I'm putting together a buck regulator (first time around) based on the lm2374 chip. TI provides gerbers of a recommended layout for this chip family, but my configuration differs a bit. I've read up on ground bounce, and nominally understand that I need to minimze changing loop areas, but am having a hard time understanding what the loop here would be. I'd like to get feedback on the PCB layout with regards to possible noise/EMI:

I've overlaid additional part names in yellow to make it easier to match up with TI's recommendations / general components. You can ignore the air wires, one is an enable line that hasn't been hooked up to the MCU yet, the other is the battery line being needed elsewhere and eagle drawing the airwire from that particular spot.



Please let me know if/what can be improved here. Thanks.

EDIT Looking at this for the umpteenth time, I do see that I tied the EN pull-up to VIN, not VOUT. I'll make that change.

  • \$\begingroup\$ Use their gerbers for guidance. If they minimize trace length and go to a pour/plane, they consider that important. Specifically, the trace length from your pin3 to r14 and the meandering connection from the IC through your "U3" to Vout. Not saying you'll have any problems, but it's the biggest discrepancy between the two designs. Also, for what it's worth, I'm surprised they routed R1 to Vout like they did in the reference design. \$\endgroup\$ – scld Jul 15 '14 at 13:09
  • \$\begingroup\$ One side of C19 isn't connected to anything. \$\endgroup\$ – Blup1980 Jul 15 '14 at 13:29
  • \$\begingroup\$ Ya the schematic actually looks a little off. There seem to be a lot of green dots missing at what should be net connections. \$\endgroup\$ – scld Jul 15 '14 at 13:36
  • \$\begingroup\$ @Blup1980 it's connected to the same junction below the 5V symbol \$\endgroup\$ – kolosy Jul 15 '14 at 13:56
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    \$\begingroup\$ @kolosy Yes, on the schematics, but on the layout you posted the screenshot, it isn't. \$\endgroup\$ – Blup1980 Jul 15 '14 at 13:59

It's a bit rubbish your layout and a bit unfinished - C19 isn't connected thru to anything as far as I can tell from the picture you supplied. Golden rules: -

  1. Make the track length to and from the inductor as short as possible (not good on your layout)
  2. Make the diode connections on switch output as short as possible (OK on your layout except for topside ground connection)
  3. Make all ground connections as short as possible (it looks like you have a ground plane in blue but you are choosing to connect things to ground on the top layer such as C17 and C20 - I'd have a full ground plane and via down from C17 and C20 to ground)
  4. Have a separate ground path for R14 directly to pin 2 on the chip if at all possible (don't share this track with anything - i.e. star point at the main ground pin, pin 2)
  5. R15 (sense resistor and C19) take from as close to C20 as possible (on your layout it's quite a distance away and this will make the output noisy)

Additional comments - why is C19 so big? Why doesn't the artwork match the circuit diagram?

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  • \$\begingroup\$ Thanks. Ground plane - I have both the top and bottom layers as ground planes configured in eagle. Should it not be? \$\endgroup\$ – kolosy Jul 15 '14 at 14:36
  • \$\begingroup\$ The top ground will never be as good as the bottom ground so, I tend to never have a top ground for power supply circuits - leave top for components and tracks - leave bottom fully intact except for vias thru to ground - you are in danger of compromising performance. On more powerful switching regulators I'll have multiple vias from a component to ground as well. On RF circuits then top and bottom should be used in a lot of cases but it's never a 100% rule. \$\endgroup\$ – Andy aka Jul 15 '14 at 14:47

There are two high-current loops, depending on whether the switch is conducting or not:

enter image description here

Both should have the area they enclose minimized.

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