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74ACT11244: Tying Outputs Together

Alec Forbes
Expert 3500 points
Part Number: 74ACT11244

Team-

My customer is trying to use the 74ACT11244 in an atyipical operation. They want to drive a 50ohm load at 48mA by tying 2 inputs together and then tying the corresponding outputs together (as shown below) to double the current drive. 

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Is there any issue using this method given their goals? If it will work, what size of output resistors would be recommended?


Thanks,
Alec

  • 0
    TI__Guru* 96591 points

    Tying outputs together isn't all that uncommon, in fact it's on our FAQ here.

    Yes - you can definitely tie together outputs in the manner described here.

    As for driving a 50 ohm load, each output of the 74ACT11244 can drive 24mA at 4.94V (50mA abs max), which means it would take ~4 parallel outputs to drive a 50 ohm load at ~5V (100mA). This is something you can do with this device, but you should note that the total maximum current for this device is 200mA, so if you push all of the outputs to the limit, you run the risk of damaging the part.

    I understand that your desire here is to impedance match the logic device output for the 50 ohm line, but in logic this is typically not required.  Just drive the line directly (use 0 ohm resistors) and only add resistance if you see signal integrity problems. Note that if you add enough resistance to impedance match the line, the output voltage will be 2.5V, as this device is not intended for communication over 50 ohm transmission lines.