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CD74FCT245: and CY74FCT245T typical and max VOH

Part Number: CD74FCT245
Other Parts Discussed in Thread: CY74FCT245T

Hi,

The datasheet of the CD74FCT245 claims to limit the output voltage swing to 3.7V, the electrical characteristics list a minimum value of VOH, but, is it safe to assume that the max value won't go above 3.7V (without pull up resistors)? If pull up resistors were used, is it also safe to assume that the dual-diode voltage drop of the output stages will be overridden? 

On the other side, the CY74FCT245T claims a typical VOH value of 3.3V under specific load conditions. What would be the maximum expected VOH with and without pull up resistors?

Regards

JS

  • Hi JS,
    I have notified the appropriate apps engineer - he will be back with you soon.
  • Hello Jesus,

    It is not really recommended to have the pullup resistors on the outputs of these devices.
    These devices are specified to TTL type logic conditions where these VOH values are high enough to ensure that the VIH value of the next device is met.

    What might you be trying to acheive with these devices?
    I may be able to find another device that is more suitable for your application if you need a higher VOH.

    What is the application? What are the voltage levels you are trying to get to?
    What drive strength is required?

    Best,
    Michael
  • Hi Michael,

    These devices (or the CY74FCT245T more specifically) are part of a design on which we are updating some legacy CPLDs that work with 5V logic (but with 3.3V TTL tolerance). We don't have physical access to the boards nor we have any hardware specifications that justify the decision of using pull up resistors to 5V on the output of these transceivers or why these were picked instead of a full 5V I/O device.

    The new CPLDs are not 5V tolerant and looking at the documentation of these transceivers it appears that it would be safe to keep them instead of using a level translator o replacing them with more appropriate devices. Our concern is that despite being rated for a typical VOH of 3.3V, a maximum value (not specified) could go over the maximum tolerable level of our CPLDs and damage them.

    Regards,

    JS

  • Hello JS,

    At low loads the VOH is very likely to be higher than 3.3V, so if you can't tolerate greater than that, I would not recommend using this part for that application, but I can help to suggest even pin to pin compatible parts that might work in your application.

    So what voltage is connected into the input of the device and what voltage would you like on the output?
    There are devices that could be used where the inputs can tolerate 5.5V and if powered from 3.3V, will ensure that the output is <3.3V.
    www.ti.com/.../sn74lvc245a.pdf would be a good one to look at.

    Let me know what you think or if I can better help!
    Michael