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Electronics => Beginners => Topic started by: chrismec on March 31, 2021, 12:37:33 am

Title: 5V LDO immediately going into current limit
Post by: chrismec on March 31, 2021, 12:37:33 am

Hello all, first post here. I've recently run into an issue that I have never come across before. I have an MIC29501 (https://www.microchip.com/wwwproducts/en/MIC29501 (https://www.microchip.com/wwwproducts/en/MIC29501)) that I am using to power a raspberry pi 4. It is a 5V 5A regulator that can handle 24V continuous on the input. The problem I am having is the regulator immediately goes into current limit upon power up and starts generating a lot of heat and eventually goes into thermal shutdown. This is with an input voltage of 24V. With a lower input/output differential things start acting correctly. It seems this device cannot deliver 5A with a max continuous input voltage of 24V. How would I know this looking at the datasheet? I knew the differential would be an issue with a linear regulator but felt I was pretty safe in choosing an LDO. Can anyone point me to where in the datasheet I should have know this? Thank you : )

Title: Re: 5V LDO immediately going into current limit
Post by: MikeK on March 31, 2021, 12:42:14 am

(24V - 5V) x 5A = 95W!!!!

How big is your heatsink?

Title: Re: 5V LDO immediately going into current limit
Post by: Dubbie on March 31, 2021, 12:46:32 am

What temp do you think the core might reach in a second or so given 95W dissipation?

Check out the table in the datasheet.

Title: Re: 5V LDO immediately going into current limit
Post by: chrismec on March 31, 2021, 12:55:24 am

booooooo, what a dummy I am.  :palm: Don't know why I was thinking LDO had some magic sauce for large differentials. Ima switch to a proper buck converter. Thanks!

Title: Re: 5V LDO immediately going into current limit
Post by: Brumby on March 31, 2021, 01:01:55 am

Quote from: chrismec on March 31, 2021, 12:37:33 am

I knew the differential would be an issue with a linear regulator but felt I was pretty safe in choosing an LDO.

LDO = low drop out.  This means the minimum difference between input voltage and output voltage can be low and you can still have regulation.  It does not define the operating principles - which is what you seem to have assumed.

You worked that out.   :-+

Title: Re: 5V LDO immediately going into current limit
Post by: Dubbie on March 31, 2021, 01:03:57 am

Just to make the most of this lesson you have learned,
check out this video from Dave showing how to run the numbers.

https://www.youtube.com/watch?v=8ruFVmxf0zs (https://www.youtube.com/watch?v=8ruFVmxf0zs)