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Universal voltage-level translation

Discussion in 'Electronic Design' started by [email protected], Jul 20, 2006.

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  1. Guest

    Hi !

    I'm designing an universal JTAG programming/debugging dongle. I've
    found it a bit problematic to find good logic level shifters for my
    design. Basically I need output buffers interfacing 3.3V MCU with
    "device under test" which may operate at 1.8V-5V.

    My needs are as follows:

    - all buffers are controlled by MCU with 3.3V I/O
    - outputs can operate at any voltage between 1.8V and 5V, output Vcc is
    taken from the "device under test"
    - outputs have to be three-state kind. I need 3 signals with HiZ
    controlled in common, and 2 signals with separate OEs.

    Quick research led me to the following:

    1. I can use dual-supply level translation from TI, series SN74LVC1T45,
    SN74LVC2T45 etc. they have the right voltage ranges, unfrotunately no
    three-state capabilities (only bigger ones, 8bits wide and wider).

    2. Maxim's dual-supply translators, like MAX3390 - there are many
    different input/output configurations so I could choose something for
    me, however there are 2 requirements complicating the thing:
    - one of the supply voltages (called VL) always need to be lower or
    equal compared to another one (VCC) - I can eventually use 5V MCU I/O
    - THREESTATE has to be driven from VL side :( I need it to be driven by
    MCU, so from VCC side.

    3. I've found NC7SZ125 from Fairchild. It's three-state buffer with VCC
    range 1.65V - 5V, and overvoltage tolerant inputs - quite ok for me.
    Still I need 5V MCU I/O to drive its inputs, if I want to support full
    VCC range, but I can live with it (additional 3.3V -> 5V shifter would
    do the trick). Does anyone know similar parts, with such a nice VCC
    range and 5V-tolerant inputs ? Maybe more than one buffer per chip ?

    Anyone here designed similar thing ? Any suggestions welcome..

    BR,
    Przemek
     
  2. Klaus Bahner

    Klaus Bahner Guest

    do the trick). Does anyone know similar parts, with such a nice VCC
    Have you looked at TI's 74LVC2T45, which are dual rail 1.65 to 5.5V
    bidirectional transceivers. Might be overkill for your application, but
    the solved all level shifting problems I've encountered so far.

    Klaus
     
  3. Guest

    Yes, I even mentioned this part in my post - problem is that it lacks
    OE input (HiZ mode).

    BR,
    Przemek
     
  4. Jim Thompson

    Jim Thompson Guest

  5. Tim Shoppa

    Tim Shoppa Guest

    Many commercial programmers meet similar needs with not digital
    buffers, but with analog parts, e.g. op-amps (sometimes with push-pull
    pairs on the output for extra current push or to explicitly tri-state
    the output) and comparators rigged as schmitt triggers, and
    digitally-programmed power supplies setting the rails. I think some TI
    JTAG app notes show example circuits.

    Do you have any specs for rise/fall time, capacitive load, ?

    Tim.
     
  6. Jim Thompson wrote:

    I've just found it today, however I've already seen most of TI shifters
    datasheets :) Still it's a good review of what TI offers.

    Best Regards,
    Przemek
     
  7. I've seen fast comparators used for inputs, already. Yet I've never seen
    outputs done in analog way. Any schematics,articles,app notes welcome.
    As above, any pointers welcome. I'll search TI's site later today.
    Not yet, really :)

    BR,
    Przemek
     
  8. Jim Thompson

    Jim Thompson Guest

    Personally, I just roll my own. Unfortunately they're not stand-alone
    chips, they're part of a much larger ASIC. I think there's a hidden
    example in one of my USB patents.

    ...Jim Thompson
     
  9. Tim Shoppa

    Tim Shoppa Guest

    I searched my desk but didn't see what I thought I remembered from TI's
    app notes.

    But if you look at Microchip's TB013 they show nice drivers for
    delivering arbitrary programming voltages with op-amps and push-pull
    drivers. A small change lets you tristate the drivers.

    Tim.
     
  10. Tim Shoppa wrote:

    I've finally found a little time to review it - the schematic there seems to
    contain OP-AMP voltage followers with push-pull output stages, i.e. simple
    current amps.

    According to the app note, it's mainly needed due to capacitive loads on
    VPP/RESET line. In my case I will not need such a high drive capabilities,
    yet thanks for this pointer.

    BR,
    Przemek
     
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