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CY74FCT157T: The O/P pins are driven at 2.5V, but Vcc=3.3.V

Rasif Rasheed
Intellectual 370 points
Part Number: CY74FCT157T
Other Parts Discussed in Thread: SN74LVC157A

Hi,

I am using the MUX to choose between two SPI interfaces. The input signals are driven at 3.3V and the Vcc=3.3.V. But the O/P pins are being driven at 2.5V.

Can you please explain the reason?

The schematics is as attached above. 

Regards,

Rasif

  • 0
    TI__Guru* 96712 points
    Hello Rasif,
    The CY74FCT157CT is only defined to operate between 4.75 and 5.25 V. Please see the Recommended Operating Conditions table.

    It's not surprising to me that the output is lower than the supply -- this is clearly defined in the Electrical Characteristics table -- V_OH with a supply of 4.75V and a drive current of 15 mA has an output voltage of 2.4V (min), 3.3V (typ). That's a drop of (typically) 1.45V, and up to 2.35V.
  • 0 in reply to Emrys Maier
    Intellectual 370 points

    Hi Emrys,

    Thank you. Can you provide an alternative device that can work on 3V3 with a similar footprint? 

    Regards,

    Rasif

  • 0 in reply to Rasif Rasheed
    TI__Guru* 96712 points
    Hi Rasif,
    We have a parametric search table to find similar devices here: www.ti.com/.../products.html

    Unfortunately, you just have to know that this device is under "Encoders/Decoders" - but after that it's pretty easy. Just find a device that also is the '157' function and verify the pinout matches.

    For example, the SN74LVC157A is a great choice to replace this device.