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BQ77915: CTRD as power switch

Jeffrey Blum
Prodigy 60 points
Part Number: BQ77915

Tool/software:

I've got a client who wants a battery-powered system to have a rocker power switch and a software shutdown process, and I'm hoping to use the BQ77915's CTRD input to force the DSG relay off when the system is supposed to be shut down. My plan is to use a pair of optoisolators to get those signals to the protection circuit without compromising its ability to isolate the battery at the low end. If I connect the transistor sides of the isolators in parallel between CTRD and protection-side signal ground and connect a 10Mohm resistor from CTRD to BATT+, will that safely allow either signal to short CTRD to ground, and only pull CTRD up to BATT+ when both signals are off?

  • 0
    TI__Mastermind 34980 points

    Hello Jeffrey,

    That sounds like it would work okay. As long as the signals are within what is shown in the datasheet EC table and in Figure 9-10. CTRC, CTRD Voltage Levels. Then you should be able to control it. In your case it seems you would pull to Vss to enable the FETs, and pull up to BAT to disable the FETs. Correct?

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Prodigy 60 points

    I believe so. I've attached the schematic below; PGND is the ground reference outside the protection FETs and GND1 is connected to VBAT- through a net tie per the EVM schematic. The SHUTDOWN signal has a pull-down resistor on a different sheet. J201 goes to the switch through pins 1 and 2.

  • 0 in reply to Jeffrey Blum
    TI__Mastermind 34980 points

    Hello Jeffrey,

    That looks okay to me, now my only comment is that to power the opto-coupler there likely would be a few mA of current to bias it ON. If that is okay, I think this should work!

    Now, keep in mind that the battery voltage is not required to get this to work, as long as it is within the ranges from the datasheet (Shown below), the FETs should be able to turn-on:

    Best Regards,

    Luis Hernandez Salomon