Modelling a charge-pump in SIMULINK

Hi, I am interested in modelling a CP in my PLL in with circuit components. I tried connecting an ideal current source to a an ideal switch, but SIMULINK wo't allow it. I saw in Dick Benson's AMS package he connected an ideal switch to to an ideal voltage source, but I don't understand how that modells a CP circuit. I am not interested in modeling the CP as a mathematical gain, since that pretty much forces me to write the loop filter as a transfer function, and not implementing it with resistor and capacitor elements.

             Thanks in advance for anyone who can help,

                             Itai