I am using a comparator (TS3021) as a zero crossing detector (ZCD). The ZCD toggles its output based on the zero crossings of a sine wave generated across a transformer coil. One end of the coil is connected to the +ve input of the comparator via a voltage divider and the other end of the coil is connected to the -ve input of the comparator. The comparator is powered from a 2.5V LDO output supplied by a 3V battery source. The voltage swing of the sine wave is less < (VDD-VSS) of the comparator and doesn't violate the common mode voltage requirements (rail-to-rail) of the comparator. I tested this configuration and I was able to see the comparator output toggle at the zero crossings.
The comparator doesn't have a shut-down/enable pin and consumes around 73 uA of quiescent current. I do not need the ZCD to operate continuously, hence I tried to disable the comparator by inserting an NMOS transistor between the negative supply and the VSS pin of the comparator to cut the tail current. I connected the gate of the MOSFET to VSS to test if it disables the tail current. I noticed that the output of the comparator still toggles, but doesn't go all the way to the negative rail. Also, the VDS voltage is around 0.2V. When I connect the gate of the MOSFET to VDD, I am able to resume normal operation with full rail-to-rail output swing. My question is regarding the case when gate of the MOSFET is tied to VSS. Why is the output still toggling when I have cut (I assume I have) the tail current? Is there a better way of implementing shut-down functionality?