Designing Your PDN to Work Within Your PCB Layout Guidelines

I enjoy watching the occasional episode of Star Trek here and there, and I get a kick out of noting the inconsistencies in their technology. In one episode they may be firing all of their energy weapons at the bad guys, moving at warp 3-billion to time travel and showing no regard as to where their power was coming from. In the next episode though, someone will plug in a space toaster to warm up their breakfast in the galley, and the whole ship will blow a fuse.

I know that they wrote those storylines to create dramatic tension, therefore, adherence to sense can be a casualty. It amazes me though to see how often in fiction, or even in real life, that we assume that power is something that can be taken for granted. For years we designed printed circuit boards (PCBs) the same way. Throw in a power plane and a ground plane, sprinkle a couple of bypass caps where there’s room, and you’re good to go.

With today’s more advanced designs the stakes are much higher. Adequate power isn’t a given, it must be planned for. By taking a look at some of the issues that designers have to contend with when laying out a printed circuit board, you can learn how those issues affect the design of the power distribution network () on the board. Then you can start optimizing your designs with all the available tools.

Layout Concerns for PCB Designers Today

As a PCB designer, you have to contend with many design requirements today that many years ago we had only just scratched the surface of. Most digital designs today are in the realm of what we would have considered high speed back then. Now designers routinely deal with the placement and routing constraints that at one time was cutting-edge design technology.

Components have to be placed as always to fit inside enclosures, avoid obstacles, and match up with connectors, plugs, and other exterior interfaces. In addition, though there is a greater need for the precise placement of components to maximize their performance with other devices. On top of that, component placement also needs to be mindful of EMI and thermal issues as well. With the available PCB layout design space continually reduced, the shrink as well.

Routing also has more constraints on it now than before. Smaller trace widths and spacing, shorter lengths, diff pairs, and other signal integrity issues all increase the complexity of the routing. In addition to all of that, power and ground planes have to be designed in order to supply an uninterrupted return path for transmission lines.

Design Requirements can Easily be Forgotten

The increasing difficulty in creating robust PDNs is in large part because of the requirements of the design. Vias fanouts and mechanical obstacles tend to take up needed space which means that there is less space to devote to the power routing. Then, with the increased circuit densities of today’s smaller designs, the power planes and traces often get trimmed down in order to accommodate the additional trace routing.

Another problem area is the fine-pitch and high pin-count devices in use on today’s designs. Their multiple power pins are often supplying active components that are constantly switching and causing current spikes. Without an adequate power supply, these spikes can create noise or ground bounce. Therefore it is important to ensure that these pins have a good solid connection to power.

How You Can Design a Solid, and Stay Within Your PCB Design Guidelines

Fortunately, there is a way that you can get the help you need to design a strong and reliable on your circuit board. By using a power distribution network to verify that power is being delivered evenly across your board, you will get real-time feedback on your high speed design while it is still in layout. A PDN Analyzer^™ will give you the ability, while you are laying out your designs, to see problems such as:

• Insufficient current densities and voltage drops.
• Marginal voltages on critical power pins.
• Insufficient or excessive areas of copper.
• Isolated areas of copper.

Problem areas like these are highlighted in your layout so that you will be able to correct them before sending the design to manufacturing. No longer will you need a prototype build to see if you have a good in place. That verification will be done for you while you are still in the design phase.

For PCB design software which has a PDN available that will do the power distribution checking that I’ve talked about here, look into Altium Designer^®. This checking is on top of your layout enabling you to see and correct errors in real-time layout before you finalize the design.

Would you like to find out more about how Altium can help you with the power delivery network design on your next printed circuit board layout? Talk to an expert at Altium.