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BQ76200: DSG does not switch on

Carlo
Expert 7715 points
Part Number: BQ76200
Other Parts Discussed in Thread: CSD19536KTT

Hi team,

Good day.

Our customer is using the BQ76200 and encountering some problems in his design. Here is the full query:

"I'm using the BQ76200 for a BMS application. I'm now 2 weeks in and started building the schematics given by the datasheets. Because of some problems with the delivery of some products I'm not able to use the precharge. I will give my schematic as an attachment so you know what I have built.

Type of FETS: CSD19536KTT.

Now I have found some problems and don't really know how to fix them. The first one is that the Charge FET is always turned on even when the enable is pin is low. Is this normal?
The second thing is the discharge won't come on at all even if CP_EN is high.
Do you guys have any tips for testing and can you help me with these complications?

If there is anything wrong with the schematic please let me know so maybe I can fix them in time for the school deadline!

Your assistance is appreciated.

Regards,
Carlo

  • 0
    TI__Guru 54715 points

    Hi Carlo,

    The schematic looks much like the EVM schematic with addition of realistic resistors to the gates and the Zener & resistor between DSG and PACK pins as discussed in the application note https://www.ti.com/lit/pdf/slua794 , so I would expect it should switch.

    1. It is not normal for the charge FET to be always on.

    2. As noted above it would seem DSG should switch.  CP_EN alone will not turn on DSG.  DSG_EN will (should) turn on DSG

    Some suggestions for test & inspection:

    • It is easier for the BQ76200 to turn on DSG if you connect LOAD+ to BATT+ for test if you suspect a load issue on DSG.  This way you can see the function more easily without LOAD+ transitioning.
    • You might inspect the charge pump waveform compared to the application note to see if it toggles as if one of the outputs is shorted.  Troubleshooting will be easier with an oscilloscope than with a meter alone.
    • Remove the resistors from the CHG and DSG pins to the gate for test.  
      • The Rgs resistors will keep the FETs off.  Verify the FETs are off and not shorted/damaged.
      • The BQ76200 will not be loaded, verify that the CHG and DSG pins swing as controlled by the enables
    • Check the FET pinout.  CAD tools vary in how they assign pins on FETs, but the pin assignment in the schematic clip does not match the CSD19536KTT data sheet.  The BQ76200 may not be able to drive the incorrect pin if there is a mismatch:
      • FET source is pin 3 in the data sheet, pin 2 on the schematic
      • FET gate is pin 1 in the data sheet, pin 3 on the schematic
      • FET drain is pin 2 in the data sheet, pin 1 on the schematic

    Hope something here is helpful.

  • 0 in reply to WM5295
    TI__Mastermind 28680 points

    Hi Willy,

    Thank you for your response. Let me continue this inquiry from our customer. Here is his feedback.

    Lately I have send a request for some help for switching the FET's. That is now working which is the furthest i have been yet.

    The DISCharge FET is switching nicely ( example DC = 15 volt when switched is becomes 25 Volt). What i want is that when i dont turn on DISCharge the Gate is 0 Volt. Right now it stays 15 Volt and the FET is giving through the voltage.

    Do you have any tips to help me??

    Regards,

    Danilo

  • 0 in reply to Danilo A.
    TI__Guru 54715 points

    Hi Danilo,

    I'm not sure I understand completely but suspect there may not be a load on the PACK+ in the customer test.  From the application note https://www.ti.com/lit/pdf/slua794 

    When DSG turns off it will pull to the PACK pin level turning off the FET.  But this does not discharge PACK+ and could leave the PACK+, PACK pin and DSG at the BATT+ voltage.  Leakages or something like a 10 Mohm meter impedance will discharge the voltage but it may be very slow.

    The BQ76200 does not have an automatic mechanism to discharge the PACK+.  An external circuit could be made to come on when DSG_EN was low.  The BQ76200 does have a significant bias current from the PACK pin when PMON is enabled, see the application note section 2.3 and figure 8.  That bias current would pull down PACK faster than leaving it to drift down, but would need some separate control to turn it on when desired.