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SN74ACT244: About output at power up

Tao_2199
Genius 5740 points
Part Number: SN74ACT244

Hello,

Regarding to output on SN74ACT244, my customer is asking some question.

They will replace TC74ACT24 to SN74ACT244( 3-State Buffer).

(Question)

In case of TC74ACT24, at power up and no input, short pulse(glitch?:22usec) is occurred on each outputs.

When Vcc is more than 1.5V, it is occurred.

Please refer below wave capture. Each inputs are pull down.

They have a concern of this short pulse.



 

 

In case of SN74ACT244. is there possible to short pulse is occurred on above condition?

If so, is there possible cause?

 

Regards,

Tao 2199

  • 0
    Guru 298820 points

    The TC74ACT244 datasheet says:

    Operating Ranges

    Supply voltage VCC 4.5 to 5.5 V

    Note: The operating ranges must be maintained to ensure the normal operation of the device.

    1.5 V is outside the operating range.

    The SN74ACT244 has the same recommended operating conditions, so it also can have glitches during power up. Its datasheet adds:

    To ensure the high-impedance state during power up or power down, OE should be tied to VCC through a pullup resistor

    The only way do avoid garbage on the outputs is to have a microcontroller (or a discrete delay circuit) to pull OE low only after the power supply has stabilized.

  • 0 in reply to Clemens Ladisch
    Genius 5740 points

    Hello Clemens,

     

    I informed your answer to my customer and they could understand about output at power up.

    Many thanks!

     

    Regards,

    Tao 2199