• State
  • Locked Locked
  • Replies 1 reply
  • Subscribers 63 subscribers
  • Views 304 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

BQ77PL900 Wake up in SCD?

Travis Pollok
Prodigy 110 points

So I have the circuit attached to 9 cells and woke it up by applying 20V across the PACK terminals momentarily.  When I ask for the status byte, I receive 00000001.  Going to the datasheet this corresponds to it being in current protection to to a short circuit in discharge. 

I then tried to recreate this result using a different board and using the dummy resistor batteries we have.  I tried just shorting the PACK terminals to see if that had an effect but was glad to see it didn't turn on.  So I turned it on using the 20V across the PACK terminals momentarily and got the same status byte. 

So first question is, would applying a power supply of 20V across the PACK terminals wake the part into a short circuit in discharge mode?

After that we turned off the dummy batteries and turned them on again.  I wanted to see if the chip resets out of SCD after shutdown mode.  So this time I attempted to wake it up the usual way of shorting it momentarily with a long wire across BATTERY+ and PACK+ but this time the BQ chip popped and smoked into what I assume is to an unusable state. 

Second question, did I do something wrong here and why is this protection circuit such a pain to work with?

  • 0
    TI__Guru 54715 points

    #1: With a 9 cell pack, I would guess your battery voltage is in the 27 to 36V range.  When you momentarily connect 20V from a supply to the pack side, the PL900 will wake up and turn on FETs.  Most likely there is a current surge into the filter caps of your 20V source as the higher voltage battery forces current which causes the SCD event.  I typically must use a diode when doing this or the cells drive current into the bench supplies I typically use causing sizzle/smell/smoke from the supply and it must be replaced.

    #2: If you connect a wire from the battery+ to the pack+, there may be a current surge into a load filter capacitor (like on the EVM), but this may be lower or shorter than the SCD event limits, so there may not be a protection event.  There might be a spark if you don't use a series resistor, but there should be no damage.  I don't recognize anything wrong from your description, certainly a pop and smoke from the IC is unexpected and bad.  You might review all connections carefully to see if there are unexpected connections, extra wires or multiple ground connections.  The IC will likely need to be replaced. 

    Unfortunately I have made a wrong connection error repeatedly, I find many things hard until I understand and am familiar with them.