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LSF0108 Question

Mike Hassanpour
Prodigy 115 points
Other Parts Discussed in Thread: LSF0108, SN74LVC1G125, SN74AVC16T245, SN74LVC2G125, SN74LVCH16T245-EP, SN74HCT244

Hello,

 

My application calls for a Bi-Directional voltage translator Buffer with the following parameters. On the low voltage side +3.3V the FPGA will drive the line with a 510 Ohm pull-up to 3.3V rail and the output side has a 100 Ohm pull-up to +5V. We are currently using IDT74FCT​16245TPAG8 and getting 2V output when the FPGA is driving. I can't check to see what the FPGA sees (on the 3.3V side) when the 5V side is driving. We need at least 2.5V or preferably >=3V on the 5V side when FPGA is driving  that line. Do you offer a better solution? I have looked at your FET switches (LSF0108) and they could work for us except the FPGA can't sink more than 12mA.

 

Appreciate any help you can provide on this.

 

Thanks,

 

Mike 

  • 0
    TI__Guru* 96681 points
    Hi Mike,

    The LSF0108 can definitely be used to translate from 3.3V to 5V on a bidirectional data line.

    I have a few questions:
    How fast is the signal to be translated?
    What is the capacitance on each side of the translator?
    What is the drive strength on the 5V side?

    Also, a block diagram or schematic would be very helpful.
  • 0 in reply to Emrys Maier
    Prodigy 115 points
    these I/O's have 2uS on and duty cycle of 33% to 50%. I am not sure of the capacitance loading, from what I understand the long cable (800') is a very low capacitance and it's connected to the 100 Ohm Pull-up on the 5V side. If we can even drive 30mA of current thru the 100Ohm resistor, we will be ok. The datasheet says something about the driver needs to be able to handle the total current and that would mean the FPGA should handle the large current and that's not acceptable solution. I think the 16245 device can work in this application but will need to parallel 2-4 I/O's to support the VOH of 3Vat 30mA?

    Thanks,

    Mike
  • 0 in reply to Mike Hassanpour
    TI__Guru* 96681 points

    The LSF device is passive, so unfortunately it won't provide its own drive current.

    You could use a pair of SN74LVC1G125's after the LSF.  They can sink/source > 32mA at 5V.

    Here's how I would build it:

    You could also use a direction controlled translator such as SN74AVC16T245 and tie together the outputs as you suggested previously to reach your desired output current.

  • 0 in reply to Emrys Maier
    Prodigy 115 points

    Hi Emrys, Is there an octal version of the SN74LVC2G125? I did a search and didn't see an octal or 16 bit version of these buffers. I need 24 buffers in my application.

    Thanks,

    Mike

  • 0 in reply to Mike Hassanpour
    Prodigy 115 points
    Hi Emrys, I think SN74LVCH16T245-EP will work for the bi-directional data along with LSF0108. I also need a 22 unidirectional buffer, wondering if the FPGA side is 3.3V and the buffer side is pulled up to 5V, can I expect the VOH to be at least 3V with 32mA of drive on the 5V side? Seems to me that I might need LSF0108 in front of the unidirectional buffer also to get the full 32mA drive at the 5V side.

    Thanks,

    Mike
  • 0 in reply to Mike Hassanpour
    TI__Guru* 96681 points
    Hey Mike,

    You could use an SN74HCT244 - which is an octal buffer w/ 3-state outputs -- this has the added benefit of working with 3.3V inputs while the supply is set to 5V.

    One thing I would recommend keeping an eye on is the maximum allowed current through Vcc/Ground on a part. it's fairly easy to violate that number when you are ganging outputs for high-drive (ie some parts offer 32+ mA drive current per channel, but can only handle ~100mA per Vcc/Gnd pin).