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[FAQ] What is the difference between IOFF and VCC isolation? What are the conditions to guarantee it?

ShreyasRao
Guru 57370 points

FAQ: Logic and Voltage Translation > Logic Technology >> Current FAQ

The IOFF specification guarantees the I/O ports are in high impedance whenever either one of the power supply is at pulled to ground (0V) or close to gnd(<100mV as specified for the AXC devices). The Ioff is not guaranteed when the power supply is left floating. The Ioff spec is listed in the datasheet electrical specs table.

Detailed information about live insertion with our logic devices can be found in the Logic in Live-Insertion Applications application report

Vcc isolation is a feature of the devices which support the Ioff partial power down spec.

 An important point to note is that the input buffers of the I/O ports of the powered up supply side are still active and this can lead to high power consumption with floating inputs. For e.g. For any device which supports Ioff, if Vcca is 0V, and Vccb is at 5V, then the B ports must not have floating inputs. Similarly, if Vccb is 0V, and Vcca is at 5V, then the A ports must not have floating inputs.

TI recommends having weak pulldown resistors to limit higher than normal Icc current, whenever the Vcc isolation feature is considered to be used in the application.