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CMOS to TTL monostable multivibrator

Discussion in 'Electronic Design' started by PPP, Oct 26, 2006.

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  1. PPP

    PPP Guest


    I am looking at Texas Instrument's CD74HC538 monostable multivibrator.
    Going thru the datasheet, I noticed that it does not have anything for
    Voh (high level output voltage for TTL loads) at VCC ranges lower than

    Does that mean that I won't be able to drive a TTL switch if my voltage
    supply range is around 3V?

    I found an analog switch also on Texas Instruments that work at 3V, but
    it is labeled as having a TTL input. I wanted to interface this with

    Any hints?

  2. Andy

    Andy Guest


    The standard TTL (SN54) is not specified with the Vcc below 4.5V -
    that's probably the reason why the CD74HC4538 is not specified neither
    with TTL loads with the Vcc below 4.5V.
    It could work, but is not specified.
    What is the part number ?
    -- Andy
  3. PPP

    PPP Guest

    Hello Andy!

    The switch part number is TS5A3159. It does not say that it has a TTL
    Control Input feature on the datasheet, but it states this in the TI
    Analog Switch Guide.

    I'm new to this CMOS to TTL interfacing. After giving it much thought,
    the TI selection guide probably meant that TTL devices can also drive
    the switch (?). I figured since CMOS devices normally have voltage high
    outputs close to VCC and voltage low outputs close to VSS (GND), the
    voltage input high and voltage input low specifications for the
    TS5A3159 will be within the CD74HC538 voltage high and low output
    range, no?

  4. Andy

    Andy Guest

    Most certainly, I have not looked through the guide, but the input logic
    thresholds of the TS5A3159 at 5V suppy (4.5 to 5.5v) match the standard
    TTL output specifications.

    Look at the input logic voltage specifications of the TS5A3159, for the
    3.3V supply. For the Vcc range from 3.0 to 3.6V, the input logic high is
    specified at 3Vcc, from the specs at 2Vcc and 4.5Vcc (CMOS loads!), one
    can safely derive that at 3 to 3.6Vcc its outputs will finely drive the
    TS5A3159 digital inputs (powered at the same Vcc).

    When interfacing TTL & CMOS, one also has to pay attention to the
    input/output current specifications, and ensuring the required switching
    speed is not quite simple.
  5. neon


    Oct 21, 2006
    who cares what the voltage is providesd is surpasses the voltage threshold. 3v hi is good believe it or not TTL things to watch for is fan-in fan-out capability. basicaly is specified as unit loads. confused want to know more?
  6. John Fields

    John Fields Guest

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