LM2679-12 voltage drops under load

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Reply to "LM2679-12 voltage drops under load"

Anonymous
on 05/06 at 06:35 PM

Category(s):	power.national.com
Subject:	LM2679-12 voltage drops under load
Message: (Notified via email on every direct response)
I've got a board with 2 LM2679-12 regulators. They are both identical circuits but independent of each other. They operate at 24V and the intended load current should be 5A max. The inductor value is 33uH and the output caps are 2x 330uF. The current sense resistor is 5.36K The max load without the voltage dropping below 11.9V is about 3.5A. The datasheet indicates that with these component values I should get the maximum 5A. With a 4.8 amp load the voltage drops to about 9.5V. I simulated with the LM WeBench software and it indicated to use a 22uH inductor and a 180uF output cap. I tried this and it made no difference from the components that I originally designed with. Does anyone have any suggestions as to why the voltage is dropping off like this? I have good thermal relief on the board but the face of the chip (plastic side) is about 220 degrees F. Would a bad thermal connection result in the chip limiting the current? The max on the datasheet is 257 degrees F. I am running out of things to try and really need to get this board up and running. Any help is much appreciated.

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LM2679-12 voltage drops under load (Anonymous)
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