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SN74HC125: Does the valid input voltage range depend on Vcc?

Adrian Torregrosa
Prodigy 90 points
Part Number: SN74HC125
Other Parts Discussed in Thread: SN74LVC125A, SN74AHC125, CD74HC4050

Hello Logic Forum.

I am planning on using an SN74HC125 being fed with Vcc=3.3V, having its OE always 0. My question is: is it safe to have its inputs range from 0 to 5V, or would it be safer to use voltage dividers instead, to reduce the input voltage range from 0-5 to 0-3.3V?

If the latter were true: would the use of another similar device instead of the SN74HC125 make it possible not to use voltage dividers?

Thank you in advance, best regards.

  • 0
    TI__Guru 55970 points
    Hi Adrian,

    Please refer to the FAQ on this question here below:
    e2e.ti.com/.../723607

    For HC technology family, you cannot have input greater than Vcc due to the positive clamp diodes.
  • 0 in reply to ShreyasRao
    TI__Guru 55970 points
    What is the usage, could you consider using a different device like the SN74AHC125 or the sn74lvc125a which accepts this overvoltage levels?
  • 0 in reply to ShreyasRao
    Prodigy 90 points
    Hello Shreyas, thank you for your kind reply.

    I have 4 results coming from other ICs which can be either 0 or 5V, which I must make available to a 3.3V GPIO. This GPIO happens to have a SW-controlled pull-up/pull-down set of resistor, which depending on the operating system can or cannot be configured or disabled. If I am able to disable them I could use a couple of resistors for each result in order to reduce the 5V to 3.3V. However, if I cannot disable them, the pull-down resistors seem to be active by deault, and that results on my 5V turned into 0.8V, which results in a LOW when it ought to be high.

    Therefore I thought of using a buffer, such as the SN74HC125, fed by 3.3V, between the 0-5V results and the GPIO. This should override the GPIO's pull-down resistors. But according to your explanation I would need voltage dividers before the device.

    As opposed to that of the SN74HC125, reading the SN74AHC125 datasheet I see that there are two lines:
    VCC Supply voltage 2 5.5
    VI Input voltage 0 5.5
    So, if I got it right, I can feed the SN74AHC125 with Vcc=3.3V and it would work fine with 5V inputs. Am I right? Would the CD74HC4050 be a valid alternative too? Maybe the SN74AHC125 is a better choice because the CD74HC4050 has a very wide DC Input Range, 0~15V.

    Maybe a bit off-topic, however: do you know of any document I can read in order to get an understanding of the the differences between the different IC families? HC, AHC etc.

    Best regards.
  • 0 in reply to Adrian Torregrosa
    TI__Guru 55970 points
    Adrian,

    You can read about the different families and related meaning of each value given in the datasheet, in this app note : www.ti.com/.../szza036c.pdf

    You could either go with SN74AHC125 or the SN74LVC125A which also tolerate higher voltage on the inputs even with lower Vcc.
    You could also consider using the CD74 device, but since they are little older technology family, I would not recommend them.