Opamp comparator output interfacing with digital circuit
According to the datasheet, VIH, Voltage-input-high, varies with VCC, and it's 1.5 volts when the chip is powered with 2 volts.  This means your first strategy won't work.

If you have a rail-to-rail op amp, you might be able to power it between +Vcc and ground, depending on how close to ground the output of your particluar op amp can go.  Check VIL on the data sheet for your PLL to figure this out, as well as your op amp data sheet

I also suggest that you arrange for some hysteresis on the input, to guarantee nice clean transitions.  You should consider a comparator instead of an op-amp.