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[ SN74LS244 ] Terminate Unused Input to Different Voltage

Kenichiro
Mastermind 9165 points
Other Parts Discussed in Thread: SN74LS244, SN74LVC244A

[ SN74LS244 ] Terminate Unused Input to Different Voltage

Hi Expert,

I understand that it should be prevented to leave unused input pin open (floating) for CMOS input, and it's suggested to connect either Vcc or GND.

However, what about connecting to the different voltage than Vcc?
As indicated in the drawing below, SN74LS244 is powered with 5V, however, unused input is connected to 3.3V via a resistor.

Can we recommend this connect?

Thanks,
Ken

"Must-Check" FAQ before posting...
Floating the inputs of Logic devices
e2e.ti.com/.../2945.floating-the-inputs-of-logic-devices

  • 0
    TI__Guru 58370 points

    Hi Kenichiro San ,

    The 'LS244 is bipolar device and not CMOS device. The bipolar device the unused input should not be connected to gnd. This is suggested to be connected to Vcc via a series resistor.There will be excessive input current and icc as well.

    you could refer to designing with logic app note page 11.

    On another note, if you could choose sn74lvc244a which was recently released in tiny package(X1QFN) it allows for space savings as well as very low input leakage and Icc .

  • 0 in reply to ShreyasRao
    Mastermind 9165 points

     Dear ShreyasRao,


    Thank you so much for your prompt reply and correction!
    I see that LS244 is bipolar (Not CMOS), good.

    So, my customer does as below for three unused inputs.
    Please correct me if something wrong.

    - 3 unused input (#4, #15 and #17) connected to 3.3V via a 4.7kohm resistor.
    - Leave open 3 outputs (#16, #5 and #3)


    Thank you for confirmation!
    Thanks,
    Ken

  • 0 in reply to Kenichiro
    TI__Guru 58370 points
    Ken ,

    The rail being 5V for the device operation , I would suggest to be connected to rail instead of 3.3V .
    outputs can be left open.