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LM5118 Regulator soft start capacitor not charging fully when larger load is attached
So I am using an LM5118 as a voltage regulator. Vin = 26V nom, 42V max. Iout = 5A max. When using a load that draws 3A or less of current the circuit works perfectly. However, when a load that draws more than roughly 3A when the power board is turned vout = 12V Iout roughly 2A. If the load is disconnected and then reconnected the regulator will operate correctly outputting 24V and the > 3A output current.
To try and debug this I have been checking the mosfets and some other components on the board. The soft start capacitor will with no load attached store 1.36V on start up which is greater than the 1.24V reference voltage. However, when a load that draws more than 3A is attached before the power board is turned on the soft start capacitor will only store 0.44V which is less than the 1.24V reference voltage at the error amp. I am assuming this is why the output voltage is only 12V, but I don't know why the soft start capacitor is not powering up fully until the load is detached than reattached. I am using a 0.1uF capacitor.
Any suggestions would be appreciated.
Did you try to simulate your design on Webench?
WEBENCH Simulation failed, large enough mosfet not found (this must be a glitch, because I assure you there are mosfets big enough). Problem was solved by using a 1uF capacitor at the soft start pin instead of the suggested .1uF capacitor.
I'm glad you found a resolution. I'm not exactly why Webench didn't work in your case. You could try the quick start Excel calculator in the product folder under the LM5118.
I'm not sure either Ron. I used the quick start calc and it worked great.
I'm glad it worked.
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