TPS745-Q1: How to calculate Maxmum Cload for Inrush current

Jimmy Chen

Prodigy 60 points

Part Number: TPS745-Q1

Dear Sirs,

We would like to know what is the maximum acceptable value of output Cload.

Iinrush = ICL(Output current limit)

C = Cload

dV = Vout(NOM)*0.95

dt = tSTR(Startup time)

So, Cload = Iinrush current*dt/dV = ICL*Start time/Vout(NOM)*0.95 = 515mA*200us/(3.3*0.95)V = 32.85uF

Is my calculation method correct? Or will the start time change due to the Cload increase?

So if the Cload is greater than 1uF, I can't refer to this table. And should I refer to the Cout value on the Recommended Operating Conditions table? The Cload should be within 1~220uF. Is this correct?

over 2 years ago

0 Juliette over 2 years ago

TI__Mastermind 18466 points

Hey Jimmy,

In short, the output capacitor can range from 1-220uF and your math looks correct:

The app note where you can find more information (https://www.ti.com/lit/an/slva670a/slva670a.pdf), suggests using the max inrush current spec for your application.

Using the minimum current limit spec may be useful, but may need to be adapted when testing on bench.

our datasheet shows a similar equation:

It may be beneficial to use the minimum output capacitance and monitor what the inrush current looks like for your application, and then calculate the capacitor that will help you bring that to an acceptable range.

Other helpful App Notes: Taming Linear Regulator Inrush Currents

Best,

Juliette

0 Jimmy Chen over 2 years ago in reply to Juliette

Prodigy 60 points

Hi Juliette,

Sorry, let me clarify my question. The maximum Cload I calculated is 32.85uF. And the maximum Cload in the datasheet is 220uF. I would like to know which is correct. Because our design is 53.6uF.

Thank you very much.

0 Juliette over 2 years ago in reply to Jimmy Chen

TI__Mastermind 18466 points

Hi Jimmy,

Sorry for the confusion!

The maximum Cload is 220uF. The 32.85uF would be the minimum capacitance needed if your inrush current was 515mA and you wanted to minimize that peak.

Best,

Juliette

0 Jimmy Chen over 2 years ago in reply to Juliette

Prodigy 60 points

Hi Juliette,

"The 32.85uF would be the minimum capacitance needed if your inrush current was 515mA and you wanted to minimize that peak. "

But if the Cload is larger than 23.85uF, will the inrush current increase and exceed 515mA? If so, will it cause current limiting?

0 Juliette over 2 years ago in reply to Jimmy Chen

TI__Mastermind 18466 points

Hey Jimmy,

No, I don.t think this is likely to cause the inrush current to trigger current limiting. What is your load current?

Best,

Juliette

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TPS745-Q1: How to calculate Maxmum Cload for Inrush current