Video: PCB 201 with Atmel’s Paul Rako

In this Atmel Edge episode, Analog Aficionado Paul Rako demonstrates how to place a switching power supply on the same circuit board with analog and digital circuits.

“It’s a fairly high-level clever trick to lay out a switching power supply on a board that has analog and digital and some delicate circuits,” Rako explains.

“What did my two friends – Jon Dutra and Alan Martin – come up with? You use a top-side copper pour on your circuit board to make a local ground for your switching regulator. And then you just connect it at one place, at the bottom, at the ground reference of the output capacitor.”

To illustrate his point, Rako highlights a four-layer circuit board.

“So this is top, signal, then there’s ground, then there’s power plane, then there’s bottom signal. Design it four-layer. When you get that figured out, then you can spin it down to a two layer. A buck regulator, has an input voltage. Got an input capacitor. Then you’ve got a switch,” he continues.

“Usually it a FET transistor, or sometimes it’s inside the control IC. Here’s that control IC. Then you’ve got a catch diode, which causes a lot of problems. It gets hot. Sometimes it’s inside the IC. Sometimes it’s a synchronous. The basic thing with a switching regulator is this inductor. Then you’ve got an output capacitor. And always put those arrows and feathers on your circuit so people understand what’s coming in and what’s going out.”

As Rako notes, the inherent problem with a switching regulator is its fast-changing currents, di/dt.

“Those fast-changing create electromagnetic noise. If you let them run in the ground plane they’ll go out and affect other circuits on your PCB. So the trick is you pour a top-side copper pour,” he added.

Interested in learning more? Be sure to watch the video above for a full PCB 201 run-down.

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