One of the great marketing and design tools the semiconductor industry has created is the reference design. One great way to quickly learn how to use a component for the first time is to look at the reference designs. For those engineers that learn best from well-documented examples, reference designs provide an excellent way to quickly come up to speed with new components.
Although reference designs do a great job of showing how to implement components in a schematic, they might implement the wrong circuit board design guidelines. Before copying the layout style from a reference design into a custom PCB, take note of the potential hazards to your new product. If you’re not careful, you might end up with a reference design that can’t be sourced, functions incorrectly, or will never pass EMC testing without major changes.
Why PCB Reference Designs?
Although PCB reference designs are great tools that show how to use a component, there is one important thing to remember about reference designs: they are marketing tools first and foremost, a secondary benefit of using reference designs is to educate. Semiconductor vendors create these designs with the goal of selling their chipsets. In an ideal world, the semiconductor vendor is hoping your design will be totally reliant on their components, even as you scale up to high volume production.
In some cases, the evaluation product for a specific component will double as a reference design. This is certainly possible for more advanced products like large FPGAs or application processors. These PCBs may be packaged as an entire module, and the vendor will provide everything needed to manufacture, program, and configure the reference board. Users can then develop an application with the reference product, or they can totally reverse engineer the product and build something custom.
What’s in a Reference Design?
Inside a reference design package, you are likely to find some of the following elements:
- PCB design files
- PCB manufacturing files (usually Gerbers)
- Firmware for the main chipset
- Datasheets and app notes related to the main chipset
- Other technical material, such as results from test studies
- A user manual for the reference design
- Any required software, such as the vendor’s IDE or apps
Because there is so much information available in any reference design package, which data should you trust and which should you toss out? A little bit of scrutiny and understanding of the purpose of a reference design will go a long way.
Check the Date and the BOM
There is one simple thing you can do to see how valuable a reference design is going to be: check the date it was last updated! If a design was last updated several years ago, it might have hidden sourcing and design problems that aren’t obvious. If an open-source design is very old, don’t be surprised if you find any of the following:
- Obsolete/EOL components
- Out-of-stock components
- A newer reference design might be available
- Out-of-date design practices were used
- Old symbols/footprints were used
Regarding the last point, footprints are probably the most important, even in new reference designs. Don’t be surprised if the footprints in an open-source design do not match entries in the BOM or the part descriptions. Make sure to review footprints before reusing any components or sections of the PCB layout.
Start With Schematics
Remember, a reference design is meant to show potential customers how they can quickly get started using a component they may be unfamiliar with. These designs show exactly how to adapt that component into a specific application. This end of the reference design package is thoroughly qualified by a group of engineers who will know everything about the main components.
This reference oscillator circuit for an application processor shows many of the important connections needed to ensure the main chipset will work properly.
While reviewing the schematics, it’s also important to review the BOM for any obsolete parts. Older reference designs are much more likely to have obsolete components that need to be updated. Even in this case, once replacements are made, the reference design is still viable.
Where it starts to get questionable is in the PCB layout. In fact, some reference designs will simply not include a PCB layout, they might only include some general PCB layout guidelines. Semiconductor vendors can guarantee that the schematics in reference designs accurately represent the correct working connections, but they can’t provide the same guarantees for a custom PCB based on that design. For this reason, it’s important to scrutinize the PCB layout in a reference design (if it exists) before adding to the reference PCB.
Scrutinize the PCB Layout
Although the schematics in reference designs are normally very thoroughly qualified, be careful with the PCB layout. Reference designs will receive varying levels of testing and qualification in terms of noise, mechanical stability, and even DFM. Some reference designs are only tested for functionality before being released publicly, while others will be an evaluation product that has passed significant testing before being made available for sale.
If a reference design is not made available as a product that you can purchase directly from a semiconductor vendor, then you should be skeptical as to whether the PCB layout will have excessive EMI. PCB reference designs are not required to receive any kind of certification for emissions testing. Beyond some prototypes for testing functionality, the device may not have gone through more than a few prototyping runs, so there could be DFM problems that make scaling to high volume production difficult.
For this reason, it’s better to start with reference designs that you can actually purchase. Some reference designs are also intended to be evaluation products, not just application examples. This is something commonly found in reference designs based around FPGAs, MCUs, application processors, and some ASICs. If you’re just adding to an existing layout for one of these evaluation designs, then you’re much more likely to be successful as these layouts must meet certain standards before they can be sold on the open market.
If you decide to use a PCB reference design to create your new product, make sure you use OrCAD, the industry’s best PCB design and analysis software from Cadence. OrCAD users can access a complete set of schematic capture features, mixed-signal simulations in PSpice, and powerful CAD features, and much more.