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TPS25982: Capacitor inrush current

Hans
Genius 12865 points
Part Number: TPS25982

Hello Team,

with a target slew rate of 0.2V/ms I calculate a capacitance of 23nF (used 22nF, C0402C223K5RACTU) but measured 110V/ms.

Even with 4.7nF I got 40V/ms which is far faster than 1.4V/ms with 3.3nF (see below):

Can you explain this, please? Is this part useable for USB3 power? Something is wrong here, right?

Thanks and Best Regards,

Hans

  • 0
    TI__Guru** 105361 points

    Hi Hans,

    Looks unusual..

    What is the Cout ? Can you share startup waveform with Vin, Vout, Input current , EN voltage

    Are you observing similar slew-rates for multiple devices ?

    Best Regards,

    Rakesh

  • 0 in reply to Rakesh
    TI__Genius 12865 points

    Hi Rakesh,

    Cout = 120uF electrolytic capacitor (10V) for Vout = 5V.

    Vout drops when connecting a USB stick/flashdrive. Therefore the idea was to limit inrush current. Screenshot not yet available.

    Thanks and Best Regards,

    Hans

  • 0 in reply to Hans
    TI__Guru** 105361 points

    ok Hans. Will wait for the scopeshots.

    BR, Rakesh

  • 0 in reply to Rakesh
    TI__Genius 12865 points

    Hi Rakesh,

    please find 2 plots below. Both are taken when connecting a USB3 Stick. But the error appears sporadically! It looks like the TPS25982 not designed for use as USB power switch? 

    Output Voltage drop when connecting USB Stick:

    Output Voltage shut down when connecting USB Stick:

    What I don't understand is that I see faster slew rate with bigger capacitors (see below). 22nF should be slower than 40V/ms, right?

    Is there a limited allowed capacitance range for this capacitor?

    1nF

    190 V/ms

    4.7nF

    40 V/ms

    22nF

    110 V/ms

    Can you explain this, please?

    Thanks and Best Regards,

    Hans

  • 0 in reply to Hans
    TI__Guru** 105361 points

    Hi Hans,

    If you insert load (having capacitors) on a live output of eFuse, the eFuse would trip due to circuit breaker event and tries to recover again. In this recover mode, the slew rate is NOT a function of dVdt capacitor. Please verify this by probing current as well.

    For your function, this problem can be solved by using LDSTRT feature of TPS25982.

    BR, Rakesh