Hi John:
I appologize for the long delay but I had to complete some other work. Back to the issues with my implemenation of the TPS5450. I was not vey clear in my past posts. The test fixture I made had very fast rise and fall times; much faster than I required and I was naive about the implications to the design of the reguator especially since it is internally compensated. The module is not yet available but I now have some current waveforms from the mfg that indicates it is about 2.5A in 50us or 50mA/us.
In any case, I modified my test fixture to at least slow down the leading edge of the MOSFET switch and have attached new waveforms in a form more like you requested.
I am now testing with a 1ms, 4A pulse that has a rep rate of 10ms. The rising edge is now much slower at about 1.5A/us. Not so bad on the leqading edge but problems on the falling edge (vertical at 200mV/div AC)
but the falling edge is still very fast at about 20A/us
If I look over a longer time, I see lots of loop instability as it seems to take about 8ms before the output starts to become stable.
Given that my target application may require slower a di/dt but needs to be able to handle at least 5A for short durations, do you believe I am ok with TPS5450? Do you believe the poor transient response on the falling edge is beacuse of the high di/dt or poor matching of the output LC? Earlier you mentioned lead compensation using a small capacitor across the upper divider resistor. Will that improve my performance based upon what you see now? If not, how do I best "tune" my output LC to match the internal compensation? The data sheet shows that for the voltage mode type 3 compensation networ there are two zeroes at 2170 and 2590 Hz. and a first pole at 2165 Hz with many other poles much higher in frequency. How do I takle the alignment? Do I assume the 2165 Hz is the dominant pole and take the average of the two zeroes around 2400Hz. I asume the output network looks like a series L feeding a shunt comboination of the output C in series with the capacitor ESR. I can solve for the double pole and single zero but not sure what my goal is?
Thanks for your help,
Craig