When should I use soft-start on the LTC4269-1? I am using the LTC4269-1 on a circuit that has a lot of capacitance on the secondary. It's about 1.5 mF, but could be more in future designs. What I have noticed is the Vcc oscillates for small values of Ctr capacitance. If I reduce Rtr to 12.1 kΩ and increase Ctr to 60 uF, I don't get any voltage sag below 12 V and the part starts up fine. The problem then is the time it takes for Ctr to charge up initially can be too long for the PSE. It should be less than 300 ms, but I'm taking 414 ms.
Can I use soft start to reduce my Ctr value and still avoid a sag on the Vcc during power bootstrapping because the output current is limited? What I'm thinking is change Ctr to 22 uF like on the demo board, and set some reasonable soft start capacitor value. My goal is to keep the time it takes for Ctr to charge below 300 ms and still have no sag on the Vcc below 12 V during startup.
Here is what I see when I add 1 uF to the SFST pin (approximately 70 ms of soft-start time).
It appears soft-start does not help me to get rid of the voltage sag and still maintain a short ramp of the Ctr voltage during start up.
Here is the functionality I want. Ctr voltage rise time (Vcc) of less than 250 ms. No voltage sag during the power bootstrapping period (Vcc > 12 V). When the LTC4269-1 powers on, limit the output current until my bulk capacitance on the output of the secondary have filled up. Then operate as normal. How can I accomplish this?
I also tried 100 nF for soft-start. Here's what I saw.
I repeated the measurement about 10 times. Sometimes it looked like this too.
Added plots for 100 nF STST cap
[edited by: current view at 8:42 PM (GMT 0) on 13 Dec 2019]