input impedance of an LDO

hello guys..
My first stage is a charge pump then followed by an LDO.
I'm having trouble on what value of output resistance to be used for the charge pump simulation before LDO is connected.
Any suggested approximation for the input impedance of the LDO?

Thank you......

what are those charge pump and LD0
at least send here schematic

Operating in dropout or close to it, impedance will be
Vdrop/Iout more or less. If it's a PMOS LDO then you
are pushing on the source of a cascode which is low
impedance. So your impedance depends on the pump
and the load. You might want to work on the LDO first
as well as representing load-cases "generically".

You may have to do a few corners (low Vdrop, low current;
low Vdrop, max current; high Vdrop, low current; ....) to
capture the likely variation in stability and pump-up
behavior. Perhaps permuted with whether you are trying
to pump while on, vs enable after pumped exceeds
undervoltage. I've done some charge pumps that pump
less effectively when voltages are low (sync rect style)
and really take off after per-stage charge (voltage) gets
past VT. Swinging from subthreshold to well driven, as
it were.