I am designing and building a power supply with TPS54531 switching regulators from TI. I am double LC filtering the output. The 5 volt PH output pin feeds a 9.2uH inductor, then to a 47uF ceramic capacitor then through a 0.47uH inductor and finally to a 15uF ceramic capacitor at the output. My question is where should the V_sense resistors connect, before or after the secondary LC filter?
My question is where should the V_sense resistors connect, before or after the secondary LC filter?
It should almost certainly NOT go after the 2nd LC filter because you'll turn the circuit into a power oscillator.
With feedback systems you have to take care of not introducing too much phase shift or delay or negative feedback (desirable) becomes positive feedback (turns into an oscillator).
The regular LC filter in a switcher introduces anything up to 180 degrees of shift and this is compensated by good chip design and compensation component selection. See this: -
184.108.40.206 Compensation Components
and this: -
Slope Compensation In order to prevent the sub-harmonic oscillations when operating the device at duty cycles greater than 50%, the TPS54531 device adds a built-in slope compensation which is a compensating ramp to the switch-current signal.
Note the parts about slope compensation. Adding another LC network within the feedback loop is asking for trouble.