I am using the LM5575-Q1 Buck Converter in a Buck Boost Configuration in WEBENCH. You can easily recreate this design by using these WEBENCH input parameters: VinMin = 20V, VinMax = 28V, Vout = -25V, Iout = 0.25A. (The negative 25V output corresponds to the "buck as buck-boost" configuration.
The WEBENCH analysis looks fine, except that WEBENCH has calculated the Rramp resistor value to be 360 mOhms. Based on the formula for Rramp in the LM5575-Q1 datasheet, I don't believe this makes any sense.
I think the reason for this miscalculation MIGHT be the fact that the math in WEBENCH is incorrect due to the unusual buck-boost configuration with its negative output voltage.
Could someone take a look at this and see if the Rramp value calculation in WEBENCH is correct? (I'm pretty sure it is wrong).
Thanks, David
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David Hildebrand
Analog Field Applications
South Region
Texas Instruments, Inc.
Office: 214-479-0917
Cell: 972-841-6499
david.hildebrand@ti.com
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