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LM5088: Current range ambiguity on Webench

Rohit J
Intellectual 850 points
Other Parts Discussed in Thread: LM5088

Hi,

This is an urgent request. I want to use LM5088 for Vin = 9 ~ 16V -> 5V and test it over 0.1 ~ 7A.

The initial parameters make me select an Io max = 7A.

In the simulation tab, the tool allows me to select Rload between 10 ~ 0.5 ohm which translates to a current range of 0.5 ~ 10A, when I run for Bode plot. 

On the other hand, the tool allows me to select between 0.01 ~ 10A when I run for Load transient plots.

The datasheet doesn't really specify any minimum current range. Naturally, my components will be sized for Io = 7A.

I want to know how the circuit will perform for a 7A ~ 0.1A transient and vice versa as well as the bode plots for both.

Thanks!

  • 0
    TI__Intellectual 2360 points
    Hi Rohit,
    The simulations are automatically set up with input voltage and loads based on your design inputs. For example, Bode plot has a default load of 7A (in your case), and Load Transient is set to transition between 1/7A and 7A (again, in your case).

    You can change those load values if you like. Select your desired Simulation Type (Load Transient, for example). If you've already run the simulation type before, it will display previous results. The button next to "Design Version" will be active, so click "Show Latest Design Version" to be able to set up a new sim. Then click the output load symbol; a form will appear having the title "Component Simulation Parameters." Select I1 to see the nominal initial current setting. Enter your new value, click "Save changes." Do the same for I2. When you're done editing, dismiss the form. Then click the green button "Start New Simulation."

    The maximum value of 10A appears for these load values because that's the highest current the LM5088 application has been configured for. It may be above the target max load of your design, so you can see what may happen when the load is too high.
  • 0 in reply to Wanda Garrett - WEBENCH Design Center
    TI__Intellectual 850 points
    Hi Wanda,

    Thanks for your response !

    It was not a problem with exceeding the maximum current but the minimum one. The data sheet provides no information for minimum current requirements whereas Webench places a limit. Do you know why that would be the case?

    Also why does it have different current ranges for Bode plot and Load Transient? The maximum for both is the same i.e. 10A. It is the minimum current value that I can't understand.
  • 0 in reply to Rohit J
    TI__Intellectual 2360 points

    Hi Rohit,

    The Load Transient simulation uses the transient model, where the Bode Plot simulation uses the average model.  The two models do have different minimum operating currents due to the way they behave.  The transient model works best (converges best) with load current no lower than 10% of the design's maximum, so this is the minimum that is shown in the load current configuration control.

  • 0 in reply to Wanda Garrett - WEBENCH Design Center
    TI__Intellectual 850 points
    Hi Wanda,

    So is it safe to conclude that the minimum currents in the simulation models are not a reflection of the device capabilities at light loads but a limiting factor of the models themselves? Do you advise any way around this?

    Best,
  • 0 in reply to Rohit J
    TI__Intellectual 2360 points
    Hi Rohit,
    To run a simulation of a WEBENCH design without the protective limits, or with a different circuit configuration, you can start with your LM5088 WEBENCH design and take it into the Edit environment. At the top of your design view, in the navigation bar, there is a button titled "Edit." Click that, to copy your design into the Schematic Editor tool.

    (1) From there, click the Sim button in the navigation bar. Your design will be already set up for a startup simulation. Click "Start New Simulation" to see the timing of the startup - you need to know this for the next step. When the simulation has completed, identify the node for your output voltage and click the probe to see the waveform. For example, if the node is #9, click VV9 to see the waveform.

    (2) Identify when the output voltage has settled to its final value. For my example, it was settled at t=3ms, and the full window was 5ms.

    (3) Now, using the navigation bar "Edit" button, go back to the Schematic Editor. Add a current source in parallel with your Rload resistor.
    To do so: Place Part > Instruments (esim) > PULSE CURRENT SOURCE. Put the current source in a clear spot on your schematic, and click Rotate until it's the correct orientation. Then move it to be in parallel with Rload, and add wires to connect it to the circuit.

    To set the pulse parameters for the current source, click its schematic symbol, and see on the left a form for Electrical Simulation Parameters. Change I2 from 0 to -6.9A (because Rload give 7A load, and you want to test at 0.1A). Change Td to 3m, and PW to 1m (assuming that timing works based on step (2) above.

    (4) Go back to the Sim view and run a simulation again. You will be able to see the load transient response with load from 7A to 0.1A to 7A.