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LM27403 and Low Loads

Electronnuts
Prodigy 160 points
Other Parts Discussed in Thread: LM27403, CSD17573Q5B

Building a DC/DC power rail based on the LM27403 with the major driving factor being efficiency. Im looking to achieve over 95% with a 5v power source and .8-1v core voltage output with total load being under 15 amps. Typical load will be between 2-10 amps. 

Webench tells me that the LM coupled with CSD16323Q3C /CSD17573Q5B  High/low MOSFETS  switching  200khz and a 1.3uH inductor with 1.8mOhm DCR will get the job done with over 94% efficiency in my typical load range and 96% at nominal load of 5A. 

The LM data sheet (section 9.2.2 and 9.2.5) shows the buck coupled with Power Block NextFETS (which combine high/low mosfets into one). For my application is there a reason to go with the power block route? I don't see any way to generate schematics or simulate efficiencies with these power block mosfets. 9.2.5 shows an example of a "ultra high" efficiency setup using DUAL power blocks for a 5v input rail. Would this "ultra high" efficiency be greater than what webench is showing me for application?

  • 0
    Prodigy 160 points

    So I found the design tool for the LM27403 which you can manually input the specs of CSD87353Q5D Efficiency is about the same but save lots of space so I think I will go this route. Now issue is choosing right inductor. Specs for LM27403 + CSD87353Q5D say a 300nH inductor...but isn't this way too low? Im getting over 100% ripple current with 6A of load and a 500nH inductor.

    Does the buck have skip mode for low loads or something? Or do these example circuits just assume there will never be any low loads? If I used a 300nH inductor like it suggests the inductor ripple current would be at 11A at 1v output....this is no small load and the circuit can't be expected to operate at only >11A. 

    What am i missing here?

  • 0 in reply to Electronnuts
    Prodigy 160 points
    Can someone from TI chime in here and explain what would happen if load dropped below 11A on this circuit? And whether low loads is even viable with this design?