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TPS2492: Question about TPS2492 Design Calculator's fault timer.

Seokmin Hong
Intellectual 485 points

I am trying to use TI's Hotswap Design calculator, and have a confusing point.


Choosing Ct and turn-on time is like below in the datasheet.

For my case

 Cout : 1000uF

Plim : 100W

Ilimit : 25A

Vcc(max) : 29V

then If i calculate with these parameters, then ton will be about 4.28ms.

I choosed use external soft-start control because load current is high.

 

I think target fault time should be larger then ton time. Is this right?

So, then I set my system's target fault time for 5ms.

Now, I put these parameters in design calculator.

Target fault time : 5ms

then calculated Timer capacitance becomes 31.25nF

And if i put 30nF for the actual timer capactiance,

then SOA Margin becomes "RED", that I don't have enought margin for the SOA.

.

But If I reduce target fault time, then ton is too high so I cannot reduce this.

I don't know what to do.

  • 0
    TI__Guru** 105361 points

    Hi Hong,

    The fault timer starts counting time when the device enters into power limit or current limit. If you use external soft-start control, timer value gets decoupled from the turn-on calculations. You can set low timer value may be around 1ms.

    Please share the filled-up design sheet for review.

    Best Regards, Rakesh

  • 0 in reply to Rakesh
    Intellectual 485 points

    TPS2492_Calculation.xlsx

    Dear Rakesh.

    In the datasheet below, there says Ct programs the fault time and should be chosen so that the fault timer does not terminate prior to completion of start up.

    Isn't this mean fault time should be longer than ton(turn on)time?

    I am confused.

    As you said, If fault timer starts counting time when the device enters into power limit or current limit,

    then why below datasheet says and calculates fault time and CT value more than ton time?

    Datasheet calculated ton time for 416us then choosed Ct value. 

    As I said in my first question, my calculated ton time is 4.28ms.

    If I use soft start control(dtdv), then is this mean I can make fault timer shorter then ton time? (Regardless of ton?)

    I attached my design sheet.

    Please, refer to this and suggest appropriate fault time for this design.

    And Please, clarify me for the relation between fault time and ton time.

    I want to know clearly about this hot swap ic.

    Thanks very much for helping me.

  • 0 in reply to Seokmin Hong
    TI__Guru** 105361 points

    Hi Hong,

    The tun-on time calculations discussed in Section-4 is applicable ONLY if we use power limit or current limit startup. It is not applicable if we use dv/dt startup, in which we can set fault timer shorter then ton time.

    I have attached the modified design sheet with lower fault timer to improve SOA margin. 

    modified_TPS2492_Calculation.xlsx

    Best Regards, Rakesh

  • 0 in reply to Rakesh
    Intellectual 485 points

    Thanks very much Rakesh!