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i think that they r used to decrease the dynamics and charge sharing (i.e. when the up signal change the voltage of the drain of the MOS with UP-bar have the sdame voltage as the drain of the MOS with UP , so the current will move smoth and no dynamics is formed ) note i am really just guessing .
Yea its to mitigate charge sharing problem between the drain of PMOS, NMOS and loop filter. Basically it fixes the drain voltage so that the drain voltage does not pull to gnd or Vdd when the switch is off.