TPS1663: tdVdT Formula: V x C equals time?

Thomas Krannich

Part Number: TPS1663

Tool/software:

Hello,

I have some problems understanding the fomula for inrush calculation in the TPS1663 eFuse. The datasheet gives two formulas:

(page 16, chapter 9.3.1)

(1) I(inrush) = C(out) x V(in)/tdVdT

(2) tdVdT = 20.8e3 x V(in) x C(dVdT)

And here is my problem, the unit of C is Farad (F) and F = As/V (Ampere seconds per Volt).

So if F gets multiplied by V, the result must be As (Ampere seconds).

This means, according to the formula [tdVdT] = As.

All in all, I can't see how this makes any sense. I suppose tdVdT is actually meant to be a time? Otherwise in equation 1 this would lead to I(inrush) being without any dimension at all.

What exactly is tdVdT?

11 months ago

0 Amrit Jit 11 months ago

TI__Mastermind 35991 points

Hello Thomas,

Check on design calculator below.

https://dr-download.ti.com/design-tools-simulation/calculation-tool/MD-QV4CzjZsl1/01.00.00.0B/slvc765b.zip

Thanks\Amrit

0 Thomas Krannich 11 months ago in reply to Amrit Jit

Prodigy 10 points

Hello Amrit,

thank you for your reply. I'm already using the calculator. But my question was a completely different one. I'm trying to understand the datasheet formula and to me it looks wrong. (as described above)

0 Amrit Jit 11 months ago in reply to Thomas Krannich

TI__Mastermind 35991 points

Hello Thomas,

Formula is correct. Refer the explanation below.

dVdT has a multiplying factor of 25 shown in below image. So, for formula I=C*(dV/dT), put dV as VIN/25 and I as 2uA

You will get tdVdT = 20e3 x V(in) x C(dVdT)

For these block approximate values are given. if you put exact value, you will get

tdVdT = 20.8e3 x V(in) x C(dVdT)

Let me know if you have further questions.

Thanks

Amrit

0 Thomas Krannich 11 months ago in reply to Amrit Jit

Prodigy 10 points

I appreciate you taking your time, thanks!

My actual question is still not answered. How can the multiplication of a capacity with a voltage result in time?

[V(in)]x[C(dVdT)] = V x As/V = As

... how can that be the resulting dimension of t(dVdT)?

Or is t(dVdT) not a time but something else?

0 Amrit Jit 11 months ago in reply to Thomas Krannich

TI__Mastermind 35991 points

As described in above formula

current(I) has been replaced with 2 uA. Ampere is preset in formula.

I hope that cleared. equation 2 is a simplified formula.

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TPS1663: tdVdT Formula: V x C equals time?