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SN74CBT3306C: Bus Switch clamping output?

Trey German8
Expert 1920 points
Part Number: SN74CBT3306C
Other Parts Discussed in Thread: TINA-TI, TS5A23166

I designed a 3306 into a analog bus isolation application.  Device is powered with 5V and I'm trying to pass 0-5V signals through it.  I've noticed when the signal I'm passing is between 3.75-5V,  the device clamps the output to 3.75V.  

My initial though is that this is some type of diode clamp or maybe the -2v overshoot protection kicking in, but in any case its very frustrating.  This product was found in the analog switch/multiplexer family on ti.com and the datasheet and product page specify 5V output levels.  I also didn't see a spice model on the product page, but I'm downloading Tina-TI just incase its included with the package.

Could someone clarify why this is happening?

Thanks,

Trey

  • 0
    TI__Expert 3900 points

    Hi Trey,

    This can be a confusing one, but you are on the right track! The datasheet specifies up to 5.5V tolerable input voltages like you mentioned. However, the device utilizes the -2V undershoot protection like you mentioned. This circuitry prevents the output of the device from reaching 5V. Please see the graph for more information:

    This graph is pulled out of a TI app note which explains this in more detail. For more info on this part start at page 11.

    Will a device such as the SN74LVC2G66 work for your system?

  • 0 in reply to Dakotah
    Expert 1920 points
    Thanks Dakotah!

    Thats close, but is lacking one key feature we really need. The 3306 has that Ioff feature which makes the pins high impedance when the device is unpowered. We designed the part in because our application was drawing current from a 3rd party system when our application was unpowered. This in turned caused faults in the 3rd party system.

    All I really need is something that passes 0-5V signals when powered and is Hi-Z when unpowered. Bandwidth and current are low.
  • 0 in reply to Trey German8
    TI__Expert 3900 points
    Hi Trey,

    Powered off protection feature (Ioff) is a common need by many applications.

    See if the TS5A23166 will work for your applicaiton. It is still a 1:1 2channel device that can operate from 0-5V and also has the powered off protection feature.