描述

Will driving the I/Os of an unpowered bank cause damage to the part? Can a 7 Series device be plugged into an active system?

How much current can the clamp diodes sink?

解决方案

7 Series devices do not support hot-swap. When they are plugged into an active system and Vcco is not yet powered, clamp diodes on both HR and HP I/O will sink current to the unpowered Vcco rail.

There are two aspects of the device specifications that must be considered when determining if a device could be damaged:

1) Ensure the Iin specification from the data sheet is not violated. Unpowered pins still have clamp diodes present that will be forward biased when a pin voltage is applied. It is important to note that both aspects of the Iin specification must be considered: both the per-pin current limits and the per-bank limits. The IBIS model includes an IV curve for these diodes and can be used to determine clamping current.

2) Voltage applied to pins in an unpowered bank can reverse power the bank in some circumstances. When reverse powering the bank through an I/O it is still relevant to observe the Tvcco2vccaux specification.

NOTE: If Vccaux is powered with Actual Vin < Absolute maximum Vin: this is okay as long the difference between Vcco and Vccaux (Vcco - Vccaux) does not exceed 2.625V and as long as the Iin specification is not violated.

Assessment Example

Clamp/ESD diode threshold will range from 0.43 to 0.64V. The worst case of 0.43V is at Tj = 125 deg C. An unpowered part will remain close to ambient temperature.

For a C grade device, with Tj = 85 degrees C, assume diode drop = 0.53V. Then, Vcco could be reversed powered by Vin up to the Maximum Vcco- Diode drop at Tj = 85 degrees C (that is, 3.465V - 053V = 2.935 V).

At 85 degrees C, this condition could affect long-term reliability of the device if the power distribution system of the PCB does not sink enough current.

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