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SN74BCT760 Max Vin - Diode Protection?

David Hildebrand
Expert 4860 points
Other Parts Discussed in Thread: SN74BCT760

Does the SN74BCT760 have an input protection diode connected between each input and Vcc?  I know that some CMOS logic families such as the SN74HC and SN74AC do include these diodes that limit the positive input voltage to Max Vcc + 0.7V.  The SN74BCT760 datasheet shows an ABS max of 7V for the inputs, but it also shows an ABS max of 7V for Vcc, so it isn't clear to me where there is a protection diode to Vcc or not.

I have a customer scenario where the SN74BCT760 is being driven by an old schottky TTL device.  The logic levels line up with the TTL device providing 2.4V & 0.4V output levels and the SN74BCT760 accepting 2.0V and 0.8V input levels, providing 400mV of noise immunity.  This circuit works just fine, but they have had some SN74BCT760 devices fail destructively.  The driving TTL device is separately powered and it can have its supply come up before the Vcc for the SN74BCT760 supply comes up.  If a protection diode to Vcc is present, this would be a problem since the driving TTL device would attempt to power the SN74BCT760 up through the protection diode.

Thanks, David

---------------------------------
David Hildebrand
Analog Field Applications
South Region
Texas Instruments, Inc.
Office:  214-479-0917
Cell:       972-841-6499
david.hildebrand@ti.com
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  • 0
    TI__Mastermind 24765 points
    Hey David,

    Since the device doesn't specify that the input voltage in terms of Vcc the device should be able to handle up to 7 Volts across the input without having power and it won't damage the device, thus you are correct.

    You mentioned they had some SN74BCT760s "fail destructively". Could you elaborate on this? If they are having a problem with the device we may be able to shed some light on the problem.

    Thanks,
    Daniel