LM2677 Start-up / Response to ON/OFF input..

Jesper Kristensen1

Other Parts Discussed in Thread: LM2677

Hello Group.

We're using the LM2677 for a 12V => 8V/5A DC-DC design, where we have mainly used the components suggested by the datasheet and used on the Evaluation Board, but where we have some problems with the Current Step Response at the output.

- Has anybody here any experiences about eg. problems getting down to the ~150mV shown in the Datasheet?

And another thing:

- I cannot find any figures or curves stating a response time when using the ON/OFF input - any experience here?
Is it (almost only) simply the reactive components and the load defining the time in practical life?

Thanks.

Best regards - Jesper.

over 9 years ago

0 Frank De Stasi over 9 years ago

TI__Guru 57555 points

Can you be more specific about the 150mV ??

Regarding the start-up, yes it is dominated by the soft start and the charging of the output caps.

0 Jesper Kristensen1 over 9 years ago in reply to Frank De Stasi

Prodigy 130 points

Hello Frank.

Thank you for attending to this subject.

There are two different Step Responses recorded on page 8 in Figures 17 and 18.
The latter shows the maximum step increase of load from 200mA to 5A (x25), and a dip of appr. 150mV is observed:

Thanks.

BR - Jesper.

0 Frank De Stasi over 9 years ago in reply to Jesper Kristensen1

TI__Guru 57555 points

The output cap and its ESR will have the biggest effect on the output voltage transient.

0 Jesper Kristensen1 over 9 years ago in reply to Frank De Stasi

Prodigy 130 points

Hello again.

Thanks for answering.

I think I'll rest my case until further notice.
My colleagues have found out that our response problem most probably comes from the combination a partly saturated coil and too much capacity at the output.
At least, we'll try to follow this lead.

Thanks.

Best regards - Jesper.

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LM2677 Start-up / Response to ON/OFF input..