LM2679-5.0 Switching Regulator Design

Thread Starter

jma2388

Joined Feb 5, 2024
6
I am designing a constant 5 volt output switching regulator that bucks 12 to 24 volts down to 5 volts while supporting 5 amps. It is used to boot up a Raspberry Pi with a 10 inch screen. At 12 volts, the Pi will boot up fine. At 24 volts, around halfway through the bootup, the regulator will drop from 5 volts to around 3.5 volts, and that will cause the Pi to restart the bootup sequence. The current adjust resistor is set to 5.6 kΩ, which would allow almost 6 amps. I am at a loss for what is wrong.

Shown below is the schematic for the regulator. It is nearly the same as the recommended design from Texas Instruments.
1707162418052.png
 

LowQCab

Joined Nov 6, 2012
4,075
It's probably overheating from too much Voltage-Overhead.

Just because the maximum-ratings allow You to temporarily
get away with pulling huge Current, at a high-Voltage, doesn't make it a good idea.

Heat-Dissipation-Calculations must be done.
.
.
.
 

Thread Starter

jma2388

Joined Feb 5, 2024
6
It's probably overheating from too much Voltage-Overhead.

Just because the maximum-ratings allow You to temporarily
get away with pulling huge Current, at a high-Voltage, doesn't make it a good idea.

Heat-Dissipation-Calculations must be done.
.
.
.
It actually isn't getting hot at all. It isn't warm to the touch. The reference design which is identical to mine, states it is rated from 8 volts to 40 volts input, so I'd assume it would work for those ratings.
 

LowQCab

Joined Nov 6, 2012
4,075
SMD Devices can heat-up, and cool back-down, extremely fast,
especially if there are issues with soldering the Heat-Sink area to the Board, (which You can't see ),
or inadequate surface-area/thickness in the Board design.

And, I agree, where was the Chip purchased from ?
.
.
.
 

Thread Starter

jma2388

Joined Feb 5, 2024
6
We once had (2009) an almost exact same problem.
To make a very looooooooooooong story short, the IC was counterfeit.

Where did you purchase it?
I bought it from DigiKey. I've seen a lot of people say they bought them from eBay in other threads. Hopefully, DigiKey didn't screw me over.
 

Thread Starter

jma2388

Joined Feb 5, 2024
6
SMD Devices can heat-up, and cool back-down, extremely fast,
especially if there are issues with soldering the Heat-Sink area to the Board, (which You can't see ),
or inadequate surface-area/thickness in the Board design.

And, I agree, where was the Chip purchased from ?
.
.
.
Bought it from DigiKey. Should be a legit chip. There is solder on the back of it, it may not be enough. Is there a way to verify that overheating is the issue? I do have a thermal camera, but the refresh rate probably isn't spectacular. The board is 2 oz and the traces are decently wide.
 

crutschow

Joined Mar 14, 2008
34,455
I also suspect overheating.
The efficiency is shown to be about 87%, so for a 5A (25W) output, it can dissipate 3.25W of power.
If the device is not on a heatsink, it will likely overheat and shut-down.
 

Thread Starter

jma2388

Joined Feb 5, 2024
6
I also suspect overheating.
The efficiency is shown to be about 87%, so for a 5A (25W) output, it can dissipate 3.25W of power.
If the device is not on a heatsink, it will likely overheat and shut-down.
Is there a good way to verify that overheating is the issue besides adding a heat sink and seeing if that fixes it? I have left the display running for an hour plus and it still was not hot to the touch.
 

crutschow

Joined Mar 14, 2008
34,455
Is there a good way to verify that overheating is the issue besides adding a heat sink and seeing if that fixes it? I have left the display running for an hour plus and it still was not hot to the touch.
Since it doesn't appear to be overheating, perhaps it's going into current limit during startup.
Try reducing the value of the Rset resistor and see if that helps.
 

Thread Starter

jma2388

Joined Feb 5, 2024
6
Since it doesn't appear to be overheating, perhaps it's going into current limit during startup.
Try reducing the value of the Rset resistor and see if that helps.
I can try that. 12 volts works perfectly, but anything above 14 volts, give or take, always fails. That makes me second-guess everything.
 
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