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SN74LV1T125: Level Translation for PWM

tedex45
Prodigy 190 points
Part Number: SN74LV1T125
Other Parts Discussed in Thread: SN74LVC1G57, SN74AHC1G125, SN74LVC1G97, SN74LVC1G99, SN74LVC1G98, SN74LVC1G125, , SN74LVC1G58, SN74LV1T34, SN74AHCT1G125, SN74LVC1T45

Hi, I have a micro-controller with only 3.3V tolerant inputs. I would like to accurately interface it to 5V PWM inputs and also drive out 5V PWM outputs. 

What TI device would you recommend for this application. 

  • 0
    Guru 256980 points

    To convert from 5 V to 3.3 V, use a buffer with a supply voltage of 3.3 V that has a 5V-tolerant input. All devices in the AHC(T), LV1T, and LVC families have 5V-tolerant inputs, so you could use SN74AHC1G125, SN74AHCT1G125, SN74LV1T34, SN74LV1T125, SN74LVC1G125, SN74LVC1G57, SN74LVC1G58, SN74LVC1G97, SN74LVC1G98, or SN74LVC1G99.

    To convert from 3.3 V to 5 V, use a buffer with a supply voltage of 5 V that has a TTL-compatible input. All devices in the AHCT and LV1T families have TTL-compatible inputs, so you could use SN74AHCT1G125, SN74LV1T34, or SN74LV1T125.

    For any kind of conversion, you could use a dual-supply translator such as the SN74LVC1T45.

  • 0 in reply to Clemens Ladisch
    Prodigy 190 points
    Thanks for the quick reply. Am I correct in understanding that the SN74LVC1T45 is able to drive 5V PWM signals from 3.3V PWM inputs and also shift 5V PWM down to 3.3V PWM? Thus I wouldn't need 2 different devices, I can just control the direction of transfer?

    Looking at the devices you listed, I saw that some have a Schmitt Trigger. What parameters should I look out for in deciding for a device that accurately interfaces to 0-5V PWM signals. In my application, I would just be using input capture or generate high frequency PWM waves.

    Thanks again!
  • 0 in reply to tedex45
    Guru 256980 points
    Using a single SN74LVC1T45 and switching its direction is possible only if your hardware actually uses the same signal for I/O in both directions.

    (The SN74AHCT1G125, SN74LV1T34, and SN74LV1T125 appear in both lists.)

    None of these devices have 'real' Schmitt triggers, but a proper PWM signal should not need them. But if your signal does have edges that are slower than allowed in the datasheet, then you have to use a real Schmitt trigger buffer (*17 in any of these families).

    Which family to use depends on how much drive strength you need.