SN74AVC1T45: Vcc condition question

Hello Shidara-san,

K.Shidara said:

1, It is no problem to input signal without VCCB power in terms of IC broken possibility, right?

You need to be certain that "without VCCB power" means that VCCB = 0V. Just turning off a supply often results in VCCB > 0V, and the following will not apply.

This device has over-voltage tolerant inputs and includes the VCC Isolation Feature. If either supply is at 0V, then all I/O pins are in the high impedance state. This is clearly defined in the Electrical Characteristics table as a valid operating state:

K.Shidara said:

2, It is no problem to supply VCCB only without supplying VCCA in terms of IC broken possibility, right?

Section 9 of the datasheet states:

SN74AVC1T45 Datasheet said:

Pin A and direction control pin are support by VCCA

This means that when the VCCA supply is ramping from 0V to 1.2V, the control circuitry is not completely powered and could cause unknown output states. 

As per section 11.1 in the datasheet, VCCA must be powered up first or at the same time as VCCB. The datasheet states:

SN74AVC1T45 Datasheet said:

A proper power-up sequence always should be followed to avoid excessive supply current, bus contention, oscillations, or other anomalies.

These issues are typically not a major concern for damage or reliability reduction at such a low voltage (below 1.2V), however it is very likely that power will be consumed well beyond the rated specifications during this time, possibly up to several milliamps.