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BQ25628E: Slow start timing

Sake Kroondijk
Prodigy 20 points

Part Number: BQ25628E

Can you be a bit more specific on the slow start timing? How long does it take to reach the full load input current?

  • 0
    TI__Mastermind 34330 points

    Hello, 

    Thank you for reaching out via E2E. Can you help to point to the page/section of the datasheet you are referring to in your question? I want to ensure I am answering about the appropriate feature. Are you possibly asking about converter 'soft start' rather than 'slow start'?

    Best Regards,

    Garrett 

  • 0 in reply to Garrett Kreger
    Prodigy 20 points

    Hello Garret,

    Sorry for the confusion. Yes, I am referring to the soft start timing as mentioned in the datasheet section 9.3.3.5.

    Best regards,

    Sake 

  • 0 in reply to Sake Kroondijk
    TI__Mastermind 34330 points

    Hi Sake, 

    Thank you for the confirmation. Please see my explanation below. 

    The BQ25628E buck converter provides soft start at initial start up of switching. This feature is most relevant when the battery is not connected or completely discharged. When a charged battery is connected it will initially provide power to SYS output then after VBUS input is connected SYS power source will seamlessly transition from BAT to VBUS. 

    When you attempt to power up the device from VBUS with no battery and SYS initially at 0V (or any value below VSYS_SHORT) SYS current draw must initially be less than 500mA. If the SYS current draw is initially too high SYS output voltage will not be able to ramp up as expected. 

    Best Regards,

    Garrett 

  • 0 in reply to Garrett Kreger
    Prodigy 20 points

    Thank you Garret.

    I am more concerned about the VBUS input current behaviour at start up. In my application the battery is always connected.

    The input VBUS supply is a wireless Qi power receiver. This receiver should initially not be loaded with a current >500 mA @ 5V and the output should smoothly ramp up to full power. I admit that 'smoothly' is a weak requirement, but can we expect a smooth input current rise from the BQ25628E? Can you provide a specification for Input current ramp up in A/sec or msec soft start from start to full input current?

    Best regards,

    Sake    

  • +1 in reply to Sake Kroondijk
    TI__Mastermind 34330 points

    Hi Sake, 

    There is not a specification on the input current ramp up. My best recommendation would be to initially set the input current limit to 500mA to ensure the BQ25628E does not attempt to pull too much current. Then increase the limit to your desired level for charging at a later time. 

    For your reference I have taken waveforms of input current ramp up with all registers set to default ( input current limit set to 3.2A & ICHG set to 320mA) and a 500mA load at SYS. Please see below. 

    -2nd waveform is a zoom in of the 1st step up in the above capture. 

    Best Regards,

    Garrett 

  • 0 in reply to Garrett Kreger
    Prodigy 20 points

    Thank you Garret. We will follow your recommendations.

    Best regards,

    Sake