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BQ76952: Using FUSE pin to drive a circuit

Harry McNally
Intellectual 440 points
Part Number: BQ76952

Hello Matt and others

We have a requirement for the FUSE output to drive a small circuit rather than the example 5K series resistor to a capacitor and NFET gate that heats and opens the chemical fuse.

I haven't found a maximum drive current for the FUSE pin on the datasheet.

If I want to drive a 0.1uF decoupling capacitor and a circuit consuming 100uA or so, do you have data for the output impedance and maximum current I can safely consume from the FUSE pin ?

With that I can determine the rise-time and if the FUSE pin can sustain power to the circuit.

Thank you and all the best
Harry

  • 0
    TI__Guru* 81035 points

    Hi Harry,

    The electrical table in the spec is with a parallel RC load in mind. So the FUSE pin should be able to supply at least 6V across 5k, or 1.2 mA (depending on BAT).

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Intellectual 440 points

    Thank you Matt

    A 1.2mA supply current will be fine.

    I used the risetime on the the FUSE Pin Functionality data to arrive at 240 ohms for the pin output impedance.

    If the external circuit has a typical 100nF X7R decoupling capacitor across the fuse pin my model shows a 33mA surge at turn-on.

    I am still using the signal for safety control so is that surge going to stress the pin beyond acceptable limits ?

    I see a recent post:
    https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1021406/bq76952-1-2-volts-on-fuse-pin
    which shows Figure 1-5 FUSE Pin Equivalent Diagram which switches BAT voltage; hence my being prudent about stressing the internal FET.

    All the best
    Harry

  • 0 in reply to Harry McNally
    TI__Guru* 81035 points

    Hi Harry,

    The image from the other post comes from this Pin Equivalent diagram document: https://www.ti.com/lit/pdf/sluaaf2 

    We haven't tested to failure to see where this pin would be damaged, but the datasheet Abs max section says the current flowing into this pin must be limited to 2mA. 

    Matt

  • 0 in reply to Matt Sunna
    Intellectual 440 points

    Hi Matt

    Thank you for the pin diagrams; I can get a better understanding of the device.

    I don't think the 2mA in-flow current limit will be an issue unless the FUSE pin turn-off means that the decoupling cap charge flows back into the pin. It doesn't look like that would be a surge in the same way as the FUSE turn-on.

    I interpret from the simplified diagram that the CONTROL block adjusts the high side FET as a linear regulator to present 6V (typ) at the FUSE pin. If my capacitive load damages that FET then I may get BAT voltage at the FUSE pin and I don't think I have a way to test for that damage.

    Is there anything in the device modelling that can determine if a 100nF capacitive load at FUSE is within the design limits ? Or a way to test it in your test fixtures for the part ? This will not be a repetitive transition of course !

    If it cant be known then that is a showstopper and I should not pursue the design.

    Thank you and all the best
    Harry

  • 0 in reply to Harry McNally
    Intellectual 440 points
    Harry McNally said:
    and I should not pursue the design.

    Sorry to quote myself, but I meant that circuit element; not the whole BQ76592 design. We are committed to the BQ but I wont be able to apply it in several applications that need the non-standard use of the FUSE pin.

    I am hoping you can make a definitive assessment.

    All the best
    Harry

  • 0 in reply to Harry McNally
    TI__Guru* 81035 points

    Hi Harry,

    I'll try to look into this more. I cannot make a definitive assessment without digging deeper with the design team. That may take a few days to get some of their time.

    Regards,

    Matt

  • 0 in reply to Matt Sunna
    Intellectual 440 points

    Thank you Matt

    My request is an unusual use of the pin so I appreciate that you are considering it for me.

    If the design team can arrive at limits for the pin then I can consider increasing or reducing the circuit values.

    If it takes some time to consider, there is plenty to be going on with here Slight smile

    All the best
    Harry

  • 0 in reply to Harry McNally
    TI__Guru* 81035 points

    Hi Harry,

    I discussed with the design team. By design, the FUSE pin can drive up to 5 mA and sink up to 2 mA of current. A one time 33 mA load surge will not damage the device (the output current will be limited to 5 mA by the metal width), but repeated current like this can cause device damage. Or a long-term current > 5 mA could damage the device. 

    If you use a resistor in series on the FUSE pin output, it should limit the peak current.

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Intellectual 440 points

    Thank you Matt

    The fact that the metal imposes a 5mA limit means I need to consider the rise-time anyway and a series resistor isn't imposing further limiting so it can be added.

    All the best
    Harry