LM25148: LM25148 chip will not switchV .

Hello

Please provide the link to the previous post.

Thanks

LM25148: Follow up question on CC/ CV control - Power management forum - Power management - TI E2E support forums

Hello

Let me assign this to our Application engineer.

Thanks

Hello,

Can you confirm the exact operating conditions where you observe this behavior? What do you mean when you say it shuts off after you adjusted the load slightly?

What is the VCC voltage during this condition?

Best reagrds,

Ridge

Vcc is steady at 30V from a PSU.  we Checked the LDO output from the chip and verified a steady 5V output . We use this to drive our op amps for feedback control.   When we checked the gate and the Sw node there was no waveform present.  In the testing we were verifying the feedback loop to ensure constant current and constant voltage, To test this we applied a variable resistor and then adjusted the resistor to increase resistance.  It was during this change in load that the chip shut off . In other words no more switching signals to the external mosfets.  We have not been able to bring it back online since. 

Hi Ricardo,

Are you able to test the converter on its own by connecting the FB divider without the CV/CC network? The first step should be to determine if the buck design on its own is stable.

What is the specific current at which the device shuts down? What is the output with no load?

Please confirm Vout, Fsw, and the Iout range and provide any waveforms you have captured regarding this behavior.

Best regards,

Ridge

Hello Ridge,  We have tested the converter in another sample board and it works fine.  The issue was when we tested a sample board say proto board 1, with the varying load test it shut off. Now it will not switch at all, so minimum current draw, and no switching with or without the load.  Proto board 2 and 3 are working fine but we have not performed the varying load test to avoid the issue with proto 1. 

Hi Ricardo,

Please provide the parameters requested above. What is the expected load when the device shuts off?

Are you able to share the board layout?

Best regards,

Ridge

Sorry Ridge, I think there is some confusion, to clarify: 

During nominal operation , the supply outputs up to 13.5V at which time the Voltage control kicks in, the current is variable, up to approximately 7.4A. The load is variable depending on the state when the power is connected.  For this unit after our test, we have tried open , we have tried regular, light and heavy load, the unit just will not switch so there is nothing waveform on the inductor at all. 

Regarding the PCB layout , not at this time I can try to see if we share snippets, however the circuit can be found there: LM25148: Follow up question on CC/ CV control - Power management forum - Power management - TI E2E support forums

Hi Ricardo,

Thanks for sharing the operating conditions. 

I am looking into this issue and will respond next week with feedback.

Best regards,

Ridge

Hi Ricardo,

C21 in your design will act as an additional pole in the system and may be making it marginally stable. Try removing C21 and re-testing the design. 

Best regards,

Ridge

Hi Ridge, Thank  you for the suggestion , we removed it from the non working unit and it is still not working. Out of curiosity we also removed it from a working unit and the unit still powered up and regulated , though not sure how it will respond to transients. 

Hi Ricardo,

For the board which is not working, you should try to replace the FETs and the buck controller IC  then retest. 

Also, check a CV to CC transient to make sure that the response is well damped.

Best regards,

Ridge

Hi Ridge,  We will double check the Fets and if they need to be replaced we will , they tested fine but will lift the gate to perform a more thorough test.  If the buck controller IC is the problem, is there any reason why a changing load would do that?