soft-start circuit of UC2845A

Hello Takahiro-san

The soft start for this part is implements as shown in the diagram below:

The approximate soft start time is equal to the RC product of CSS and RSS.

This is an approximate forumula and is suitable for implementing a soft start function where no soft start pin is available on the controller.

Regards

John

Thank you for your support!

Soft-start time=Rss x Css

For example, soft-start time is 5 sec.

Rss=100kΩ

Css=50μF

Is this understanding correct?

Best regards,

Takahiro Nishizawa

Yes but 5 seconds is a very long time and most power supplies have soft start times in the order of a few milli seconds.

It is not reasonable to expect the approximate formula I gave you to be accurate for such a long time.

Regards

John

Thank you for your answer!

We find this application note in UC2845A web page.

This is for UCC38C42.

http://www.tij.co.jp/jp/lit/an/slua257/slua257.pdf

Is the soft-start information(p.14) of this application note applicable to UC2845A?

Best regards,

Takahiro Nishizawa

Yes.

Thats is correct.

Section 4.3 details the soft start technique

Regards

John

Thank you for your answer!

We understand that this application note is applicable to UC2845A.

Tss=Rss x Css

But our custoemr used this formula.

We don't know why they use it.

Is this formula correct?

   Tss=3600 x Css(µF)

Best regards,

Takahiro Nishizawa