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TXB0108: Using level shifter for 5 V to 3.3 V for Interfacing with Zynq 7000

Tapajit Mondal
Prodigy 165 points
Part Number: TXB0108
Other Parts Discussed in Thread: TXU0104, SN74LXC8T245, SN74AHC541

Tool/software:

Hi all, 

Planning to use a TXB0108 level shifter for converting 5 V to 3.3 V  to interface with my Zynq 7000 FPGA. Schematic photos as attached.

The inputs are mainly I/O signals generated in TTL format which i have to convert to 3.3 V for use in my FPGA 

Is there any other circuitry which is needed for this, or is this the right implementation. 
Any resistors, capacitors etc  

Also do we need isolators for removing noise for such inputs to FPGA?

I ask Because these inputs are being generated in a circuit which has high voltage components and there may be noise on the path from there to here.

The Connector on the right is the Mating Connector for the FPGA.

  • 0
    Guru 318050 points

    The TXB does not support these supply voltages; VCCA must not be larger than VCCB.

    The high level of TTL signals is 3.3 V. Are you using TTL signals or 5 V signals?

    The TXB is designed for auto-bidirectional applications, which results in many restrictions; see section 8.3 of the datasheet. For unidirectional signals, you should use unidirectional translators like the TXU0104 or SN74LXC8T245. For translating to a lower voltage, you can also use any buffer with overvoltage-tolerant inputs like the SN74AHC541.

  • 0 in reply to Clemens Ladisch
    Prodigy 165 points

    Dear Sir, 
    We are using 5 V signals logic .

    1) In the applications section of the TXU0104 datasheet uni-directional translators, all the examples have VCCA < VCCB. my application requires me to go down from 5 V to 3.3 V. Would this component be suitable for my use then?  

    2) Would you recommend using an optocoupler to provide extra isolation or do these unidirectional translators provide sufficient noise immunity for the FPGA Digital Inputs?  

    3) In bi-directional translators like SN74LXC8T245, there is the provision of a direction pin, can we use such translators by fixing the direction pin acc to our requirement. 

    Thanks

  • 0 in reply to Tapajit Mondal
    Guru 318050 points

    1. Yes; the TXU translators support any combination of supply voltages.

    2. Optocouplers are useful only for galvanic isolation, i.e., when the grounds are different. All logic devices require a decoupling capacitor near each supply pin; this also reduces noise.

    3. Yes; you can connect the DIR pins directly to VCC or GND.