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LM5118 Buck Boost, constant input, variable output, simulation challenge

Tom Minnis
Prodigy 60 points
Other Parts Discussed in Thread: LM5118

Is there a way to freeze the chip and topology and at least manually change the value of one part and run the simulation?  Web Bench assumes you want to design a fixed output Voltage with a variable input Voltage range.  I want to simulate a design that has a fixed input Voltage and by changing the feedback resistor that goes to ground, change the output Voltage.  For the load, I would like to put in a resistor rather than calling out a current.  The next think I would like to do is replace that Voltage divider resistor going to ground with a programable current sink to set the output Voltage.

  • 0
    TI__Intellectual 940 points

    Tom,

    Unfortunately I am not aware of any way to do what you are asking with Webench.  You could try using the Excel Quick Start in the LM5118 folder on the TI website.  There you could change the values you want and get a static picture of what is happening.  It isn't a simulation, just a calculation of currents and external values based on you input.  I have used it to do worst case analysis for a customer who had a variable voltage output and a constant current application.

    Regards,

    Ron Crews

  • 0 in reply to Ronald Crews
    Prodigy 60 points

    Ron;

    I gave it a try and it will help me get a little further down the road.  In my case, Rfb1 will be replaced by a current source which may affect the values calculated for the compensation network.  The load impedance will be Rfb2 instead of the parallel combination of Rfb1 and 2.

    Thanks

    Tom

  • 0 in reply to Tom Minnis
    Prodigy 60 points

    If I were to adjust the output Voltage to 73 or 75 Volts with 48V on the input, which pin on the LM5118 would be over stressed?

  • 0 in reply to Tom Minnis
    TI__Intellectual 940 points

    Tom,

    Since the datasheet specification for the maximum output voltage is 76 volts, you should be ok. 

    Ron Crews

  • 0 in reply to Ronald Crews
    Prodigy 60 points

    This is confusing because WEB Bench wouldn't let me put in an output Voltage over 70.  Again, just for my education, what part of the controller would be in trouble if I exceeded the output Voltage spec?  Assume I wanted an output Voltage of 100 with an input of 48.  Which pin on the part would be in trouble?

  • 0 in reply to Tom Minnis
    TI__Intellectual 940 points

    Tom,

    Webench is most likely allowing for voltage spikes which can be present, especially in transient conditions.  The Vout pin (which is an input) is connected directly to the output and would be where a problem would occur if the voltage exceeded 76 volts.  Anything over that and nothing is guaranteed.  It might work with one IC and fail with another.

    Ron Crews

  • 0 in reply to Ronald Crews
    Prodigy 60 points

    Yes, I see that but couldn't the sample of the output Voltage be divided down so that input never sees anything above 76 Volts?  Just curious why the chip design needs a direct connection to the output.

  • 0 in reply to Tom Minnis
    TI__Intellectual 940 points

    Tom,

    You cannot divide the output down since the output voltage is used for several functions.  First it is used to set the current used in emulated current mode control.  Also, it is used to determine which mode to be in and where to start the transition between modes.  It uses Vin and Vout in a multiplier circuit to determine duty cycle and from that when to start transitioning between modes.  Dividing it down would destroy that function.

    Ron Crews

  • 0 in reply to Ronald Crews
    Prodigy 60 points

    Thanks!  I just had to ask!  Are there any parts you could recomend that would take me from 48V on the input to a variable output between 24 to 120V delivering 40W to the load?