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Since the NC pins are not connected to the IC chip, there should be no problem even if the pins are open, but we recommend connecting them to ground from the standpoint of heat dissipation.
If the termination regulator has a VTTS pin the phase margin can be measured using FRA. Insert a 100Ω resistor between VTT and VTTS and measure the gain-phase of VTT relative to VTTS by supplying an AC signal around 10mV across the resistor. Refer to the technical materials for details.
If forward bias is supplied to the diode between the VTT output and VTT_IN an excessive current will flow, possibly causing deterioration and breakdown.
Since the order of voltage supply follows no sequence, there should be no problems turning ON/OFF any pin. There also is no problem with shorting and using the VCC and EN pins.
The VTT pin will show high impedance when the VTT output is OFF. And when a voltage not exceeding the VTT_IN voltage is supplied, a minute leakage on the order of nA flows to ground.