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BQ2000T regulator

Timothy Starr
Expert 2445 points
Other Parts Discussed in Thread: BQ2000T, BQ2000

Hi-

Is it OK to power the BQ2000T from a 5V regulator which is reference to system ground rather than the local VSS on the other side of the current sense resistor?

What is the switching frquency of this device? Thanks and Best Regards,

-Tim Starr on behalf of JK@N

  • 0
    Prodigy 5 points

    4452.pmp2781_revA_sch_June_7.pdf

    You may need some form of clamp mechanism (zener) on VCC-VSS if powered that way.
    The current sense SNS resistor  will be between VSS and System ground.
    Consider the possibility of a large current transient in the charger output.
    This current could make the VCC of the chip exceed its 7V abs max, or go below the 4Vminimum operating point.
    This may be a remote possibility, just a consideration.

    Local capacitive decoupling of VCC-VSS at the IC is needed. 1-10uF
    The IC regulates to +-50mV on SNS, so this 100mVpp noise will show up on the VCC supply.

    There are instances of BQ2000 being used directly from a 5V DC+ DC- where DC- is system ground.
    Local 10uF decoupling on VCC to VSS was the only requirement.

    See PMP2781 attached.

  • 0 in reply to Ed Walker
    Expert 2445 points

    Thanks Ed. A few more questions about the BQ2000T if you would:

    Is it OK that 7S NiMH cells get programmed (via the resistor divider)  for 14.1V even though they're full at 11.2V? The idea is that the dT/dt mechanism will terminate charge. Is it safe?

    If we do that, what will cause teh chrager to resume charging?

    What is the mzimum duty cycle for the BQ2000T?

    Regards,

    -Tim Starr on behalf of JK@NM

  • 0 in reply to Timothy Starr
    Prodigy 5 points

    The resistor divider is to detect Over Voltage.
    When no cells are in place, the charger output will go high, indicating there is no load on the charger.
    As a pack is inserted the charger output will transition below the OV point and tell the charger to start.

    A bad cell could be high impedance, so OV also prevents charging a pack with bad cells.

    You expect the max cell voltage for NiMH to be 1.6V/cell.
    A good NiMH will terminate BEFORE THAT.
    dT/dt temperature termination is a little better for NiMH because their -dV/dt full charge slope is less than a NiCd.

    dT/dt termination for NiMH is widely used.

    Once a proper termination has been determined, to start a new charge:
    1) cycle input power to the charger (reset)
    2) pull out charged pack, output sees OV, put discharged pack in, pulls output below OV.

    A charge of C/2 (0.5) or greater must be used to force the chemical reactions for Temperature Rise, and -dV/dt.