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BQ24195: VBUS has a almost 4.5V voltage, after adapter was plugged out

Jesse Lee
Expert 1985 points
Part Number: BQ24195

Hello

My customer is using BQ24195 as Battery Charger in his application.

And he finds an issue with BQ24195.  

1)  when he plug out the adapter,  Vbus voltage is almost 4.5V, but is not 0V.

and BQ24195's register indicates that it is still in charging mode, which is an abnormal phenomenon.

Vbus=4.5V,  BQ24195's register value list as belowed:

get_reg_00 = 0x05

get_reg_01 = 0x10

get_reg_02 = 0x98

get_reg_03 = 0x00

get_reg_04 = 0xC8

get_reg_05 = 0x8C

get_reg_06 = 0x03

get_reg_07 = 0x10

get_reg_08 = 0x84

get_reg_09 = 0x00

2) Customer forces off and reset on Q4 of BQ24195 through I2C, and BQ24195 quit out charging mode .

get_reg_00 = 0x05

get_reg_01 = 0x10

get_reg_02 = 0x98

get_reg_03 = 0x00

get_reg_04 = 0xC8

get_reg_05 = 0x8C

get_reg_06 = 0x03

get_reg_07 = 0x10

get_reg_08 = 0x00

get_reg_09 = 0x00

off adapter:STATUS_LOAD_NORMAL

could you help to solve this issue ?

Thanks very much!

Attached is customer's circuit.

dw697-cb-v1.pdf

  • 0
    TI__Intellectual 960 points
    Hi Jesse,

    I'm working to replicate this issue in the lab. We're concerned about the amount of capacitance on the VBUS line, which can cause a boost-back effect onto the VBUS line.

    Can you provide the following information:

    1. Waveform on the SW node
    2. Long-term voltage on VBUS: does it continue at 4.5 volts, or does it decay over time?
    3. Any SYS load for this test
    4. VCC input voltage for this test

    Regards,

    Parker
  • 0 in reply to ParkerHolloway
    TI__Intellectual 960 points
    Just to follow up on my previous post, this is not a suggestion that the issue is a boost-back effect. We would need more information before finding the best resolution for your undesired behavior.
  • 0 in reply to ParkerHolloway
    Expert 1985 points

    Hi Parker
    Thanks for your reply.
    and I just get below feedback:
    1. I find that i cannot attach the picture. The frequency of waveform on SW pin is 724.5kHZ, and voltage level is 4.64V.
    The frequency is 1.445MHZ when BQ24195 works normally.
    2. 4.5V on VBus always stays on , unless reset Q4 through I2C or plug out battery.
    3. There is no load on sys pin, because we don't connect sys with other circuit.
    4. VCC is 5V or 12V , the issue can be repicated at the same ratio.
    we find that the issue can be replicated much more easily when the battery voltage is at alost full status or complete full.
    THX

  • 0 in reply to Jesse Lee
    TI__Intellectual 960 points
    Jesse,

    I was unable to replicate the customer's issue as described above.

    I applied a similar capacitance as well as identical configuration of the registers and setup pullups/pulldowns, and could not create a situation under which the SW node continued to switch past the disconnect when disconnecting from the adapter at either 5 or 12 volts.

    I would recommend:

    1. Reduce the capacitance at VBUS and PMID. For reference, on the EVM, these values are 1uF for the VBUS, and 9uF on PMID.
    2. Reduce the resistance on the BTST pin, in the schematic it is shown at 33 ohms. For reference, the value on the EVM is 0 ohms.
    3. Applying the input voltage directly to the VBUS test point on the circuit to rule out any issues originating from the supply portion of the circuit.

    Parker
  • 0 in reply to ParkerHolloway
    Expert 1985 points
    Parker
    Thanks for your suggestions.
    I will do the test based on your suggestions.

    And I want to know what the voltage level of battery is when you are doing the test.
    we can replicate the issue easily when the Battery Voltage level is 4.1V or above than 4.1V.
  • 0 in reply to Jesse Lee
    TI__Intellectual 960 points
    Jesse,

    My battery voltage ranged from approximately 4.1 to 4.3 volts during the test.

    Parker
  • 0 in reply to ParkerHolloway
    Expert 1985 points
    Hi Parker
    Good news!
    we fix the issue after we change the value of resistance from 33phms to 0 ohms.
    Thanks very much for your help!

    And is it the boost-back effect, or any other root cause ?

    what is the effect on Q1 or other relative circuit inside the BQ24195 when the resistance value is too much ?

    Thank you very much !
  • 0 in reply to Jesse Lee
    TI__Intellectual 960 points
    Jesse,

    Glad to hear it.

    This particular portion of the device and circuit are the bootstrap used to refresh the HSFET part of the switcher. Using a resistor here both causes an unwanted voltage drop on the bootstrap circuit and also keeps the circuit from being able to refresh as required.

    Parker