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bq77908a Discharge Overcurrent and Discharge Shortcircuit Not Working

Paul Yarr
Prodigy 50 points
Other Parts Discussed in Thread: BQ77908A, BQ24630

I am having an issue with a bq77908a. I am testing a design that is heavily based on the bq77908a EVM schematic but with a series FET configuration. There have been mistakes in copying from the EVM schematic, so there many be more I haven't found yet.

The issue I'm having is that the bq77908a does not seem to detect over current or short circuit in discharge. I have set OCDT to 25mV (across a 10mOhm sense resistance implies 2.5A threshold) and OCDD to various times, currently set to 2000ms. I have enabled polling of the device using the companion software to monitor the status flags. Polling is at 5 second intervals such that there is minimal interruption from the ZEDE signal.

I have connected the battery pack output to an electronic load initially set to 0A constant current. As I increase the current load and using an oscilloscope I am able to see that the voltage across the sense resistance increases as I would expect with current. However, with the sense voltage at a level constantly above 25mV, the bq77908a does not detect overcurrent in discharge to shut off the CHG and DSG FETs.

Performing the same test, but increasing the load over the short circuit threshold (50mV => 5A) I get the same result, the fault is not detected.

However, if I simulate a dead short, the chip recognises this as a discharge short circuit and turns off the FETs accordingly.

Can anyone suggest a possible reason for this behaviour? 

  • 0
    TI__Guru 54715 points

    Be aware of the items in http://www.ti.com/lit/pdf/slua612 with series FETs

    If current detection does not occur at the threshold either it is not connected right, not set right (it operates from the EEPROM) or the part is bad.

    If you poll during normal operation you will have problems. Perhaps not on the first board you test, but eventually.  Polling is disruptive to normal operation.  Polling requires set of ZEDE which makes the part respond with minimum delay, one sample of noise can trip a fault. 

     

  • 0 in reply to WM5295
    Prodigy 50 points

    Thank you for the reply.

    I have read through the app note for the series FET configuration.

    I have had the part replaced in an attempt to rule out a bad part, this has not yielded any different results.

    I believe I have set the EEPROM correctly for the sense resistance. For, a sense resistance of 10 mohm, a OCDT of 25mV implies an over current threshold of 2.5A. I am using the software provided by TI and a Total Phase Aarvark to interface to the EEPROM.

    Using an oscilloscope across the sense resistance I can see the correct voltage relative to current. To use the reference designators on the EVM board (http://www.ti.com/lit/ug/sluu854/sluu854.pdf) as a reference, if I connect the oscilloscope across C22 I do not see the same voltage as I do across the sense resistance. I have put this down to oscilloscope probe loading, would you expect this to be the case?

    I understand that polling the ZEDE would cause problems for normal operation. However, in my case I do not get any over current detection, surely if the ZEDE signal is high and therefore delay is minimal this would help the over current detection to detect?

     

  • 0 in reply to Paul Yarr
    Expert 1085 points

    I had a similar problem in testing the bq40z50EVM (gas gauge): the charging current to the battery pack was about 300 mA less than the programmed charge current from our charger (bq24630).

    I was also using an oscilloscope to monitor some voltages. It turned out that the oscilloscope and the PC connected to the EV2300 formed a ground loop, and my missing 300 mA was going through the scope lead ground connection!

    Try measuring the current through your scope's ground lead (assuming your scope is also AC powered) to see if it's bypassing some current through it. If it is, an isolation transformer on the scope should help.