My question is about transient response or compensation component selection for stability on this buck-boost converter. I have made a prototype with -12V input ( range of -9V ~ -15V) , and 3.3V output converter with LM5000. Con output current range is 25mA ~ 250mA. With Rc=1k, Cc= 0.47uF, this converter is stable, but load response slow. its output voltage is within desired range with a fix load. It is not good on load step response.
With Vin=-12V, and load changes from 28mA to 138mA, output voltage has about 0.5V drop ( about 0.5 ms width).
With Vin =-12V, and load changes from 28mA to 244mA, output voltage drops about 1V with about 0.8ms width.
I understand that I have set a pole at 0.4Hz with Rc=1k, Cc = 0.47uF, which is not within the range of 10hz to 100Hz that is suggested in LM5000 datasheet ( page 13 of 17 ). I have tried to make it within 10 to 100 HZ range, say Rc=102k, Cc=4.7nF ( and other Rc and Cc values) , the circuits are not stable. it gets oscillation with heavier load ( say 150mA or more ), it also has ringing at start up with 28mA load. My calculation shows that the pole formed by load ( 132 Ohm with 20uF cap ) fp is around 60Hz, and the right half plane zero is 7.3khz. Since we are using ceramic output caps, the output capacitor ESR zero is higher than RHP, which is 7.3Khz. The phase lay of Gp(s) at fzRHP will approach -270 degree and the crossover frequency fc has to be selected below the 60 Hz ( fp). I think this is why I have to set a pole at 0.4HZ with Cc and Rc to make this converter stable.
My questions are:
1. Is there anything that we can do to improve load step response for the converter?
2. is there anything that I have done wrong to make it can not meet 10 ~ 100Hz range requirement in the datasheet while choosing Rc and Cc.
Please see the schematics of this power supply. Rcc is R104 and Cc is C177 in this schematics.
startup waveforms with Rc= 1k and Cc= 0.47uF with 244mA load:
output voltage load step response ( output current changes 28mA to 138mA ), Rc=1k, Cc= 0.47uF:
output voltage load step response ( output current changes 28mA to 244mA ), Rc=1k, Cc= 0.47uF:
start up waveforms with Rc =102k and Cc = 4.7nF: