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BQ77216: Miss-understanding regards Cout and Dout specification

richard wilson
Intellectual 555 points
Part Number: BQ77216

Hi , 

I want to use this device to open P-Channel Fets on the high side during a failure. So I assume that I would use the active low version of the device to drive N channel Fets that would be normally on and these would pull down the gate of the P channel Fets, turning them on in the normal state. During OV, the output would go active low, turn off the N channel and thereby turn off the P Channel. however, I am having a reasonably hard time understanding the data sheet. The data sheet has a specification for source and sink for an open drain output which doesn't make sense to me.

Can you simplify the output specifications?

Specifically - this specification on page 7 of the user data sheet for part number BQ77216

VOUT_AL Output Drive Voltage for COUT and DOUT, Active Low Vn - Vn-1 or V1 - VSS > VOV, where n = 2 to 16, VDD = 58 V, IOH = 100 µA measured into COUT, DOUT pin. 250 400 mV

IOUT_AL_L OUT Source Current (during OV) Vn - Vn-1 or V1 - VSS > VOV, where n = 2 to 16, VDD = 58 V, OUT = VDD. Measured into COUT, DOUT pin. 0.3 3 mA

IOUT_AL_H OUT Sink Current (no OV) Vn - Vn-1 and V1 - VSS < VOV, where n = 2 to 16, VDD = 58 V, OUT = VDD. Measured out of COUT, DOUT pin. 100 nA

Is (Vn - Vn-1 or V1 - VSS > VOV) just saying that any cell that exceeds V max causes the output to become active and I don't understand why there is an output current from an open drain drive.

Thanks for any help in advance. 

Richard

  • 0
    Intellectual 555 points

    After writing this question I found myself asking whether it is better to use one of the devices to drive N channel FETs on the ground side of the battery pack? Do you have nay relevant schematics?

  • 0
    TI__Expert 3990 points

    Hi Richard,

    I am not the best to answer this, I have passed your question to a colleague more knowledgeable on this device. 

    My guess is the 3mA max spec is intended for when active-low and the COUT/DOUT is driving low (when OV is triggered), this is the current the device sinks.  The conditions mention the current flowing into the COUT, DOUT pin, which would then make sense. 

    My guess on the next line is the 100nA is the max current that flows into the device when an OV is not triggered, and the pin is being pulled up externally.  The conditions mention the current flowing out of the COUT, DOUT pin, which would fit that case.

    I think the text about  (Vn - Vn-1) or (V1-VSS) > Vov or <Vov is just referring to the case when an OV fault is triggered or not.

    To your follow-on question, I'm not aware of a design that incorporates that approach myself, but maybe one of my colleagues may be aware of one and can respond when back in office.

    Thanks,

    Terry

  • 0 in reply to richard wilson
    TI__Prodigy 600 points

    Hi Richard,

    Terry's explanations below is the intend of the datasheet conditions.  As for the use of the protection, usually customers would use BQ77216 with two external signal NFETs to drive a fuse. If you would like to use it for power FETs drive, high side PFETs with active low BQ77216 and two external signal NFETs would work, which is similar to your original idea.  

    "My guess is the 3mA max spec is intended for when active-low and the COUT/DOUT is driving low (when OV is triggered), this is the current the device sinks.  The conditions mention the current flowing into the COUT, DOUT pin, which would then make sense. 

    My guess on the next line is the 100nA is the max current that flows into the device when an OV is not triggered, and the pin is being pulled up externally.  The conditions mention the current flowing out of the COUT, DOUT pin, which would fit that case.".

    Best,

    Xiaodong

  • 0 in reply to -Terry
    Intellectual 555 points

    Thank you Terry,

     If you can check on a low side application , that would be useful.

    Cheers

    Richard

  • 0 in reply to Xiaodong Cai
    Intellectual 555 points

    Thank you Xiaodong.

  • 0 in reply to richard wilson
    TI__Expert 3990 points

    Hi Richard,

    I have not found a good example that I can point you to, for the device driving low-side FETs directly.

    Terry