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Original question:

TPS22971: Is these circuit can get linearly rampup waveform?

TPS22971: Input capacitance guideline when load switch turn on

Toshihiro Watanabe
Genius 10235 points
Part Number: TPS22971
Other Parts Discussed in Thread: TINA-TI

Hello,

In below post, adding input capacitor of TPS22971 helps the voltage drops due to inrush current when trun-on. Is there any guidance how much input capacitor would be good? I think there is input capacitor value and output load capacitor value relationship. 

https://e2e.ti.com/support/power-management/f/196/p/750791/2773844#2773844?cm_session=7511e2c5-7f96-436d-b5b1-3c8ba0f1e619&cm_type=link&cm_link=0aa7e5fd-8474-4ac4-8924-8a27b3d4db96&utm_campaign=7511e2c5-7f96-436d-b5b1-3c8ba0f1e619&utm_medium=email&utm_source=contactmonkey&utm_content=sales%201%3a1&utm_term=RE%3a+%E5%88%A5%E4%BB%B6%EF%BC%9A%e3%80%80PoE%E5%90%91%E3%81%91%E9%9B%BB%E6%BA%90%E3%82%92%E6%A4%9C%E8%A8%8E%E3%81%97%E3%81%BE%E3%81%99%E3%80%82

Best regards,

Toshihiro Watanabe

  • 0
    TI__Genius 10855 points

    Hi Watanabe-san, 

    Thanks for following up on E2E!

    We usually recommend a 10:1 ratio between input capacitance and output capacitance. In other words, the input capacitance should be 10X the output capacitance.

    However, this only really becomes an issue when the output capacitance is high. There are two potential concerns to think about if COUT > CIN:

    1. Normally, the CIN capacitor is used to help the power supply handle large amounts of inrush current. During startup, if CIN isn’t large enough, the sudden change in current can cause the input voltage to sa
    2. If VIN suddenly drops while the device is enabled, there could be a brief moment where VOUT > VIN. If this happens, this could potentially bias the body diode inside the device and cause reverse current to flow upstream. Will there be a condition where VIN could suddenly be removed while the device remains enabled?

    Thanks,

    Arthur Huang

    To find the latest information on Power Switches, visit www.ti.com/powerswitch

  • 0 in reply to Alpha H
    TI__Genius 10235 points

    Hello Arthur-san,

    For #2, its case doesn't occur.

    For #1, Load C is big so may be difficult. We would like to check the behavior by simulation. Pspice model is available but it seems not to be able to use TINA-TI.

    Is there any way to simulate the turn-on behavior?

    Best regards,

    Toshihiro Watanabe

  • 0 in reply to Toshihiro Watanabe
    TI__Genius 10855 points

    Hi Watanabe-san, 

    Are you able to use the TINA Macro Wizard tool to convert the PSpice model into a TINA file? 

    If not, could you comment more on the load / output capacitance? How much output capacitance is in the system? What is your upstream PSU?

    Thanks,

    Arthur Huang

  • 0 in reply to Alpha H
    TI__Genius 10235 points

    Hello,

    I think the acro Wizard tool is below but error occurs because of Encrypted. If simulation is not possible, I will discuss this to the customer.

    Best regards,

    Toshihiro Watanabe

  • 0 in reply to Toshihiro Watanabe
    TI__Genius 10855 points

    Hi Watanabe, 

    Feel free to email me at a-huang@ti.com, I might have a work-around. 

    Thanks,

    Arthur Huang