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WEBENCH® Tools/TPS63805: Soft start, ramp time

user201583
Prodigy 140 points
Part Number: TPS63805

Tool/software: WEBENCH® Design Tools

Dear Sir or Madam

I want to increase the capacity of the output capacitor (Cout) to increase the ramp time of the TPS63805.

Let me ask you three questions:

1. "Soft-start, Current limit ramp time (Tramp) = 224us typ." is written in 8.5 Electrical Characteristics of the data sheet (SLVSDS9C).

  Is this the value when Cout = 22uF?

2. In section 9.3.5 of the datasheet, "The larger the output capacitor, the longer it takes to stabilize Vo."

  Can you tell me the relationship between the capacitance of Cout and the ramp time?

  (For example, if Cout doubles, does Tramp also double?)


3. Is the upper limit of Cout 100uF?

Thanks,

Hasegawa

  • 0
    TI__Expert 7925 points

    Hi Hasegawa,

    Thank you for reaching out. To answer your questions:

    1. The current limit ramp time is fixed and does not depend on the output capacitance.
    2. The current limit ramp time is fixed. The current ramps up to the switch current limit, and stays there as long as the output voltage does not reach the set voltage. The output voltage ramp-up time depends on the input and output voltages and the output capacitance. There is no simple relation between these parameters, since the converter can change mode during the output voltage ramp-up. What are your input and output voltages?
    3. There is no limit for the output capacitance. Higher capacitance will increase the output voltage ramp-up time, and make the transient response of the converter slower.

    Best regards,
    Milos

  • 0 in reply to Milos
    Prodigy 140 points

    Hi Milos-san,

    Thank you for your answer.


    The input voltage is 3.3V +/- 10% and the output voltage is 3.3V.
    TPS63805 is used to stabilize the output voltage against fluctuations in the input voltage.
    In order to match the voltage ramp rate specification of the device connected to the output of TPS63805, I want to make the ramp-up time of the output voltage about 1ms / 0 to 3.3V.

    Best regards,
    Hasegawa

  • 0 in reply to user201583
    TI__Expert 7925 points

    Hi Hasegawa,

    To extend the soft-start time, please refer to this application note: 

    http://www.ti.com/lit/slva307

    The additional circuit described in the app note extends the startup time and reduces the inrush current.

    Best regards,
    Milos

  • 0 in reply to Milos
    Prodigy 140 points
    Hi Milos,

    Thank you for introducing the application note.
    I would like to try.

    Best regards,
    Hasegawa