LM27342: EMC comply problems

Francesco Da Re

Part Number: LM27342

Hello, we are facing unexpected EMC problem with this component.

We have followed all the common design rules but we found 2 MHz harmonics (the switching frequency, of course) everywhere.

Pcb is a 4 layer with standard power planes in the middle layers. Any suggestions?

Francesco

over 5 years ago

0 Anthony F over 5 years ago

TI__Mastermind 35115 points

Hi Francesco,

Are you debugging a radiated noise or a conducted noise problem? Also is there a specific standard you are trying to meet or is it noise affecting some other components performance?

I may also need to take a look at your schematic and PCB layout.

Since the noise problem is related to harmonics of the switching frequency, an input LC filter might be needed to help filter out the noise going back onto the input rail. This mostly reduces conducted noise but can also help with radiated noise. The app note linked below is a good reference for input filter design that might be useful.

www.ti.com/.../snva489c.pdf

Best Regards,
Anthony

0 Francesco Da Re over 5 years ago in reply to Anthony F

Intellectual 260 points

Hu Anthony,

we are debugging radiated noise.

In the meantime I re- runned TI WEBENCH design tool. I saw that, for my power supply range the tool do not place the diode on the BOOST pin.

Removed that diode the radiated noise is much lower! Now it should be below limit.

Thank you

0 Anthony F over 5 years ago in reply to Francesco Da Re

TI__Mastermind 35115 points

I'm glad you find a solution to this. For future reference, can you share what voltage were you charging BOOST from initially?

I will guess it was higher than the internal 3.9 V BOOST LDO. I am guessing it was higher because higher BOOST-SW voltage would cause the HS FET to turn on more quickly resulting in faster edges at the SW node.

0 Francesco Da Re over 5 years ago in reply to Anthony F

Intellectual 260 points

Anthony, it was 5V0.

Power management

Power management forum

LM27342: EMC comply problems