LM25148 DC-DC Converter Current Feedback RC Filter Value........

[Smokey]

https://www.ti.com/lit/ds/symlink/lm25148.pdf
Check out this app note from TI for the LM25148.

Page 16:
https://www.ti.com/lit/ug/snvu773a/snvu773a.pdf



R11 is the current feedback resistor.  5mOhm sets the overcurrent trip at about 10A.  All good here.

R17 at 49.9Ohm and C34 at 33pF make an RC lowpass at 96.6MHz.  http://sim.okawa-denshi.jp/en/CRtool.php
This seems crazy high.  The switching frequency of the converter only goes up to 2MHz.

Is this a mistake or is there a reason to have the current feedback filter set that high?

[moffy]

The resistor and capacitor R17/C34 are not used in the typical application on page 32 of the datasheet, they are probably there for glitch removal i.e. spikes present on the switching edges and do not form part of the control loop, if that is their reason, then you would want them well outside the control loop frequency so that they don't compromise stability.

[fourtytwo42]

Refer to data sheet 9.2.1.2.4 Current-Sense Resistance para 5 "The CS-to-output propagation delay " you will see the effect of CS pin delay's such as this RC network upon peak inductor current, that is why the values are small.

The reason for the RC network is switching noise falsely triggering the comparator, normally there will be blanking circuitry within to ignore this but if the switching noise is severe some external filtering might also be required hence the additional RC.

The amount of external filtering required is external component (particularly mosfet) and layout dependent.

As noted in the data sheet any delay on the CS pin should be minimized.