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BQ76PL455A: BQ76PL455A Design issues (?) . FAULT_N pin is always low

user4911928
Prodigy 20 points
Part Number: BQ76PL455A

Hello, i have kinda a lot of problems with the design. First of all i get around 4 volts from the npn transistor output so i decided to supply VCC rail via an external supply. But the thing is that FAULT_N pin is always driven low. So im kinda rookie and i wonder what the issues of the design are. I would be glad if anyone helps me out . The schematic is in the attachment

/cfs-file/__key/communityserver-discussions-components-files/196/5826.BMS.PDF

  • 0
    TI__Genius 11480 points
    Hi,

    If you are getting 4V at the VP output, then the IC would likely not work correctly. Have you been able to read any of the fault registers, to indicate where the problem may be?


    Thank you for sharing the schematic.. that helps things move along more quickly. One potential issue i see is there there are no pull up/pull down networks on TX/RX/Wakeup/FaultN. I strongly recommend adding those.

    NC1 and NC2 both need to be left floating. If i am understanding correctly, you are tying them to VP. Specifically, tying pin 36 of the IC (a no connect pin) to VP can problems, and may have damaged the device.

    If you are a rookie (even if you are not a rookie! these devices are big and complicated) , I would suggest investing the time up front to read through the documentation: www.ti.com/.../technicaldocuments (specifically the design considerations document). There are also application hints in the datasheet that i would suggest looking at.
  • 0 in reply to David Wiest Jr
    Prodigy 20 points
    Hi,

    Thanks for the very quick response. NC pins are looking like they are connected to the logic voltage power rail but actually they are not connected anywhere in the layout. That was a quick solution for the moment that i realized after i finished constructing the ic for the particular cad software that i use , I selected NC pins as power rails (silly mistake) so software was giving errors to tie them to the power rail. To sum up, they are floating. I'll try pulling-up/down the comm pins and then try to read fault registers again.

    My best regards
  • 0 in reply to user4911928
    TI__Genius 11480 points
    Hi,

    Understood, thank you for clarifying.

    Roughly how much current are you pulling through the VP regulator? What else is attached to that node?

    I am also concerned that i don't see any power on the VIO pin. Typically, this is connected to the same 3.3V or 5V supply that powers the MCU, and the UART pins are pulled up to VIO. You may crash the CVRef pin, depending on how much current you pull from it. I am not familiar with that MCU, but i think it is typical for voltage reference pins to only be brought out for decoupling, and only used internally in the part.