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BQ24780S: Current limit characteristics

Part Number: BQ24780S

Our customer is working on BQ24780S and testing the current limit characteristics.

See an attached file.

/cfs-file/__key/communityserver-discussions-components-files/196/2043.BQ24780S--Current-Limit-Characteristics.xlsx

 

Test Condition

Vin = 19V

Load : Electric load CCCV mode 10V / 4A

BQ24780S setting :

           Charge Voltage = 14.8V

           Charge Current = 1024-3520mA

 

When the external current limit is enabled, OCP gradually seems to work from 2496mA setting point, while the customer sets OCP to 3A.

Is it correct behavior ?

 

よろしくお願いします。

Thanks and Regards,

Metropolitan (Tokyo) Area FAE  Kenji Ninagawa

  • If the ILIM pin sets the charge current at 2.4A, the charging current should not go to 3A.
    What voltage is on the ILIM pin?
  • Could you see an excel file I attached in previous post, which has also customer's circuit ?

    According to the circuit, ILIM pin voltage = 0.6V, that is, 3A OCP should be set.

  • From the schematic, I don't find any issue. To debugging this:

    1. Force ILIM voltage is other lower or higher value to see the current limit at same point or different. This can tell us does the ILIM regulation fully control the charging current or not? If not, we need check what other factors limit the charging current? What is the charge current register setting?

    2. If ILIM pin fully control the current, I want to run charger into turbo mode and monitor IDCHG pin to check if VSRP-SRN current sensing has any issue or not?

  • Could you see again the excel file I update, please?

    This file originally has also register value.

    /cfs-file/__key/communityserver-discussions-components-files/196/8004.7888.BQ24780S--Current-Limit-Characteristics.xlsx

    Then, the graph of “Setting charging current vs Measurement charging current” is drawn.

    In the graph, you can see some differences between Disable and Enable for external OCP setting.

    At the Enable condition, when ILIM=3A, at around 3A, there is larger difference between setting and measurement, while disable condition has no difference.

    Why does this difference occur ?

    Thanks and Regards,
    Metropolitan (Tokyo) Area FAE  Kenji Ninagawa

  • Ok. Understand now. The ILIM pin current limit has soft knee. That is what you see: with 3A ILIM setting, the real charger current drops from 2.496A 0x14H setting.

    This is because the ILIM current limit control is an analog circuit and diode OR with several other control loop. When the charging current is close to the set point, the ILIM buffer signal higher enough to make the diode pass a small current to the loop and limit the current.

    Thanks,
    Wang