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BQ29700DSET toggels on/off

Vitalij König
Prodigy 60 points
Other Parts Discussed in Thread: TPS630250

Hello,

I do have a problem with using BQ29700DSET as cut off guard for a Li+ battery voltage at 2,8V in combination with TPS630250 Buck/Boost-Converter there are to scenarios which are unacceptable for my circuit:

1. When the Li+ battery has low charge e.g. 3V the inrush current at power on is that high that Voltage of Li+ battery drop for a small period of time under 2,8V (I saw it on scope it goes down to 0.9V) and

the BQ29700DSET switches off. Because of that I can't use the whole capacity of the Li+ battery. What can I do? I thought a about an extended soft-start could these be a right solution and if yes how I can add it to TPS630250 ?

2. When the Li+ battery reaches the threshold voltage 2,8V of BQ29700DSET it switches off as aspected but then the battery voltage recovers without load above 2,8V and I can switch on the device again these I can repeat endlessly.

  But my goal is that once the battery reach 2,8V BQ29700DSET switch off an only switch on again when the battery was charging about 2,8V and not because of voltage rising with no load.

  Is there any possibility to do that with BQ29700DSET?

thanks a lot for your advice!

  • 0
    TI__Guru 54715 points

    The TPS630250 has soft start as a feature, you might inquire in the power supply forum if its operation is not clear.

    It sounds like the bq29700 is doing what it should.  It is a very simple protector and will protect and recover as situations change.  It would be good to understand if the protection is short circuit, over current or under voltage. The delay is different for the events, recovery will be similar. The only adjustment on the bq29700 is to select a different version.  If the soft start of the converter does not solve the issue you may want to consider a more sophisticated protection solution in the bq27742.  It has limits which are adjustable and gauging is available, of course it is more complex to set up.

     

  • 0 in reply to WM5295
    Prodigy 60 points

    The protection is primary only for undervoltage of Li+ battery to stop deep discharge. I don't have a microcontroller to generate I²C and use your recommendation bq27742. As you know a Li+ battery voltage is dropping at a certain load and when it reachs 2,8V bq29700 is power off the circuit but then there is no load anymore on the battery and it recovers quickly upon 2,8V again so bq29700 powers on the circuit again.  This will repeat again and again until the Li+ battery has that low capacity that it even without load can't reach 2,8V. Even if I use for example bq297001 with undervoltage point at 2,5V I will also have the same problem right? What can I do to avoid this behavior? 

    Ideal would be for me: battery is with load dropping to 2,8V -> bq29700 cut off -> battery without load recover maybe to 3,2V -> bq29700 still cut off-> bq29700 only switch on when a charger between the Pack+ and Pack – terminals in the system is connecting. 

    please advise

    thanks a lot!

  • 0 in reply to Vitalij König
    TI__Guru 54715 points

    Other hysteresis values are not available.  It may be possible to run slightly longer by increasing the capacitance, but if the battery voltage is collapsing with load current the protector will trip and recover when voltage rises.

    Generally a system should not operate the battery beyond its acceptable limit and the protector should never trip. It seems your system should recognize the battery is spent and stop operation at a higher unloaded voltage  before the protector trips from the pulse load.  If the battery is still considered usable where the protector trips, perhaps you need a lower voltage level on the protector.  

    The bq29700 is a simple protector and does not have the decision thresholds you describe.  The more configurable protection in the gauge mentioned earlier might be an option if you had a method to set up and control it.