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Original question:

SN74CBT3384A: CBT vs. CB3T for translating 5V to 3,3V (bi-directional).

SN74CBT3384A: Level shifter or switch

Muthu A
Intellectual 330 points
Part Number: SN74CBT3384A

Hi,

In my case, I need very minimal propagation delay. My application is 5V <-> 3.3V bi-directional bus. Normal level shifters (ex: LVC, AXC families) specifies at least 4ns delay. switches like CBT offers <0.5ns delay. can I go ahead with CBT instead of a common level shifter. Also, I'm not able to understand how CBT and similar families provide bidirectional data flow without a specific DIR line. how it is automatically sensing at ~200MHz data rate. could you please explain.

Thanks & Regards

Muthu A

  • 0
    TI__Mastermind 46327 points

    HI Muthu,

    The CBT line of parts only can do down translation, such as 5V to 3.3V. To covert 3.3V to 5V external components would be required. This could still provide a solution, but the external components would need to be factored in to your overall propagation delay budget.  

    The part itself is essentially detailed in the datasheet in the picture, posted below for reference: 

    They are essentially just N-MOSFET switches. This means they can only pass up to Vg - Vt, which for this part works well for 5V to 3.3V. However if you apply a 3.3V signal, a 3.3V signal will appear at the other end of the switch, so external components would be required to shift up the voltage, which could add propagation time. 

    The switch is bi-direction because of it's architecture. The N-MOSFET's used in the device have very similar Drain and Source characteristics, so a signal applied B going to A is extremely similar to a signal A going to B. Essentially Bi-Directional in this instance means both A and B are inputs or outputs, the device physics and operating conditions will determine which direction current flows in every enabled switch.

    Finally, the part doesn't detect the datarate. It's 200MHz bandwidth is determined by the parasitic capacitances of the part. Essentially all MOSFET's have parasitic capacitances on them, with high enough frequency these begin to act like shorts. For a Multiplexer specifically this means frequencies >200MHz can have greater than 3dB of losses because of low impedance pathways created in the part at higher frequencies.

    Best Regards,

    Parker Dodson