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tps62085 adding capacitor in feedback network

Nicolas Bruneau
Prodigy 490 points
Other Parts Discussed in Thread: TPS62085

Hi,

I'm wondering if adding a capacitor to the feedback network would help the tps62085 to reply even faster to fast transient? (see schematic below)
In case it doesn't help, would it indeed bring instability to the DCS control core?

Thanks in advance for your support,
-nico

  • 0
    TI__Guru**** 150570 points
    The simplest way to improve transient response is to add more Cout. If you still need better, we can have a specific discussion.

    A Cff may not help very much with DCS-Control, unless there is a lot of Cout. This app note explains this in detail for another device: www.ti.com/.../slva466.pdf
  • 0 in reply to Chris Glaser
    TI__Prodigy 490 points
    Interesting app note, thanks Chris!

    But, still not all clear...

    I understand that having more Cout will help the converter to regulate BUT at some point, when Cout gets too big the loop is getting unstable. I suppose it's because the DC/DC can't, for a given Inductor, push energy fast enough to the output...
    Is it the point where you can improve the reactivity of you converter by placing a Cff in the feedback path? Or am I on the wrong track?
  • 0 in reply to Nicolas Bruneau
    TI__Guru**** 150570 points
    If you have a specific design requirement to meet, let's move this internal and discuss that.

    The transient response of this device is already very good. More Cout can improve it even further. By the time instability results, you will reached the point where more Cout is not helping significantly.

    A Cff can increase stability and the transient response with a large Cout. As the app note explains, the loop bandwidth must be sufficiently low for the Cff to have an impact. A low loop bandwidth is caused by lots of Cout.
  • 0 in reply to Chris Glaser
    TI__Prodigy 490 points
    Your rephrased comment + the app note is enough :)

    The design I'm working on is pretty much done yet, thanks to your information.

    thanks and best regards,
    -nico