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SN74CBT3257C: Undershoot protecting voltage and absolute max range

Oba
Genius 17485 points
Part Number: SN74CBT3257C


Hello,

I have a question about undershoot protecting function in some CBT devices like SN74CBT3257C.

I have read the below document. 

https://www.ti.com/lit/pdf/scda008

So I think I understand the basic concept of undershoot protecting. It is to protect wrong turn-on when the switch is OFF.

But I’m a little bit confused about absolute max range of the input voltage  and this under protecting voltage.

 

For example, SN74CBT3257C says that it has undershoot protecting function up to -2V.
On the other hand, the absolute max input voltage range is up to - 0.5V ~.
How to think about it?

Regards,
Oba

  • +1
    TI__Guru 52612 points

    Hi Oba,

    The abs max table gives -0.5V and 7V as limits - yes, but there are exceptions listed there as well. Essentially the clamping circuit that helps the gate voltage track down to -2V will start drawing current through it (-0.5V is were it will typically start conducting current) - if this current doesn't violate the abs max clamp spec (when under -0.5V the clamp can handle up to 50mA of current being sunk into it) 

    If the clamp conditions are satisfied then this device (along with the other CBT-C line of parts) has shown to keep bus isolation during undershoot events down to -2V since the clamp is part of the undershoot protection circuitry, whereas in other devices the clamp on the analog I/O tends to be more of a simple diode, that is what allows the -2V undershoot (it is implied that the clamp current must not be violated however still) 

    I understand it can be a bit confusing as it isn't explicitly stated in the datasheet, but with most undershoot events at or above -2V the device is able to keep its isolation while not damaging the device.

    If you have any other questions please let me know!

    Best,

    Parker Dodson