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TPS61175 – About Soft Start Function

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Guru 21045 points
Other Parts Discussed in Thread: TPS61175

Hi,

 

My customer would like to use TPS61175 with following condition.

 

[Condition]

-Input Voltage (Vin) =7.0V

-Output Voltage (Vout) =12V

-Output current (Iout) = 1.0A

-Load capacitor (Cload) = 3300uF

-Soft Start capacitor (C3) = 1uF to 4.7uF

 

They are going to use the large C3(1uF to 4.7uF) to control the rush current.

Please let me know the following questions.

- What is the maximum value for C3?

- Do you have the calculating formula of the soft start time (Tss)?

 

Regards,

Kanemaru

  • 0
    TI__Mastermind 46905 points

    1. no limit at value of C3, but when turn off the device, a long time may be taken to discharge the C3 before next startup.

    2. the soft start method is to limit the inductor current. so Tss relate to the output capacitor and load condition. i don't the formula to calculate this time.

    maybe you can apply an EVM to measure the Tss and inrush current. thanks

    thanks.

    jasper li

  • 0 in reply to Jasper Li
    Guru 21045 points

    Hi Jasper-san,

     

    Thank you for the information.

    Ｉunderstood. thank you very much.

     

    Regards,

    Kanemaru

  • 0 in reply to Hide
    TI__Intellectual 1305 points

    Hi Kanemru,

    If you use a large ss cap (larger than 1uF), there is a small design bug in the circuit. If EN is low, then the ss pin voltage will not discharge to 0 and will remains at 0.5V. It will lead to large inrush current when you start up again. Use a resistor (about 300kohm) in parallel with the SS cap can discharge the remaining voltage.

    BR

    Gene

  • 0 in reply to Gene Lu
    Guru 21045 points

    Hi Gene-san,

     

    Thank you for your advice.

    We will report the information to my customers.

     

    Regards,

    Kanemaru