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I'm looking at the LM2596S datasheet and notice that on page 7 (figure 3) it recommends a "feedforward" capacitor for the adjustable variant.

The example schematic on page 9 shows a capacitor labelled CFF:

Schematic

Underneath there is a note saying "CFF - See Application Information Section". I have looked through the application information section and searched the PDF for variants of "feedforward" and "CFF" but it does not appear to be mentioned anywhere else.

What is the function of this capacitor? Why is it only recommended for the ADJ variant of the buck converter IC?

My guess is that it acts to negate some of the resistance of R2 during transients, since current will flow across the capacitor when there is a voltage differential. Perhaps this is to speed up the feedback response time, but I'm not sure.

If the feedback voltage is instead provided by an opamp and shunt resistor for constant-current operation, such as in the example below, should this capacitor be placed in parallel with the shunt?

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ It's all here: ti.com/lit/an/slva289b/… \$\endgroup\$
    – John D
    Sep 9, 2022 at 20:18
  • \$\begingroup\$ Ah, so my suspicion about transient response was correct. Since that's the case, I suppose putting such a feedforward capacitor over the shunt would have the opposite of the intended effect, and Cff shouldn't be needed because the opamp is actively driving FB. \$\endgroup\$
    – Polynomial
    Sep 9, 2022 at 20:20
  • \$\begingroup\$ In your diagram the bottom of the shunt should be connected to ground, and the + side of the op-amp to the top of the shunt. And you're correct, for several reasons a FF capacitor across a shunt is not a good idea. \$\endgroup\$
    – John D
    Sep 9, 2022 at 20:23
  • \$\begingroup\$ Whoops, yeah, that was a drawing error! Thanks. If you want to summarise your comments in a short answer I'd be happy to accept it. \$\endgroup\$
    – Polynomial
    Sep 9, 2022 at 20:25
  • \$\begingroup\$ I've fixed the schematic :) \$\endgroup\$
    – Polynomial
    Sep 9, 2022 at 20:27

1 Answer 1

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A feedforward capacitor across the top resistor in the feedback divider will provide the addition of a zero and a pole to the open-loop transfer function.

If the zero and pole locations are placed properly, it will provide a phase boost, allowing a higher crossover frequency while maintaining sufficient phase margin.

Intuitively, high frequency disturbances on the output will have a lower impedance path to the FB pin with the addition of the FF capacitor.

So by adding the FF capacitor it's possible to improve the transient performance of the converter.

Adding a capacitor across the shunt resistor would not achieve the same results, as it would decrease the loop gain to disturbances in the current.

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