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SN74LVC2G74: Level translator

Julien Djn
Intellectual 840 points
Part Number: SN74LVC2G74
Other Parts Discussed in Thread: SN74LVC2G17

Hello,

I have an electronic board to make, those functions are :

- make acquisitions of two TTL signals (motor encoder output, max frequency is around 70kHz)

- divide by 4 the frequency of these two TTL signals

- shift the level voltage from TTL to 24V (will be plugged to logical inputs of an ECU)

For acquisition and frequency division, I plan to use a SN74LVC2G17 (schitt-trigger buffer) followed by two SN74LVC2G74 (D-type flip-flop) in serie.

Then, for the voltage level translator from 5V to 24V, I am not sure which component to use ... I was thinking using an opto-isolator like the ACPL-M43T (as I didn't find any TI isolator that allows 24V) but not sure it is a good solution.

Which component would you recommend to make the voltage level translator 5V (TTL) --> 24V ?

Thanks,

Best regards,

Julien

  • 0
    TI__Guru* 97046 points
    Hi Dejan,
    I'm afraid that 24V logic isn't a standard logic value and we don't have a translator that can go that high. Do you know what the switching thresholds are on the input? You might be able to get away with using 18V logic (CD4000 series devices) or even something lower if the thresholds are low enough.

    Otherwise, a discrete solution may be your only option - a simple inverter can easily be built with a descrete MOSFET or BJT that will handle that voltage, and you would just have to make a control signal input high enough to switch that discrete circuit.
  • 0 in reply to Emrys Maier
    Intellectual 840 points
    Hi,
    Thanks for your answer.
    Would you have any TI reference part for this MOSFET (or BJT) ?
    Best regards,
    Julien
  • 0 in reply to Julien Djn
    TI__Guru* 97046 points
    I'm afraid that TI is not in the discrete MOSFET/BJT business (with very few exceptions). You would have to look at a more general distributor.
  • 0 in reply to Emrys Maier
    Intellectual 840 points
    OK thanks for your answer, I will find the proper discrete component.

    Another question : is it necessary to use the buffer (SN74LVC2G17) between the input TTL signal and the D-type Flip-Flop input ?
  • 0 in reply to Julien Djn
    TI__Guru* 97046 points

    It will depend on the input signal integrity.  Schmitt-trigger inputs are used to clean up slow or noisy inputs prior to a standard CMOS input.

    We can define 'slow' inputs from the SN74LVC2G74 datasheet -- in the Recommended Operating Conditions table there is a value, Δt/Δv that indicates the required edge rates.  With TTL signals, you will need to have edges faster than 5ns/V (25ns maximum transition time at 5V operation)

    We can define a maximum extent for a 'noisy' input from the SN74LVC2G17 datasheet -- in the Electrical Characteristics table there is a value, ΔVT that indicates the minimum hysteresis that this device provides.  At 5V operation, the worst case is 600mV of hysteresis, so that means you can have noise up to 600mV pk-pk that the Schmitt trigger will tolerate.

  • 0 in reply to Emrys Maier
    Intellectual 840 points
    Thank you for these useful information !

    Best regards,
    Julien