SN74CB3Q3257: 74CB3Q3257 undershoot

Hello,

Thank you for your question. Is this waveform the signal at the input of SN74CB3Q3257? Could you send the waveform of the signal at the output?

The datasheet states that the I/O voltages ratings may be exceeded if the I/O clamp current ratings are observed:

Clamp current can be reduced with a series resistor. Another consideration to avoid undershoot is the proper termination of the signal. 

I would also like to point out that we have a new 4-channel 2:1 mux, TMUX1574, which has additional features such as 1.8V logic compatible control inputs and fail-safe logic. What is your application and end equipment?

Regards,
Kate

Hi Kate,

     Thanks for your reply!

       The waveform was measured on the 1B1 of SN74CB3Q3257. in currently we cannot change the chip with TMUX1574 because I hasn`t opportunity to change PWB, and this signal is a 1.05V level one.

       I hasn`t understand this sentence "The input and output voltage ratings may be exceeded if the input and output clamp-current ratings are observed.", what is the meaning of " if the input and output clamp-current ratings are observed"? should I measure the current of this issue signal? how to estimate the relationship between the clamp current  and the undershoot issue? thanks.      

"

Hello,

Thanks for letting me know that this waveform was measured at 1B1. Are you using 1B1 as an input or an output? I would like to know if this waveform is the signal before entering the switch or leaving the switch.

As you pointed out from the datasheet, the device has internal clamp diodes to handle undershoot events. The statement you are referring to is saying, it is ok if the voltage drops below 0V (undershoot) if the clamp current is less than the rating I(I/OK), which is -50mA. As long as the current does not exceed -50mA while VI/O < 0V, the device should be protected. 

Best regards,
Kate