TPS7A84: About TPS7A8400RGRR's inrush current

Qi Liu

Part Number: TPS7A84

Hi, support team

My customer wanted to replace TPS7A8300ARGRR with TPS7A8400RGRR.
TPS7A8300ARGRR's COUT is 22uF, TPS7A8400RGRR's COUT is 47uF in datasheet.
As there is the output capacitor change, from 22uF to 47uF, the inruch current is affected,
But they still use the soft start feature. so in this case do they still need to worry about the inrush current?

Thanks so much.

Best regards,
Yuki

over 3 years ago

0 Nick Butts over 3 years ago

TI__Guru 50025 points

Hi Yuki,

Using soft start will reduce the peak inrush current, but not completely eliminate it. If the CSS is sufficiently large the inrush current will be severely reduced and may not cause problems, but whether they will need to worry about the inrush current depends on the application.

Regards

Nick

0 Qi Liu over 3 years ago in reply to Nick Butts

TI__Expert 5655 points

Thank you for your kindly response.
In the application the max current required at the output is 1.6A,
the Css used is 10nF, also Cff is 10pF. and Cout in current design is 22uF,
so if change the Cout from 22uF to 47uF, which will increase duration of inrush current as well.
but could you provide the equation to calculate peak inrush current ?

+1 Nick Butts over 3 years ago in reply to Qi Liu

TI__Guru 50025 points

Hi Yuki,

Please refer to this app note for help calculating the necessary rise time to limit the peak inrush current to an acceptable level:

Managing Inrush Current

Note that the inrush current is not as dependent on the load current during regulation because until the output voltage comes up substantially the load current is also just ramping up. The inrush current is mostly dependent on the output capacitance.

Regards,

Nick

0 Qi Liu over 3 years ago in reply to Nick Butts

TI__Expert 5655 points

Is there a die change between these two parts？

0 Nick Butts over 3 years ago in reply to Qi Liu

TI__Guru 50025 points

Hi Yuki,

No the dies are the same.

Regards,

Nick

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TPS7A84: About TPS7A8400RGRR's inrush current