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BQ2002T Fast Charge Termination Issue

EDY19
Prodigy 40 points
Other Parts Discussed in Thread: BQ2002T, BQ2002

I've been developing a charger for a pack of NiMH batteries using the BQ2002T and a switching constant current boost converter.  The charge current is 600mA (~C/4 for 2300mAH AA Batteries).

I'm having issues with the charge terminating prematurely.  I have the TM pin set to VCC/2, so it should be a 320 minute charge cycle at C/4 with a top off and trickle cycle.  

The problem is, after the batteries have been charging for <1 hour, the charge will terminate.  I've monitored the TS (temperature) input of the BQ2002, and it stays right around 1.69V for the entire time so I'm pretty sure I can eliminate this as the problem

My initial thought was that the switching converter was causing enough noise on the battery line for the voltage to intermittently spike past 2V and terminate the charge once the cell voltage climbs up a bit, however the charger is going in to Top-Off mode after terminating - verified by seeing that the CC output is high for 286us and low for 4.2ms.  Looking at the state diagram for the BQ2002T, it appears that a battery voltage over the MCV of 2V should send the charger to the Trickle State (on for 286us and off for 17.9ms).  Looking at the voltage on the BAT pin with the oscilloscope shows intermittent spikes passing MCV (2.0V), but the state diagram seems to indicate that this isn't my problem?

Any help is appreciated!  Thanks!

  • 0
    TI__Mastermind 31120 points

    The bq2002T should have a 1uF decoupling cap between VCC and ground and also a 100k series resistor going to the TS pin with a 0.01uf capacitor between TS and ground.

    Make sure there is good filtering for the IC and also the TS pin.

  • 0 in reply to Charles Mauney
    Prodigy 40 points

    Charles,

    The IC does have a 1uF ceramic decoupling capacitor, although it is further than optimally placed from the IC - it's always easy to find layout errors after you find a problem!

    As for the TS pin, There is a 22k pull up to 5V and a 20k parallel resistor with the thermistor that I'm using in order to set the temperature limits at appropriate levels.  However, there is no capacitor on the thermistor line in parallel with the 20k resistor.  Do you suspect that these issues could be the cause of exiting the fast charge cycle?

    Thanks,

  • 0 in reply to EDY19
    TI__Mastermind 31120 points

    It is important to filter the TS pin properly since the termination is sensitive to millivolts.

    See this link for a schematic:  http://www.ti.com/lit/ug/sluu007b/sluu007b.pdf