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I built a circuit where I use a D-type flip flop (74LS74A) to directly drive an optocoupler (LTV-817) and the circuit seems to work fine.

I measured the output current from the Dff (Q) to the opto input and in the logic HIGH state it is 25 mA (the opto input is rated up to 50 mA). However when reading the flip flop datasheet one thing is not clear to me.

In the first part it specifies the HIGH level output current (max) as -0.4 mA, while the LOW level max is 8 mA. Then on the other page it specifies the short circuit output current (max) to be -100 mA, with a supply current max of 8 mA. My doubt was whether the max output rating is 8 or 100 mA.

As I mentioned the circuit seems to work fine so I suspect the maximum output current rating is the 100 mA, but I hoped someone could clarify the datasheet entries so I don't get any surprise failures later on.

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  • \$\begingroup\$ Please refer your datasheet \$\endgroup\$
    – Julien
    Jul 27, 2023 at 20:18
  • \$\begingroup\$ The output can sink more current when low. So why don't you drive the optocoupler's cathode? \$\endgroup\$
    – CL.
    Jul 28, 2023 at 7:42
  • \$\begingroup\$ Hi CL, thanks for your suggestion. I've added a 1K resistor at the DFF output which reduces the current to 2.5 mA and the opto still switches properly, since this is the simplest solution I think I'll go with it. \$\endgroup\$
    – meext4
    Jul 28, 2023 at 8:32

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What does note 8 says?

Note 8: Not more than one output should be shorted at a time, and the duration should not exceed one second. For devices, with feedback from the outputs, where shorting the outputs to ground may cause the outputs to change logic state an equivalent test may be performed where VO = 2.125V with the minimum and maximum limits reduced by one half from their stated values. This is very useful when using automatic test equipment

Therefore, I can conclude that you are operating this device outside it's specified range and it could fail soon.

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  • \$\begingroup\$ Hi Julien, thanks for clarifying this. I will add a transistor amplifier at the D-FF output to prevent problems and keep the output current below 8 mA. \$\endgroup\$
    – meext4
    Jul 27, 2023 at 22:16
  • \$\begingroup\$ Since you only have two states, you can easily use a mosfet to drive it. No need for a full amplifier design! \$\endgroup\$
    – Julien
    Jul 27, 2023 at 22:41

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