Has anyone successfully used the UCC28950 transient PSpice model with the chip set as a Slave and an external clock input to the SYNC pin?  No matter what I do I can't get the chip to synchronize to the external clock, it still runs at the frequency set by RT.  I have done everything the datasheets and app notes say:  RT to ground, 825k from SS/EN to ground, SYNC frequency is greater than 1.8 times the natural converter frequency set by RT.  Help!