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BQ28400 trouble enabling FETs

Bruce Hansen
Prodigy 80 points
Other Parts Discussed in Thread: BQ28400

I'm using a BQ28400 in a new design to replace the 2 separate ICs previously used for protection & gas-gauging and I'm having some trouble with it. I have implemented the example application schematic from the datasheet almost exactly.

I connect up my pair of Li-Ion cells, then connect up my charger circuit, and I see the BQ wake up from sleep (2.7V is present on the REG27 pin).
I have PRES tied to ground through a 100R resistor.

I am able to communicate with the BQ and it seems to be returning valid data, but it stubbornly refuses to enable any of the charge or discharge FETs - all of the DSG, CHG & ZVCHG pins just remain floating high.

When I issue a Manufacturer Access command (0x00) and ask for the Manufacturer Status word (0x0006) it returns a value of 0x010a which I interpret to mean that its saying that the FETs are on and that its in Normal Discharge mode - but the gate drive out of the BQ to my FETs remains floating high.

I have tried issuing a Reset (Mfg Access 0x00, Reset 0x0041) but this had no effect.

Is there something I need to do to get the BQ to enable the FETs ?
Any help or insight would be very much appreciated.

  • 0
    Prodigy 80 points

    I've figured it out, and maybe this will be useful to someone else, so I'm posting it.

    My issue was caused by the FUSE pin (17). My circuit doesn't have a chemical fuse fitted, and I had left this pin unconnected, since the datasheet for the BQ28400 simply describes it as "FUSE - 17 - O - Push-pull fuse circuit drive" and makes no further mention of it. So why would i connect it to anything? Its clearly just an output which I'm not using ...

    However it appears that if the fuse function of the BQ is not used, it is necessary to tie this pin to ground. Yes - we must tie a "push-pull output" to ground.
    Suggestion to the datasheet writers: SPECIFY THIS IN THE DATASHEET!!!!!!!!!!!!  THIS IS NOT AT ALL OBVIOUS !!!!!!!!!!!!!!!!!!!!!!!!!!!

    I only discovered this critical necessity after digging through the Extended SBS Commands section of the Technical Reference manual where there's a little footnote at the end of the FET Control (0x46) Command description which reads "If the FUSE pin is NOT used in the application circuit, it should be connected to ground directly or the FETs will NOT turn ON."

    Please, for the sake of someone else's sanity, add this little nugget of information to the BQ28400's datasheet.