LM22676-ADJ It's heating to much but its no consuming much current - Page 1
Forum section: electronics / projects

author: Velth
author_rank: Contributor
timestamp: May 26, 2023, 05:49:00 pm
content: Hello, how are you? I'm using an LM22676 in this board project, and it's getting very hot and burning. I've used this schematic in other boards before without any issues, so I feel like I made a mistake in the routing of this board.It's getting very hot and burning the LM sometimes.What is happening is the following: I have two LM22676 chips, one with a 3V output and the other with a 5V output. The one with the 5V output is getting very hot even though it's not drawing much current, around 50mA. This is happening despite the fact that my project includes LEDs, relays, and other components.Things I've already done include cutting and bringing the feedback resistors closer to the source, cutting the 5V line and leaving a longer one, but all without any effect. I would like someone to help me identify the problem so that I can fix it on the next board.I have used this schematic multiple times before and have never encountered such issues.

author: Faranight
author_rank: Unknown
timestamp: May 28, 2023, 06:08:10 am
content: Did you solder the flywheel diode in correct orientation? Happened to me once.Else, try desoldering and replacing the existing switcher chip with a new one, maybe the IC is bad?

author: timenutgoblin
author_rank: Regular Contributor
timestamp: May 29, 2023, 08:47:18 am
content: Quote from: Velth on May 26, 2023, 05:49:00 pmWhat input voltages (VIN) are you using for the LM22676 ICs? The IC requires at least 4.5V to operate or approximately 3V more than the output voltage. If the input voltage (VIN) is too low, the duty cycle of the switching element in the IC regulator will be too high to operate optimally. Is the exposed thermal pad (EP) on the IC soldered to the PCB properly?As @Faranight suggested, the IC may have failed.

author: Wolfram
author_rank: Frequent Contributor
timestamp: May 29, 2023, 11:43:53 am
content: What's your incoming supply voltage, and how is it switched? Do you have any bulk electrolytics, damping, clamping or inrush limiting on the incoming supply rail? A common problem when hot-plugging power is that resonance between ceramic decoupling caps and wiring inductance can cause a large overshoot on the supply rail. With ideal and lossless inductors and capacitors, overshoot would be 2 * Vin, but with real ceramic caps it can actually be worse than this as their capacitance changes significantly with applied voltage. The attached scope image shows a 40V supply being hot-plugged into a ceramic decoupling cap, with just the inductance of the wires in the test setup. Wiring was kept loose to get a moderately large, but not unrealistic, amount of inductance for the test. In this case, the peak reached 130 V, 3.25 times the steady-state value.

author: Velth
author_rank: Contributor
timestamp: May 29, 2023, 05:37:45 pm
content: Thank you all for the responses. Well, here are some information and what I have already done. I'm supplying it with 24V (I noticed that the heat increases if I increase the input to 12V, it heats up almost nothing, and at 32V it gets hot enough to blow the fuse). Remember that I have already made the same schematic before and didn't have any problems, so I did two things: I transferred all the components from a board that didn't get hot to this one (and it got hot), and I powered the 5V of my new board the same way as an old one that was connected (it heats up almost nothing, practically irrelevant). (Remember that only the 5V is heating up, the 3.3V is fine).Things that I think might be the issue, but I don't have exact knowledge if it can be or not: I placed the capacitors (I only have 10uF ceramic capacitors) too far from the LM, maybe that trace inside the inductor.

author: thm_w
author_rank: Super Contributor
timestamp: May 29, 2023, 11:44:09 pm
content: The layout does look terrible, normally this would not cause major issues for a simpleswitcher but who knows.You want C4 and C16 closer to the switching nodes.D4 ground via goes to a plane that appears completely cut off from U2 ground (it obviously joins up somewhere, but far away).You could try soldering a capacitor directly from D4 to L2 and see if that makes a difference.Then maybe a jumper wire from D4 Gnd to U2 Gnd.

author: Velth
author_rank: Contributor
timestamp: May 30, 2023, 02:32:01 pm
content: Thank you for the response, the design really turned out awful, I have a lot to improve. I connected the jumper from D4 to GND and added the capacitor, but there was no change. One thing I accidentally noticed was that if I don't activate the relay, it doesn't heat up. Is there any issue with having the relay so close to the power source? The relay is powered at 5V, and I have a flyback diode in parallel with the load. I've used this schematic and relay components before without any problems.

author: thm_w
author_rank: Super Contributor
timestamp: May 30, 2023, 11:41:42 pm
content: All I can see is that the relay being on would add ~100mA load to the 5V line. The same effect should happen if a 50 ohm resistor is added on the rail.Its not the inductor heating right? For sure its the IC.