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SN74AUP1G17: Is this is loading issue? How many load can SN74AUP1G17DBV drive w 4mA?

Randy Fung58
Prodigy 10 points
Part Number: SN74AUP1G17
Other Parts Discussed in Thread: SN74LVC1G17, SN74LVC2G17

Dear Support, 

I am trying figure out how many loads can the SN74AUP1G17DBVR drive. I am seeing weird behavior using other vendors.

 

Here is our circuit on U21 with NC7SP17P5X (2.6 mA at 3V) driving 3 loads.  

 Please see attachment.NC7SP17P5X_Load_Drive_Capbility.pdf

Questions?

 

  1. Why is the output (red trace) did not go directly to 3V but has a step to 1.2V then 2.2V? It sometimes hang at 1.2V and never drive up to to high 2.2V.
  2. Is this a load driving issue?
  3. Do you have a load calculation application notes on how to figure out if 2.6 mA or 4mA is sufficient to drive the 3 loads?
  4.  Will replacing 4mA device SN74AUP1G17 resolve this issue?

Please advice.

Thank you.

Regards,

Randy

  • 0
    TI__Guru* 96591 points

    Hi Randy, and welcome to the forums!

    The output of the SN74AUP1G17 will be approximately equal to the supply voltage if the output current is less than 20uA -- ie if the resistive load is: R > 3.3/20uA = 165kohm. If the load resistance is smaller than that, the output voltage will begin to drop linearly to approximately 2.9V at 4mA, and down to 2.5V at 8mA.

    Since you are seeing an output of 1.2V, I would assume the current driving at the output is ~21mA.  You probably should switch to an SN74LVC1G17 if you need that much drive current. The AUP device is intended for lower power applications.

    You could also use an SN74LVC2G17 and parallel both channels to further increase the drive strength.

  • 0
    Guru 140200 points
    Hi Randy,

    weird... looks like there is also a ground problem. Have you a good signal ground management in your application? How look the scope plots when you connect the ground tip of scope probe directly to the ground pin of U21?

    Also, is there any cabling involved? Low long is the PJACK_IP line? What loads are connected to this line?

    Is U78 all the time on? Is there any relevant ripple at the output U78?

    Kai