The “RAMP”…..
All that is doing is emulating the ramp that you would see on a sense resistor if it were placed in the hi side , in series with the hi fet. That is, I mean the ramp of the inductor current as the fet is turned on
If you don’t understand this ill send you a sim.
If you look where the sense resistor is in the LM5117, its in the “wrong” place….it senses the off state current, rather than the on state current….so you need the RAMP to give you the on state current ramp…..i hope I explained it…if not, ill send you the sim I spoke of.
As the datasheet says, when you have very small duty cycles , (because vout/vin is very low), then the on state ramp would be “destroyed” by the turn on spike through the fet…..so what they have done is clever…they are not sensing the actual inductor current at turn on…instead, they are emulating what it would be like (but without the nasty turn on spike)….the RAMP cap Is supposed to emulate what the ramp would be like in an ideal situation with a hi side sense resistor.
Another reason for emulated current mode is because at small duty cycles, the gate turn on current can “destroy” the sensed inductor ramp signal in the sense resistor……..if the duty cycle is longer then you can use “leading edge blanking time” to ignore the sense signal just after fet turn on…..but you cant do that with low duty cycles, because the on time is less than the LEB time.
Other advantages are that since your current sense is in the low side. You don’t need the extra circuitry to read its voltage.
I realise I haven’t explained well, but the datasheet, explains it poorly IMHO.
The COMP cap forms part of the error amplifier transfer function……..can you produce the transfer function for the error amplifier?....i haven’t fully read lm5117 datasheet, but error amplifiers are usually opamp integrators…….so you usually work it out by the transfer function for that opamp….but maybe the lm5117 uses a transconductance amp , I don’t know………they should give you the transfer fucntions I spoke about, then you wade through the math and get the open loop transfer function, then you can asses the stability………and yes, generally speaking, the lower your bandwidth, (crossover frequency) then usually the easier it is to stabilise the feedback loop.