A question about sizing capacitor to mitigate the ripple of a DC-DC converter output
The problem is this cheapo buck regulator runs at 2kHz and Zcap is much higher than 200kHz, ...

added


due to low current loading it appears to run in hysteric mode. Adding shunt capacitance with unknown ESR reduces ripple but also relaxation frequency making C harder to reduce ripple, thus limited by cap ESR.
this mode is essential for this class to improve phase margin under light loads. , which I leave for you to understand later. Some design dictate range of load caps inside feedback loop and ESR in order not to suppress f ripple feedback and lean towards open loop) This is why my second schematic shows the RC "outside" the feedback loop as a LPF with higher series R.




You can understand the behaviour by examining the Cap specs.

Dissipation Factor D.F. = tan δ   of General Purpose (G.P.) electrolytic caps. are standardized for 120Hz bridge rectifiers and in general implies not ultra-low ESR.   But if ESR is specified then it can be special ultra-low ESR needed to suppress (low frequency) ripple sawtooth.

Compare using a GP 1000 uF cap with 350mΩ  and one tantalum ultralow ESR 1000uF cap with 10 mΩ effect on ripple attenuation with estimated buck regulator source ESR = 100 mΩ . 


If Load regulation for 500mA on 5V is 1% then Vdc drops 1% or 50mV /500mA=100mΩ  ESR








simulate this circuit &ndash; Schematic created using CircuitLab

So what would you prefer 10~20mΩ  ultralow ESR 1mF cap or add a few Ω series ESR for your 5mA load to reduce ripple.


  N.B. First define your specs what you need



% ripple or +/-mVpp max
% load regulation error (ok)
% tolerance or +/-mV max


Then design impedance ratio to achieve it.  
Then choose parts 
*learn to remember range of ESR * C = Ts for different cap chemistry, just like batteries.*





p.s. Although I estimated ESR of Buck regulator near 100mΩ , it is more complex like Op Amp with negative feedback reducing with rising f, Zout rises with f which includes ESR and is also affected by duty factor and RdsOn of Switch.


But then with Zcap(f) on regulator output,  impedance drops with f, so net effect can be fairly flat. (hand waving argument)  


So what cap part number did you choose? and what was the ESR?