• State Resolved
  • Locked Locked
  • Replies 5 replies
  • Subscribers 63 subscribers
  • Views 1628 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

BQ2002T Unknown state between Fast Charge and trickle

David Mittmann
Prodigy 130 points
Other Parts Discussed in Thread: BQ2002T, LM317

I have 6 BQ2002T's idependently charging 6 9V NiMH's.  Sometimes the CC pin will go from Fast chage (Steady VCC) to some randomly pulsating state  and either stay in that state, or some time later going to trickle chage (286us pulse everly 18.3ms).  I can cycle the power on the device and that same chip might not go into this non periodic pulsing state.  Any ideas on this wierd state? Sometimes it only lasts a few seconds long, other times many minutes.  I assume it is a filtering issue somewhere.;

I have 10.01uF on VCC, 100nF on TS, 100nF on BAT.

  • 0
    TI__Mastermind 31120 points

    Does it happen if only one charger is operating at a time?  If so, you may need some isolation between the chargers.

     

  • 0 in reply to Charles Mauney
    Prodigy 130 points

    If I power cycle the board sometimes the misbehaving channel will work. I have not removed 5 of the six chips to see if that solves the problem. You may be right, I was leaning towards some noise/ interference problem.  Do you think higher value decoupling capacitors might help? Maybe some extra low pass filters on the sense pins (TS, Vbat)?  Do you know the minimum current the Vbat and TS pins need? For Vbat I have a 400k, 100k devider, I was wondering if this current was to low.

    Thanks for the feedback.

  • 0 in reply to Charles Mauney
    Prodigy 130 points

    I populated a board with just one bq2002T, trying to charge one 9V battery.

    It looks like it is a threshold problem with the combination of the bq2002t and the battery. If the battery doesn't fully charge past the 2V level it gets stuck in this mode (measuring pin 8 CC).  I have made the theshold for these 9V batteries from 10.4 to 9.5 and always some batteries will get the BQ2002T stuck in this mode:

    If the battery can charge higher it will get to trickle mode, but some batteries won't and the bq2002T will stay in this mode forever (well overnight anyway).

    This test was done with only one of my six channels populated on the board.  This is a really simple design and board shown below:

    If I drain that battery a bit the bq2002T cc pin 8 will look correct at a constant 5V, but when the battery reaches the Vbat threshold it will go haywire.  If I try a few different batteries some will charge to trickle and some won't.

  • 0 in reply to David Mittmann
    TI__Mastermind 31120 points

    A good termination requires constant current up to the termination threshold.  It is always good to try a little higher and lower input voltage to see how this effects the charge margin.

    There should be a pullup resistor from the input to the battery of about 10k.  I do not see this in your circuit.  Refer to the UG.

    http://www.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=sluu007b&fileType=pdf

  • 0 in reply to Charles Mauney
    Prodigy 130 points

    It looks like the constant charging current from the 10k resistor may help a bit to eliminate the undetermined state between fast charge and trickle (or top off), possibly by adding another 1mA to the up and down charging current from the unstable CC pin.  But if the batteries happen to be at just the wrong voltage when charging is initiated, it does cause the bq2002T to stay in the oscillating state for some time. I'm not sure why increasing or decreasing the input voltage would affect anything. I already have 15 volts to the LM317 and even with several diode drops this should have plenty of clearance for the 9 volt batteries.

    Thanks for you help.