM: BQ76952

deepak kumar
Prodigy 220 points
Part Number: BQ76952EVM

Tool/software:

Hi Alexis,

 We use the differential probe to check the precharge & predischarge but do not get the expected result. Result is same as previous.

Thanks with regards,

Deepak

  • 0
    TI__Expert 5090 points

    Hi Deepak,

    Alexis is out right now.  I'm not well versed on this case, but from a quick look, I wonder if you have some leakage on these nodes.

    The PCHG/PDSG drivers are hiZ when disabled.   There should be a ~10M resistor between gate and source of the PCHG/PDSG FETs, which will keep the gate-source=0V.  Then when the driver is enabled, it pulls down with limited current (~30uA or less).

    Given the currents are low, connecting a DMM to the node can cause significant leakage current and affect the voltage, so you have to be careful of the impedance of any test equipment or scope probes you attach to the nodes.

    When you say "result is same as previous", what do you mean exactly?  What is the voltage on the stack, the BAT pin, the CD node, and then the differential voltage across the PCHG/PDSG FET gate-source when enabled?

    Thanks,

    Terry

  • 0 in reply to -Terry
    Prodigy 220 points

    Hi Terry, 

                  I am not attaching any MOSFET or circuit at PCHG/PDSG pin. I am just activate the PCHG TEST /PDSG TEST & looking for expected voltage at multimeter or differential probe.

    We are supplying +47V DC at BAT- and BAT+ and can read the cell voltage,stack voltage correctly.

    Question 1 :- When PCHG/PDSG TEST is off then at PCHG & PDSG pin voltage is 40v...is it right?? I think it should be 47V DC.

    Question 2 :- When we on the PCHGTEST/ PDSGTEST using the Battery Management Studio software, the voltage on these pins drops from 40V DC to 38V DC, a decrease of 2V. is it right?? i think it should goes 8.4V down from 47 V to 38.6 V DC. 

    Thanks with regards 

    DEEPAK

  • 0 in reply to deepak kumar
    TI__Expert 5090 points

    Hi Deepak,

    If you are not attaching a FET, then you probably do not have the gate-source resistor connected to the pin either.  

    When the driver is disabled, it is hiZ.  So the pin is just floating.  It does not pull up to BAT voltage level on its own, it relies on the gate-source resistor for that.  I think the 40V you are measuring is just a floating node with some small leakage current on it.  This is not how the driver is meant to be used, so measuring it this way is not indicative of what it will be in system operation.

    When the driver is enabled, it pulls down ~8.4V below the BAT pin voltage level (can be between 7.5V and 9.7V below the BAT pin).  In system, the BAT pin is typically a diode drop below the top of stack.  I'm not sure if you have the stack voltage at 47V or the BAT pin voltage at 47V, that will make a difference of the diode drop.

    So, you might measure the voltage on the BAT pin, subtract 7.5V and 9.7V from that, and see if the resulting measured voltage on the driver pin is between those voltages.  It sounds like you are measuring approximately 38V, which seems to be within expectation.

    Thanks,

    Terry