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TPS55340: TPS55340 Softstart calculation

Wei-Hao Chen (TW-FAE)
Expert 3260 points
Part Number: TPS55340

Hi team,

Could you please provide equation of how to calculate soft-start cap and soft-start time for TPS55340 which is not specified in the datasheet. Any limit on the biggest value of Css can be used? 

Thanks.

Best Regards,

Wei-Hao

  • 0
    TI__Mastermind 34231 points

    Hi Wei-hao,

    Thanks for reaching out to us.

    TPS55340 has a built-in soft-start circuit. When the IC is enabled, an internal bias current source (6uA, typical) charges a capacitor (Css) on the SS pin. The voltage at the capacitor clamps the output of the internal error amplifier that determines the peak current and duty cycle of PWM controller. Once the capacitor reaches 1.8V, the soft-start cycle is completed and soft-start voltage no longer clamps the error amplifier output.

    As SS pin voltage is clamping the COMP  voltage during soft-start phase, the steady state COMP voltage after soft-start is related to output current, duty-cycle, so even with same SS capacitor, output capacitor, the soft-start time varies with different load current, Vout.

    A 47nF ceramic capacitor could be selected firstly and increase or decrease this cap value to get the desired soft-start time depending on the bench test result.

  • 0 in reply to Zack Liu
    TI__Expert 3260 points

    Hi Zack

    Thanks for your reply. In order words, if it's only simple current source and will clamp the COMP pin until 1.8V. 

    Then tss = 1.8V * Css / 6uA. This is correct?

    Thanks.

    Best Regards,

    Wei-Hao

  • 0 in reply to Wei-Hao Chen (TW-FAE)
    TI__Mastermind 34231 points

    Hi Wei-Hao,

    No. TPS55340 soft start circuit only clamps the COMP pin voltage during startup hence the inductor peak current will not be high.

    But the soft start time cannot be calculated using the equation. The soft start time is related to load current and needed steady state duty cycle. Customer could do bench test and then increase or decrease soft start cap.