Hi Srinadh,
The Pspice model can be found on the LMH6644 product page under the Design & Development -> Design Tools & Simulation. There you can download a zip file containing all the files for the the Pspice model.
For a brief introduction, looking at the description and applications section on the first page of the datasheet will provide you some insight on what features the device has and common applications it is used for, although that is not an exhaustive list.
To highlight a few things about the LMH6644, it features a rail-to-rail output (i.e. the maximum output voltage can be within 40mV of the supply rail voltage. Also, it has a high output current drive (around 75mA) which is useful to drive heavy output loads. Despite this high output current drive capability, the LMH6644 has a relatively low power consumption for applications with a focus on low power cost.
There is also a newer device (the OPA4863) that is a drop-in replacement to the LMH6644 with some improved specs. It has lower noise, lower offset, lower offset drift, and lower power consumption. It may be worth while looking into this device as well.
Thanks,
Nick
