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SN74LS244: Start-Up Behavior

Part Number: SN74LS244
Other Parts Discussed in Thread: SN74LS07, SN74HC244

[ SN74LS244 ] Start-Up Behavior

Hi,

My customer tests the SN74LS244 together with photo couple as drawing below.


During their initial evaluation, they have seen unexpected pulse from photo coupler output.




I assume that this is expected behavior of the combination of SN74LS244 and photo coupler. During voltage ramp-up, from certain voltage, SN74LS244 starts its operation and try to make output low during the input signal is in low level(<0.8V). Thus we have voltage difference which enough to turn on photo coupler.

To prevent this behavior, what we need to do is replace SN74LS244(3-state output) to some other device with Open-Collector or Open-drain output.
Do you have any other ideas with SN74LS244?

The reason why this behavior is unexpected for my customer is because they have used SN74LS07 which is open collector output.
With this device, output voltage of SN74LS07 just follows VCC5V, so there is no voltage difference.

It's appreciated if you can share your idea with me.
Regards,
Ken







  • I would have thought a pull-up resistor on the LS244 input would fix your issue here, or pull-up on the output of the LS07. The opto LED will then not be biased at initial power up.
  • You could try pulling the output enable pin (1G\ and 2G\) up to Vcc during startup through a 10k resistor. I haven't worked with this particular device, but this would be my first idea for a fix.

    Have you considered going with a CMOS device such as SN74HC244 or SN74LVC244?
  • Hi Raymond, Emrys,

    Thank you for your comment on my post! It helps me a lot!
    I have been talking with my customer and their solution was to replace LS244 to LS07...

    Anyway, thanks!
    Ken