LM5118: Desired 12Vout, but only getting 3.8Vout. Help/schematic review?

Calvin Wagner

Part Number: LM5118
Other Parts Discussed in Thread: LM393, 4460

I configured an LM5118 with WeBench to accompany a low voltage disconnect circuit I'm putting together. The intent is to condition the output voltage for a separate device and to provide a steady reference for the op-amp to do voltage comparison between "VCC" (battery voltage) and the output of the 5118. As of right now, the only load that the 5118 has is the LM393(+relay coil, 7mA to close). My problem is that I'm only getting ~3.8V on the output of the 5118.

I used the information provided by WeBench for the components to configure the LM5118. I attached the schematic to show what I'm doing plus a sheet with some readings I recorded.

0827.Schematic.pdf 4460_001.pdf

over 5 years ago

0 Garrett Roecker over 5 years ago

TI__Mastermind 21720 points

Hi Calvin,

Thanks for reaching out with your question and for using the LM5118 in your design. Sorry to hear you are having this issue.

Are you seeing this on multiple boards or just one board?

One other suggest is to lift the VCCX pin and see if that fixes the issue.

Thanks,

Garrett

0 Calvin Wagner over 5 years ago in reply to Garrett Roecker

Prodigy 130 points

I've got two proto boards I've made up, same behavior on both.

I lifted VCCX, pin 17 (oh the joy!), I still have the same ~3.8V out.

0 Calvin Wagner over 5 years ago in reply to Garrett Roecker

Prodigy 130 points

Attached a picture of the brd file as well if that helps any. Disregard the missing trace on coutx, somehow I left that off on the board file. I've added a jumper wire for now.

0 Garrett Roecker over 5 years ago in reply to Calvin Wagner

TI__Mastermind 21720 points

Hi Calvin

Thanks for sending the image of the layout. Based on this image it looks like the exposed pad (EP) on the back of the LM5118 is not connected to ground. This can cause the IC to not operate properly.

I suggest taking look at the LM5118 EVM to see an example layout.

Some things to keep in mind: Keep the CBOOT and VCC capacitor as close as possible to the IC. Ground the exposed pad. Consider designing a four layer board. This will help shield the noise sensitive signals and allow for easier trace routing.

Thanks,

Garrett

0 Calvin Wagner over 5 years ago in reply to Garrett Roecker

Prodigy 130 points

Ooof...You definitely hit the nail on the head there, Garrett. I have no idea how that ground got missed. Things need ground.

I think I might just copy the layout and BOM of the Eval board, and add the extra components I need on an isolated copper pour.

Thanks for the help!

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LM5118: Desired 12Vout, but only getting 3.8Vout. Help/schematic review?