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LM5117: Error on Floating Pins

Evgeni Gofman
Prodigy 100 points

Part Number: LM5117

Hi TI Team,

I am trying to simulate LM5117 buck design.

looking at the DS, I can leave the pins :VIN,CM,DEMB floating but I get an error every time I try.

VIN - left unconnected since I supplied an external VCC (client asked for it) and provided the voltage to VCCDIS.

CM  - not used, looking at the DS it can be left floating

DEMB -  Left floating : 

the ERRORS I get : 

  • 0
    TI__Guru 68581 points

    Hello Evgeni 

    Please connect the VIN pin to the input supply power and connect 10kOhm between the DEMB pin and the ground connection of the current sense resistor.

    Also, populate 100pF capactior between the CM and ground. 

    -EL

  • 0 in reply to EricLee
    Prodigy 100 points

    Hi Eric,

    I did that and it worked (there are some different problems but I will open a new ticket on it)

    I don't want to use the internal VCC. do I have to connect vin to the input supply just for the simulation to work or in real design also? 

    also I want to leave this pins unconnected in the real circuitry..

    how does this connection that you suggested effect the simulation?

  • 0 in reply to Evgeni Gofman
    TI__Guru 68581 points

    Hello Evgeni

    The VIN pin should be connected to the input power supply for both simulation and bench test. Please don't leave the pin unconnected. 

    -EL 

  • 0 in reply to EricLee
    Prodigy 100 points

    Hi Eric,

    Please  ELABORATE on your answer. 

    1. the datasheet says that VIN is to provide voltage for the INTERNAL voltage regulator. I am using an external power supply to provide the needed voltage 

         for the INTERNAL voltage regulator and also supplied the needed voltage for the VCCDIS just like the datasheet asked - why are you telling me to

         connect the VIN pin.

    2. How does this connections that you suggested effect the simulation?!

    3.Why does the TRANSIENT REFERENCE DESIGN works with some of this pins left unconnected?

  • 0 in reply to Evgeni Gofman
    TI__Guru 68581 points

    Hello Evgeni

    The VIN pin supplies not only the internal VCC regulator, but also other circuits inside the IC. Please don't leave this pin unconnected.

    Simulation model is not 100% identical with the real silicon circuit. The VIN pin in the simulation model supplies only the internal VCC regulator.   

    -EL