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BQ2973: BQ29732 not outputting correctly

Hossam Moghrabi
Intellectual 360 points
Part Number: BQ2973

Hello,

please check my circuit here: https://www.eevblog.com/forum/beginners/nimh-3x-aaa-batteries-charging-circuit/?action=dlattach;attach=1736723;image

please make sure to read my ground and vpack- correctly, as gnd was taken for the entire circuit.

my problem is that BQ IC does not output VBatt to the mosfet package. DOUT is about 2.9v and COUT is 0v with respect to battery negative terminal (here labelled vpack-).

Also, there is a voltage drop of about 2.9v across the 2 mosfets...

the circuit works only when i briefly touch the GND and Vpack-... or touch COUT DOUT to Vpack+ (battery positive terminal).

please help identify the issue, why doesn't it work by itself as long as everything wired correctly?

  • 0
    TI__Expert 3425 points

    Hi Hossam, 

    The BQ297x protector BAT pin is a power pin which requires a voltage to be applied in order for the device to function. The BAT pin will not source voltage to the MCP73832T in the schematic. 

    Since the BQ297x device is active-low logic, the 2.9V output on the DOUT pin means DSG is enabled, but when COUT ( 0V ) the CHG FET will be disabled. 

    Hossam Moghrabi said:
    the circuit works only when i briefly touch the GND and Vpack-... or touch COUT DOUT to Vpack+ (battery positive terminal).

    What do you mean by touch the GND and Vpack-? Can you explain some more on this point? 

    Best, 

    -Luis Torres

  • 0 in reply to Luis Torres
    Intellectual 360 points

    hello,

    you misunderstood the concept of my design.

    the protection IC does not supply voltage to the charger IC (MCP73832T).

    VBATT always exists since I plugged in the battery, but despite it is like 3.95v... protection IC still does not let the battery circuit be ON.

    when i briefly touch Vss with V- pins, the circuit works immediately. I noticed this was a hint in datasheet indicating the circuit won't work when first plugging the battery unless connecting Vss to V- pins briefly, or connecting the charger.

    at first i wasn't connecting the charger at all... now i connected the charger and the circuit worked as intended without touching V- to Vss.

    So I guess the BQ IC REQUIRES that when first plugging the battery that the user also plugs the charger.

    please verify and let me know.

    also, check my circuit please... tell me if there is any other way i can enhance it from BQ side.. especially lowering quiescent current and so on.

    thanks

  • 0 in reply to Hossam Moghrabi
    TI__Expert 3425 points

    Hi Hossam, 

    Apologies for the misunderstanding. To address the question at hand, yes the device requires that you short V- & VSS  OR that charger is attached when first plugging in the battery cell. 

    We looked over the schematic and there are no further recommendations. As for the quiescent current, it will be limited to the datasheet spec in normal operation as shown below: 

    Hope this helps!

    Best, 

    -Luis Torres

  • 0 in reply to Luis Torres
    Intellectual 360 points

    hello

    yes i plugged the charger and it works fine. it has no problem for me too.

    but i want to ask if any fault happened like short or under charge or over charge...etc am i required to reconnect the charger again?

  • 0 in reply to Hossam Moghrabi
    TI__Expert 3425 points

    While reconnecting the charger restores the device to normal mode in some cases, each fault condition will have a specific recovery condition which are specified in section 9.4 Device Functional Modes of the device datasheet. 

    See overcurrent in discharge fault as an example:

    Best, 

    -Luis Torres