LM5141-Q1: LM5141-Q1

Anu Shetty

Part Number: LM5141-Q1

Tool/software:

HI,

We are using LM5141-Q1 in one of our projects and designed to generate 12V .When we turn on the board at start it generates 5V instead of 12V. But when ECU is restarted meaning hard reset is applied it generates 12V as expected.

Feedback resistor and Soft start Capacitor values are correct according to 12V design calculation and we are able to get 12V output once ECU is restarted but not during the first power on.

Design values used for 12V

Feedback resistor -100k , 11k

Softstart capacitor value_56nF

Can you please comment on this issue and suggest the solution how to rectify.

over 1 year ago

0 Timothy Hegarty over 1 year ago

TI__Guru* 82567 points

Hi Anu,

It seems it's detecting a 5V fixed setting (the FB pin is seen as effectively low at startup).

Please send the schematic and a completed quickstart calculator for review.

Regards,

Tim

0 Anu Shetty over 1 year ago in reply to Timothy Hegarty

Prodigy 60 points

Please find the schematics which is designed for 12V output and 9 A current.LM5141_BUCK.xlsm

0 Timothy Hegarty over 1 year ago in reply to Anu Shetty

TI__Guru* 82567 points

Thanks, Anu. I'll take a look and reply tmrw.

0 Anu Shetty over 1 year ago in reply to Timothy Hegarty

Prodigy 60 points

Hi Timothy,

Any update..

We verified output voltage by connecting 1A load to buck controller also...but still buck controller is genrating 5V instead of 12 V

0 Timothy Hegarty over 1 year ago in reply to Anu Shetty

TI__Guru* 82567 points

Hi Anu,

Are you getting a ground bounce on AGND at startup? This would cause a fixed output setting of 5V instead of the 12V by resistor divider.

Let's setup a meeting to review the layout if needed.

Tim

0 Anu Shetty over 1 year ago in reply to Timothy Hegarty

Prodigy 60 points

Hi Timothy,

The issue has been resolved now. We removed the capacitor C133, which is located near the feedback resistors, and the Buck regulator started generating 12V as expected.

We assumed that when the Buck is powered on, this capacitor C133 would charge, causing the Buck to detect a short to ground. As a result, the Buck would set the output voltage to 5V. However, upon restarting the ECU, the charge stored in the capacitor would prevent the Buck from detecting a short, allowing it to return to normal operation and generate the expected 12V output.

Could you help us understand why removing capacitor C133, located near the feedback resistor, resolved this issue?

0 Timothy Hegarty over 1 year ago in reply to Anu Shetty

TI__Guru* 82567 points

Hi Anu,

Your explanation is correct. No need for a cap from FB to AGND. Not only does it create the issue you observed, but it also results in a pole causing phase lag in the loop, reducing phase margin.

Regards,

Tim

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LM5141-Q1: LM5141-Q1