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TPS54302: Questions about rush current and output capacitor

Kaji@PAN
Mastermind 7145 points
Part Number: TPS54302
Other Parts Discussed in Thread: TAS5132

Hi,

I confirm the operation of TPS54302.
I want to ask two questions.

(1) Rush current is generated when the current at startup is measured.
    Is overcurrent limitation functioning even at startup? Some devices I used previously do not detect overcurrent at startup.

(2) TPS54302 is supplying VDD of TAS5132.
    There is a filter in VDD, but does this filter affect the operation of TPS54302?
    The bulk capacitor C36 in the attached figure uses two 4.7 uF OS capacitors.

Best Regards,

  • 0
    TI__Expert 5745 points
    1)I don't think the current will be wake up early than the start-up.
    2)What't the output cap of the TPS54302? in your design,does the C36=4.7*2 uF, not the 22uF in the figure? If it is, i think that's ok about this design.
  • 0 in reply to Vental Mao
    Mastermind 7145 points

    Thank you for your reply.

    I will correct the contents of (1). In my measurement the inductor current is suddenly increasing during soft start. See attached below.
    Although the measurement result is 1.14A, if this current increases and reaches the current limit value, will the current limit operate?

    The output capacitor of TPS54302 uses 100uF ceramic capacitor.
    The last attached waveform was excerpted from the EVM data on TAS5132.
    So, my circuit uses two 4.7uF OS capacitors.

    Best Regards,

  • 0 in reply to Kaji@PAN
    TI__Expert 5745 points

    1)  This IC has a internal soft-start cap, and during the soft-start, the output voltage ramp up to the Vref, then the soft start stop working, Vout is determined by the Vref and FB divide resistor. 

    2) 100uF for the TPS54302 output cap is too big, 44 uF is enough. And the 22uF is also OK. So you can refer to the DS to choose the value of the TPS54302. 

  • 0 in reply to Vental Mao
    Mastermind 7145 points
    Thank you for answering.
    I understood the contents of (1).
    However, is there a possibility that IC may be destroyed if rush current flows during soft start?
    I used a product that does not work with current limit at startup. At that time the IC was destroyed by the rush current.
    Best Regards,
  • 0 in reply to Kaji@PAN
    TI__Genius 15135 points

    Hi, Kaji

    1. The inrush current is only 1.14A < OC limit (5A), there is no OCP happen.
    The inrush current can be evaluated by below equation:
    Iinrush * Tss = Vo * Cout + Ioad * Tss
    When Cout is big, the Iinrush will be big too.
    So reducing Cout can reduce the inrush current. 

    However, I think the Cout=100uF is ok, no problem.
     
     
    2. The OCP function is active during soft startup and i think there is no IC damaged risk.

    3. The filter cap C36=4.7*2 uF is no problem.