The datasheet of a DC/DC stepup IC (LTC3429) mentions for the output capacitor: "An additional phase lead capacitor may be required with output capacitors larger than 10 uF to maintain acceptable phase margin."
I want to use 3300 uF... so I guess this applies to me
What is it, where does this cap go and how can I calculate a value for it?
Thanks!
Why do you want such a large output capacitor? Are you trying to get very low output ripple? Such a large capacitor is usually overkill with a switching regulator since the capacitor's ESR is likely much higher than its capacitive reactance at the 500kHz switching frequency. If you really need to go to a larger capacitance than the recommended values, then you should use several capacitors in parallel to minimize the ESR.
The cap is Cpl (optional) in the Block Diagram on page 5 of the data sheet. It adds phase lead to the closed loop response to improve the phase margin.
It's value likely has to be empirically determined after you build it and test the phase margin.
Hi thanks for the hint. I played in LTSpice and added a 330 pF cap, it actually improves my transient behaviour. I don't see how it would be absolutely necessary though, just a bit less overshoot.
I want the large output capacitor because my load draws pulses the IC cannot handle continuously. I use a low-ESR electrolytic (panasonic FM series).
Hi thanks for the hint. I played in LTSpice and added a 330 pF cap, it actually improves my transient behaviour. I don't see how it would be absolutely necessary though, just a bit less overshoot.
I want the large output capacitor because my load draws pulses the IC cannot handle continuously. I use a low-ESR electrolytic (panasonic FM series).