charge pump in PLL - simulink matlab

hi! I have a frequency synthesizer PLL in simulink. I use a charge pump. I put voltage sources in the charge pump ( not in the enable inputs) (1 volt in the up-switch and -1 volt in the down-switch). With that configuration the system works fine. BUT....i want all the voltages from 0 volt to 1 volt and not from -1 to 1 volt. If i put 0 volt in the terminal of the down-switch, the system doesn't lock and it doesn't work at all. Any suggestions for changes in the charge pump so as to have the output of the loop filter from 0 to 1 volt and not from -1 to 1 volt?

thanks