TPS25921L - About Soft Start time(tss)

Kanemaru-san,

You are right. There is a mistake in Equation2. It should be corrected to:

tss=20.6 * 10^-5 * 12 * (CSS + 0.07), where CSS is in nF.

We will correct this typo in the next datasheet revision. Meanwhile, you can also use the calculator available here: www.ti.com/.../sluc573

regards,

-vishal

Hi Vishal-san,

I believe that “20.6 * 10^-2” is *10^-8”.

Because, I understand that this value is “I(ss)/Gain(ss)=(1*10^-6)/4.85=20.6 *10^-8”.

If no, could you please let us know the calculation method of this value?

And, according to your calculation tool, tss is ((Cdvdt_ext+0.07)*Vcc_max)/(I_dvdt*Gain_dvdt).

So, if Css (Cdvdt)=1nF, tss=4077669.961s.

[My result]

Tss=(((1*10^-9)+0.07)*12) / (20.6*10^-8)=4077669.961s

Is my understanding correct?

Regards,

Kanemaru

Hi Vishal-san,

Would you tell me about the progress on this matter?

I’d greatly appreciate your verification.

Regards,

Kanemaru

Kanemaru-san,

Let's correct Equation2 so that all terms have proper exponents. It should be corrected to:

tss=20.6 * 10^4 * VIN * (CSS + 70pF), where CSS is in F.

So, for CSS=1nF, VIN=12V, tss = 20.6 * 10^4 * 12 * (10^-9 + 70*10^-12) = 2.64ms

regards,

-vishal

Hi Vishal-san,

Thank you for your response.

I have one more question.

I would like to know about calculation method of “20.6*10^4”.

Because, our customer would like to include the variability of this value in a calculating formula.

If 20.6*10^4(typ) is the correct “I(ss)/Gain(ss)”,

-Min value is I(ss)(max) / Gain(ss)(min)=1.2/4.81=24.95*10^4.

-Max value is I(ss)(min) / Gain(ss)(max)=0.9/4.92=18.29*10^4.

Is my understanding correct?

Regards,

Kanemaru

Kanemaru-san,

1/(Iss*GAINss) = 20.6*10^4

So, min value is = 1(Iss-max * GAINss-max) = 1/(1.2u*4.92) = 16.94*10^4

max value is = 1/(Iss-min * GAINss-min) = 1/(0.9u*4.81) = 23.1*10^4

The calculator also shows tolerances in column F.

regards, 

-vishal