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BQ25703A: The current limit set is not pricise

HongRen Chang
Intellectual 390 points
Part Number: BQ25703A

When the chip work under OTG mode, the otg current is set to 3.6A. I set the electronic load to 3A. It works well under OTG voltage is 5V and 9V and it fails under 12V. It works at the beginning but fail after a little later. The voltage keep dropping and drop to 0 at last. When  I enlarge the otg current to 4.2A. It works well under 12V. Why the current limit error is so big? What can I adjust?

Thanks

BR

Ren 

  • 0
    TI__Guru 56650 points
    The electronic load's input capacitor need to be charged up. That current also count in the 3.6A. If the electronic load holds the OTG output voltage lower than 85% of the setting point about 7ms, charger will quit from OTG mode. When you increase the setting to 4.2A, it has enough output current capability to support load and bring the electronic load's input capacitor voltage up in 7ms.
  • 0 in reply to Wang5577
    Intellectual 390 points

    Dear Wang,

    I think you  mean the transient current is big when connect to the electronic load. It is a little different from my test. At the beginning, it is stable. It seems the chip runs several second and then it become unstable. I'm guessing if the loop parameter need to be changed under big current.

    And more, I think the 4.2A setting is too big for my 3A application. Is there some test can be done to verify it is caused by the load's capacitor?

    This is the wavefrom for 4A@20V. At the beginning, it is 20V. It change to this after a while. And at last it drops to 0. I notice the board temperature rise to 100 degree. Is the chip too hot and become unstable?

    Thanks

    BR

    Ren

  • 0 in reply to HongRen Chang
    TI__Guru 56650 points
    What is the battery voltage? The EVM should support 4A OTG without reaching 100 degree if the battery voltage is not too low.
    bq25703A has a very good charger status to help us debug an issue. Would you check the charger status register? Does it report any abnormal condition?
  • 0 in reply to Wang5577
    Intellectual 390 points

    Dear Wang,

    I have test the EVM board. It is about 70 degree under 4A OTG mode. When I add the fan to my board, it also works well for 4A and 5A OTG mode. My board size is only quarter of EVM board. I think the heat cannot dissapate fast and then cause the board is very hot. When I obeserve the OTG voltage shift, the SW is already not fixed. The chip works unstable.

    The battery voltage is 16V on my board. This is no problem.

    I found the current limit issue is related to sample resistor. The resistor has a big error.

    Thanks

    BR

    Ren