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TPS1663: How to derive the Cdv/dt calculate formula and Why SR maintain constant during power up?

Stanley Xu
Intellectual 1620 points
Part Number: TPS1663
Other Parts Discussed in Thread: TPS22965

Hi engineers,

I have some questions about  Cdv/dt calculation：

Formula show as blow:

A application case design requirement in datasheet show as blow:

calculation result is Cdv/dt =22nF, 

question1：What is the detailed and complete calculation process to figure out 22nF？

The IC structure diagram show as blow 

question2：How the Cdv/dt maintain fixed dv/dt(slew rate) during power up process？(such as :what's the function of 2uA CCS and 25 gain section)

question3：As my observes, the Cdv/dt is  similar with Ct in load switch, So do the two things have the same function?

question4：How to deduce SR formula of TPS22965?

                    

  • 0
    TI__Intellectual 1620 points

    Correct picture, the application case design requirement in datasheet show as blow:

  • +1 in reply to Stanley Xu
    TI__Guru** 105171 points

    Hi Stanley,

    A1) During startup, the device controls the GATE voltage in proportional to the voltage on the dVdT pin

    It uses simple I=C*dv/dt relation

    Iinrush = Cout *Vout/tdVdT  (at the end of power-up Vout = VIN)  => Iinrush = Cout *VIN/tdVdT   -- (1)

    tdVdT = 20.8 × 10^3 × VIN × CdVdT  ---(2)

    Equation (2) in the data sheet is arrived by design and by doing the curve fitting the results.

    Now with 100uF (Cout) and to limit Iinrush to 300mA 

    Using (1) & (2), we get CdVdT = 16 nF => selected 22nF nearest higher value 

    A2) During startup, the GATE is controlled as source follower to maintain constant slew-rate of the voltage 

    A3) Yes, ct in load switch and CdVdT in eFuses are similar in function

    A4) These equations are arrived by design and by doing the curve fitting the actual results.

  • +1 in reply to Rakesh
    TI__Intellectual 1620 points

    Hi Rakesh, I'm TI fae NCG, can I get your email address to discuss understanding about the feature of dv/dt start-up which I'm very curious about.

  • +1 in reply to Stanley Xu
    TI__Guru** 105171 points

    Hi Stanley,

    We are not fully aware of internal design details. Sure, you can reach me on email to discuss further.

    Best Regards, Rakesh