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LMR51610: Schematic and layout check

Matthew Pole
Intellectual 340 points
Part Number: LMR51610

Tool/software:

Hey,

I was wondering if someone could check over my schematic and layout to see if there are any improvements that may help the operation of the device and/or reduce EMI. Th egoal is a 5V ouput to power a MCU and a CAN transeiver. There is 2 voltage sources and 4 wake up sources. VIN is always active from a lead acid but it looks like Qc is super low when no enabled? The 4 wake up inputs (5-28V) that come into the PCB are fitlered and they all put through a diode and when any are on then the regulator starts and turns on the MCU/PCB. DOes this sounds and looks correct? Have I made any obcious issues? This is L1, L2 and L3 are both solid GND planes that are stitched through.

Thank you in advance!

  • 0
    TI__Mastermind 31255 points

    Hi Matthew,

    A high-frequency input decoupling capacitor should be placed at the input pin. The derated value should be 2.2uF or higher, with the voltage rating being twice as large as the input voltage. You may want to add a bulk capacitor at the input as well (10-uF could be ok).

    To me, the main concern with having multiple inputs is that enable cannot be left floating. The J? connection should be annotated, and I think it should be ok if that's populated first. 

    I think layout also looks ok to me. Typically, at lower current levels, conducted EMI is not a huge issue, and the radiated EMI can be attributed to switch node layout. I think both look ok. If you are unsure, I would recommend adding a placeholder before the diode if you are unsure of the EMI performance, and then add the bead if the initial performance is not good. 

    Thanks,

    Richard 

  • 0 in reply to Richard Fu22
    Intellectual 340 points

    Hey Richard,

    Thanks for the response!

    Sorry I am a bit confused about the decoupling capacitor requirement. I have a 2.2uF 50V capacitor at VIN (C6), are you suggesting to add another? Here are some larger images to show some bulk capacitors at the input to the PCB.

    For the multiple enables the IC enable pin has a 100kohm resistor pulling it to GND, would thi snot be enough to stop it potentially floating?

    J? which now has a designator in the one below is just a placeholder for a maual enable that is not from 1 of the 4 external wake up sources. This will normally always be left open.

    And do you mean a bead before the VIN diodes? Sorry for the confusion!

    Thank you!

  • +1 in reply to Matthew Pole
    TI__Mastermind 31255 points

    Hi Matthew,

    Thanks for providing additional information. 

    Super sorry, I got a bit confused by the initial post and in the schematic, it looked like the cap was connected to EN and not VIN. I see it now, thanks for clearing it up.

    The 100kOhm should be able to keep it pulled down. Only thing I wasn't so sure of was the sequencing, and the role of J? It's clear to me now, and seems ok. 

    For the bead, I would suggest placing it here:

    The beads could be used to prevent conducted EMI from leaking back from the LMR51610, and the CN3768 (but I think at 1A, you wouldn't really see too much).

    Thanks,

    Richard 

  • +1 in reply to Richard Fu22
    Intellectual 340 points

    Hi Richard,

    Ah perfect, no problem at all. Sounds like I am all good to go with the PCB then! The current will be below around 250mA so should be ok!

    For the beads, I guess I would need something here as well as the 12V lead acid powers the board and regulator before the DCDC kicks in

    Thank you for the help!

  • 0 in reply to Matthew Pole
    TI__Mastermind 31255 points

    Hi Matthew,

    That could also be fine, but I don't think it is necessary since you already have the bead at the input. of CN3768 (just trying to save you a component). But yes, in general at 250mA, you shouldn't need to worry too much about the EMI in my opinion.

    In future cases, if the response satisfies your thread, please help us select "This resolves my question", or similar. It helps us with our internal tracking metric, thanks!

    Richard 

  • 0 in reply to Richard Fu22
    Intellectual 340 points

    Hey Richard,

    No problem, I will mark as solved!

    Just 1 more question. What kind of bead would you normally suggest for this application? 

    Thank you!