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1.5-to-5.5V to 3.3V level translator options

Philip Pemberton
Intellectual 255 points
Other Parts Discussed in Thread: TXB0108, SN74LVC2T45, SN74LVC8T245

I'm currently looking for a level translator solution to replace a pair of TXB0108s in a JTAG/ISP pod design. The current design has an internal Vcc of 3.3V, which is translated down to 1.2V (or 1.5V depending on voltage regulator selection) by the first TXB0108, then translated back up to the target voltage (which may be anywhere from 1.5V to 5V). The problem is, the limited drive strength of the TXB0108 causes the current design to fail when pullup or pulldown resistors are present in the target circuit.

The pod has five outputs and two inputs --

  JTAG TCK/TMS/TDO output
  JTAG TDI input
  ISP DAT0/CLK outputs
  ISP DAT1 input

Drive requirement is 10mA. Translating down and then back up would be one option, or separate unidirectional "input" and "output" translators.

I have considered the SN74LVC2T45 and SN74LVC8T245 products (a '2T45 for the inputs and a '8T245 for the outputs), though these only appear to operate down to 1.65V per datasheet specification. The 74AVCH versions of these parts are specified to operate down to 1.2V, but only up to 3.6V which is insufficient for 5V targets.

Do TI offer a product which has the TXB0108's low voltage capability while also offering the higher drive currents necessary to drive cables and I/Os with attached pull-up/down resistors?

  • 0
    Guru 25210 points

    You're going to be hard-pressed to find something specified over that range. I didn't see anything in TI's portfolio and nothing in Fairchild's either.

    An option could be to build your own with transistors (like BSS138) or consider an analog comparator based circuit.

    If this isn't for a product but just something internal, you may get away with the 1.65 V parts working down to 1.5V, but beware of increase in propagation delays etc.

  • 0 in reply to Brian Boorman
    Intellectual 255 points

    I've been doing some digging and it turns out the 1.5V spec was never put into practice - the TXB0108 based hardware I have here was used down to 3.3V and that was it - the lowest voltage on the ISP or JTAG pins in anything designed or built was 1.8V (and more commonly 3.3V, occasionally 5V).

    So it looks like my revised spec is 1.8V to 5V (ideally 5.5V), which would make the SN74LVC2T45 and SN74LVC8T245 pairing a perfect choice.

    Thanks for the answer - it's nice to have a second pair of eyes to confirm that I'm not going mad, there really aren't any chips which can handle that wide a range (officially!). If I do end up designing anything with a 1.5V JTAG voltage, I'll add a couple of low-voltage bus transceivers to the PCB.

    Thanks!