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LMH7322: Turn off LMH7322 with input signals still applied?

steffen
Prodigy 10 points
Part Number: LMH7322

Hello,

I'm using a LMH7322 as comparator to convert analog signals into LVDS. The LMH7322 is powered as follows:

VCCI: +5V

VCCO: +2.5V

VEE: -5V

One input of the LMH7322 is connected to a DAC for threshold, the other is connected to the input signal (dc-coupled and parallel 50 Ohms terminated) which is within -5V to +5V.

To save power in the design I'm thinking about turning the LMH7322 off. As it doesn't have built-in capabilities to do so, the plan is to use three FETs and disconnect the device from VCCI/VCCO/VEE-supplies. But when doing so I can not guarantee there will be no input signals applied.

The datasheet specifies for maximum ratings an input voltage range of VEE-0.2V to VCCI+0.2V. But what if the supply pins are basically unconnected (because the FET is open)? Are there any effects which could harm the device when input signals are still applied?

Thank you very much!

  • 0
    TI__Expert 4555 points
    Steffen,

    The way that the ESD structure is set up on the input pins, even if you leave VCCI/VCCO/VEE- floating by disconnecting them using FETs, the input signal can still leak through and sink current through the device - meaning it is not completely off.

    Instead of placing FETS at the supply rails, you should use some sort of analog switch between the input signal and the comparator. Because this is for high speed, you should make sure this switch isn't bandwidth limiting the comparator.

    I'm also curious as to what your general application usage for this is. What exactly will your LVDS output be driving?

    Best Regards,
    Jonny
  • 0
    Guru 140200 points
    Hi Steffen,

    what do you mean by "because the FET is open"? Can you show a schematic?

    Kai
  • 0
    TI__Expert 4555 points
    Steffen,

    Would you be able to provide a schematic or more details about your application? It may help us answer the question better, otherwise, we will close the thread soon.

    Regards,
    Jonny