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LM66100: LM66100 PSpice ST Pin Operation

Larry Johnson
Intellectual 320 points
Part Number: LM66100

I plan to use the Figure 20 (LM66100 with MOSFET) in a Backup Power Circuit. I used the PSpice for TI to make sure I understood the operation of this circuit.

The attached PDF has the circuit and 2 waveforms (Waveform1) with R1 = 100 ohms and (Waveform 2) with R1 = 1K. (I also tried R1 = 10K). In this example Circuit I disconnected the MOSFET output so I could see its actual switching characteristic when controlled by the ST pin.

My understanding is that the ST output is an Open Drain MOSFET capable of around 1ma. However the simulations show the ST pin is not an open drain output. Even when I disconnect U4 gate Input and U3 MOSFET gate pin the same waveform is display with R1 = 100 and R1 = 1K.

Any help would be great, especially if you could let me know if there is an issue with the LM66100 model ST pin. I need to release this circuit design soon.

Take care.LM66100 Model Issue.pdf

  • 0
    TI__Mastermind 32650 points

    Hi Larry,

    The ST pin in LM66100 is indeed an open drain pin. The maximum rated current for this pin is 1mA.

    The workings of this pin are: If VIN > CE then LM66100 is ON and ST is floating/high-Z. If CE > VIN then LM66100 is off and ST is pulled to GND internally.

    Figure 20 describes OR-ing with an external PMOS. One major difference between the figure and your setup is that the output of LM66100 and the source of the PMOS has to be connected to the same output. Can you please make this change in your setup and retest without the SN74LVxx connected? Could you please probe VIN, CE, ST and the unified output (VOUT) and share the results?

    I also noticed that even in the 100Ohm case, the ST pin does not rise up to 5V and in the 1kOhm case, the ST pin barely rises at all. If you get the same result as before, please use the 1kOhm pull up and measure the current going into ST. Please provide that waveform to us as well.

  • 0 in reply to Shreyas Dmello
    Intellectual 320 points

    Hello Shreyas,

    Thanks for replying to my question.

    Attached is the P-Spice Simulation you requested. I believe it show that the ST pin never floats to the 5v input, thus the MOSFET never turns OFF.

    The VOUT pin never reaches +5v because the MOSFET is connected at the same time to VOUT causing a major current draw. I am hopping that something in the P-Spice model is incorrect for the ST pin or I will not be able to release my PCB design this week. Please let me know if I am incorrect about this circuit or the simulation.

    Please take care,

    Larry Johnson

    LM66100 ST Pin P-SPICE Issue.pdf

  • 0 in reply to Shreyas Dmello
    Intellectual 320 points

    Hello Shreyas,

    Thanks for replying to my question.

    Attached is the P-Spice Simulation you requested. I believe it show that the ST pin never floats to the 5v input, thus the MOSFET never turns OFF.

    The VOUT pin never reaches +5v because the MOSFET is connected at the same time to VOUT causing a major current draw. I am hopping that something in the P-Spice model is incorrect for the ST pin or I will not be able to release my PCB design this week. Please let me know if I am incorrect about this circuit or the simulation.

    Please take care,

    Larry Johnson

    7242.LM66100 ST Pin P-SPICE Issue.pdf

  • 0 in reply to Larry Johnson
    TI__Mastermind 32650 points

    Hi Larry,

    I was able to recreate your issue on my computer.

    At this time, I think the possibility that the PSPICE model is incorrect or bugged is present. I cannot confirm this at this time but I am looking into it with the help of the team at large.

    I would highly recommend you use the LM66100 EVM to run this test to confirm your schematic. Is such a workaround acceptable for the moment?

  • 0 in reply to Shreyas Dmello
    Intellectual 320 points

    Hello Shreyas,

    I do not have the LM66100 EVM, but I do have the LM66100 ICs. I will see if I have a way to make a prototype.

    Thanks for the fast simulation check, I will let you know if I get the prototype working.

    Take care,

    Larry Johnson

  • 0 in reply to Larry Johnson
    TI__Mastermind 32650 points

    Hi Larry,

    Thank you. You can acquire the EVM from our online store if needed. Do note that our EVM is set up for dual LM66100 OR-ing already and so this may help with your system.

    For now, I will close out this thread but please feel free to open a new thread if you have more questions.

  • 0 in reply to Shreyas Dmello
    Intellectual 320 points

    I did build the LM6100 Figure 20 "ORing with Discrete MOSFET" circuit. I used an Si2323DS for the MOSFET.

    The circuits works great as a Backup Power Circuit:

    1. With VIN >CE then ST = VIN voltage (example if VIN is =5v then ST = 5v [Open Drain Operation]); VOUT = VIN

    2. With CE >VIN  then ST voltage is 0v; VOUT = CE voltage (the Backup Power Voltage)

    The circuit works correctly; however the P-Spice model for the LM66100 needs to be fixed so that others do not have the same problem that I had.

    Thanks again for your help and take care.