LM3481: Performance Review

Noel Binny Abraham

Part Number: LM3481

Hi team. I've been working on the LM3481 SEPIC converter for a while now and it alwyas keeps on comin up with newer problems.

Vin:18V to 30V
Vout:24V/1A

A coupled inductor design is being used and the DRQ125-220-R is being used as the inductor. All the values of caps and resistors being used are as recommended by the Webench Design for these parameters staed above and i use a fsw of 520kHz for my application.
During testing yesterday, the borad was mounted along with a heatsink on the inductor, mosfet and diode to make it cool and test was being carried out at 0.5A continuos. The output voltage lost regulation after a while and shot up to about 100V (timestamp:2/10/2024 12:39:55 PM ) at the output. This happened for the first time and kept happening a few times. Attaching the test report here for review. I can send you the schematic and PCB on pricate chats for review but i am not able to figure out why this keeps happening. Please advice.

100V FAIL_01.txt

2/10/2024 12:39:26 PM	24.414V	0.0000A
2/10/2024 12:39:27 PM	24.415V	0.0000A
2/10/2024 12:39:27 PM	24.410V	0.0000A
2/10/2024 12:39:28 PM	24.408V	0.0000A
2/10/2024 12:39:28 PM	24.409V	0.0000A
2/10/2024 12:39:29 PM	24.407V	0.0000A
2/10/2024 12:39:29 PM	24.404V	0.0000A
2/10/2024 12:39:30 PM	24.403V	0.0000A
2/10/2024 12:39:30 PM	24.405V	0.0000A
2/10/2024 12:39:31 PM	1.1253V	0.0000A
2/10/2024 12:39:32 PM	1.0918V	0.2875A
2/10/2024 12:39:32 PM	1.1116V	0.2893A
2/10/2024 12:39:33 PM	24.420V	0.2862A
2/10/2024 12:39:33 PM	24.449V	0.4986A
2/10/2024 12:39:34 PM	24.499V	0.4986A
2/10/2024 12:39:34 PM	24.567V	0.4986A
2/10/2024 12:39:35 PM	24.582V	0.4986A
2/10/2024 12:39:35 PM	24.588V	0.4986A
2/10/2024 12:39:36 PM	24.593V	0.4986A
2/10/2024 12:39:36 PM	24.597V	0.4986A
2/10/2024 12:39:37 PM	24.595V	0.4986A
2/10/2024 12:39:38 PM	24.593V	0.4986A
2/10/2024 12:39:38 PM	24.591V	0.4986A
2/10/2024 12:39:39 PM	24.590V	0.4986A
2/10/2024 12:39:39 PM	24.591V	0.4986A
2/10/2024 12:39:40 PM	24.587V	0.4986A
2/10/2024 12:39:40 PM	24.584V	0.4986A
2/10/2024 12:39:41 PM	24.583V	0.4986A
2/10/2024 12:39:41 PM	24.583V	0.4986A
2/10/2024 12:39:42 PM	24.587V	0.4986A
2/10/2024 12:39:42 PM	24.587V	0.4986A
2/10/2024 12:39:43 PM	24.581V	0.4986A
2/10/2024 12:39:43 PM	24.572V	0.4986A
2/10/2024 12:39:44 PM	24.569V	0.4986A
2/10/2024 12:39:44 PM	24.568V	0.4986A
2/10/2024 12:39:45 PM	24.571V	0.4986A
2/10/2024 12:39:45 PM	24.569V	0.4986A
2/10/2024 12:39:46 PM	24.572V	0.4986A
2/10/2024 12:39:46 PM	24.581V	0.4986A
2/10/2024 12:39:47 PM	24.585V	0.4986A
2/10/2024 12:39:48 PM	24.587V	0.4986A
2/10/2024 12:39:48 PM	24.584V	0.4986A
2/10/2024 12:39:49 PM	24.615V	0.4986A
2/10/2024 12:39:49 PM	24.645V	0.4988A
2/10/2024 12:39:50 PM	24.653V	0.4986A
2/10/2024 12:39:50 PM	24.662V	0.4986A
2/10/2024 12:39:51 PM	24.699V	0.4986A
2/10/2024 12:39:51 PM	24.801V	0.4986A
2/10/2024 12:39:52 PM	26.585V	0.4986A
2/10/2024 12:39:52 PM	26.585V	0.4986A
2/10/2024 12:39:53 PM	26.585V	0.4986A
2/10/2024 12:39:53 PM	26.585V	0.4986A
2/10/2024 12:39:54 PM	26.585V	0.4986A
2/10/2024 12:39:54 PM	36.361V	0.4986A
2/10/2024 12:39:55 PM	36.361V	0.4986A
2/10/2024 12:39:55 PM	104.53V	0.0000A
2/10/2024 12:39:56 PM	99.575V	0.0000A
2/10/2024 12:39:56 PM	97.491V	0.0000A
2/10/2024 12:39:57 PM	30.138V	0.0000A
2/10/2024 12:39:58 PM	22.330V	0.0000A
2/10/2024 12:39:58 PM	12.924V	0.0000A
2/10/2024 12:39:59 PM	5.5436V	0.0000A
2/10/2024 12:39:59 PM	4.2647V	0.0000A
2/10/2024 12:40:00 PM	3.3554V	0.0000A
2/10/2024 12:40:00 PM	2.6785V	0.0000A
2/10/2024 12:40:01 PM	1.7490V	0.0000A
2/10/2024 12:40:01 PM	1.4227V	0.0000A
2/10/2024 12:40:02 PM	1.1597V	0.0000A
2/10/2024 12:40:02 PM	0.7724V	0.0000A
2/10/2024 12:40:03 PM	0.6300V	0.0000A
2/10/2024 12:40:03 PM	0.5133V	0.0000A
2/10/2024 12:40:04 PM	0.3382V	0.0000A
2/10/2024 12:40:04 PM	0.2732V	0.0000A
2/10/2024 12:40:05 PM	0.2195V	0.0000A
2/10/2024 12:40:05 PM	0.1753V	0.0000A
2/10/2024 12:40:06 PM	0.1086V	0.0000A
2/10/2024 12:40:07 PM	0.0839V	0.0000A
2/10/2024 12:40:07 PM	0.0639V	0.0000A
2/10/2024 12:40:08 PM	0.0362V	0.0000A
2/10/2024 12:40:08 PM	0.0275V	0.0000A
2/10/2024 12:40:09 PM	0.0214V	0.0000A
2/10/2024 12:40:09 PM	0.0142V	0.0000A
2/10/2024 12:40:10 PM	0.0122V	0.0000A
2/10/2024 12:40:10 PM	0.0109V	0.0000A
2/10/2024 12:40:11 PM	0.0095V	0.0000A
2/10/2024 12:40:11 PM	0.0083V	0.0000A
2/10/2024 12:40:12 PM	0.0079V	0.0000A
2/10/2024 12:40:12 PM	0.0077V	0.0000A
2/10/2024 12:40:13 PM	0.0070V	0.0000A
2/10/2024 12:40:13 PM	0.0067V	0.0000A
2/10/2024 12:40:14 PM	0.0064V	0.0000A
2/10/2024 12:40:14 PM	0.0061V	0.0000A
2/10/2024 12:40:15 PM	0.0059V	0.0000A
2/10/2024 12:40:16 PM	0.0058V	0.0000A
2/10/2024 12:40:16 PM	0.0056V	0.0000A
2/10/2024 12:40:17 PM	0.0053V	0.0000A
2/10/2024 12:40:17 PM	0.0053V	0.0000A
2/10/2024 12:40:18 PM	0.0052V	0.0000A
2/10/2024 12:40:18 PM	0.0050V	0.0000A
2/10/2024 12:40:19 PM	0.0049V	0.0000A
2/10/2024 12:40:19 PM	0.0049V	0.0000A
2/10/2024 12:40:20 PM	0.0046V	0.0000A
2/10/2024 12:40:20 PM	0.0046V	0.0000A
2/10/2024 12:40:21 PM	0.0044V	0.0000A
2/10/2024 12:40:21 PM	0.0037V	0.0000A
2/10/2024 12:40:22 PM	0.0038V	0.0000A
2/10/2024 12:40:22 PM	0.0040V	0.0000A
2/10/2024 12:40:23 PM	0.0040V	0.0000A
2/10/2024 12:40:23 PM	0.0040V	0.0000A
2/10/2024 12:40:24 PM	0.0040V	0.0000A
2/10/2024 12:40:24 PM	0.0038V	0.0000A
2/10/2024 12:40:25 PM	0.0037V	0.0000A
2/10/2024 12:40:26 PM	0.0037V	0.0000A
2/10/2024 12:40:26 PM	0.0037V	0.0000A
2/10/2024 12:40:27 PM	0.0035V	0.0000A
2/10/2024 12:40:27 PM	0.0035V	0.0000A
2/10/2024 12:40:28 PM	0.0034V	0.0000A
2/10/2024 12:40:28 PM	0.0032V	0.0000A
2/10/2024 12:40:29 PM	0.0032V	0.0000A
2/10/2024 12:40:29 PM	0.0031V	0.0000A
2/10/2024 12:40:30 PM	0.0031V	0.0000A
2/10/2024 12:40:30 PM	0.0029V	0.0000A
2/10/2024 12:40:31 PM	0.0029V	0.0000A
2/10/2024 12:40:31 PM	0.0029V	0.0000A
2/10/2024 12:40:32 PM	0.0029V	0.0000A
2/10/2024 12:40:32 PM	0.0028V	0.0000A
2/10/2024 12:40:33 PM	0.0028V	0.0000A
2/10/2024 12:40:33 PM	0.0028V	0.0000A
2/10/2024 12:40:34 PM	0.0028V	0.0000A
2/10/2024 12:40:35 PM	0.0026V	0.0000A
2/10/2024 12:40:35 PM	0.0026V	0.0000A
2/10/2024 12:40:36 PM	0.0026V	0.0000A
2/10/2024 12:40:36 PM	0.0026V	0.0000A
2/10/2024 12:40:37 PM	0.0026V	0.0000A
2/10/2024 12:40:37 PM	0.0026V	0.0000A
2/10/2024 12:40:38 PM	0.0025V	0.0000A
2/10/2024 12:40:38 PM	0.0025V	0.0000A
2/10/2024 12:40:39 PM	0.0025V	0.0000A

I have not been able to get the board to keep working reliably for the specs that i've mentioned above. Please do review my files and let me know if there is some underlying major issue that i might be missing.

Thanks and Regards
Noel

6 months ago

0 Niklas Schwarz 6 months ago

TI__Mastermind 21040 points

Hi Noel,

Thanks for using the e2e forum.
It is interesting that this failure seems to happen only after 20 seconds of normal regulation.
There can be several reasons that lead to a strong increase in output votlage:
- loss of FB signal: if FB is pulled low, the device will run with max duty cycle until overcurrent protection is triggered
- saturation of inductor: if the inductor is not rated for these currents, it may enter saturation, which will lead to strong current spikes and abnormal behavior
- stability issues: if the regulation is not stable, oscillation can happen that can lead to output voltage spikes. (However, this should already happen much faster than 20 seconds)
- thermal issues: slow reactions (timespan several seconds) are often related to thermal problems. There might be components like inductor or FET that are heating up extremely

I would recommend double checking the design with our Power Stage Designer tool to see if inductance, fsw and compensation are well suited.
https://www.ti.com/tool/POWERSTAGE-DESIGNER

Then I also recommend checking the current ratings of the inductor.

If possible, it would also help to measure the SW voltage, as well as FB & Isense pin to find out what triggers the abnormal behavior.

Best regards,
Niklas