With electronic devices continually getting smaller, more systems are resorting to single-board solutions instead of connecting multiple boards for their different functions. These single-board solutions result in circuit boards with analog and digital circuitry that interface together, otherwise known as a mixed-signal design. Such designs require the careful layout of their circuitry to perform as intended and minimize potential noise and electromagnetic interference (EMI) problems.
One area of the circuit board design that engineers need to pay close attention to is the layout of the integrated power supply circuitry. If not laid out correctly, the power will not be managed, creating severe problems for the board. Here we will look at some of the requirements and tactics for mixed-signal power supply design that will be helpful in your next PCB layout.
Mixed-Signal Design Requirements
As signal speeds have increased on printed circuit boards, so has the potential for noise, EMI, and other signal integrity problems. This problem is heightened in a mixed-signal design because the noise of analog signals can corrupt digital circuits and vice versa. Once you add in the power supply circuitry for the circuit board, you can end up with a very complex environment. It must be designed carefully to avoid compounding the signal and power integrity problems.
Power supplies in mixed-signal designs must deliver clean power throughout the power delivery network of the board to avoid the following problems:
- Power ripples caused by the rapid switching of a switch-mode power supply can affect other circuitry in the form of crosstalk on those signals.
- Ground bounce due to multiple digital signals switching simultaneously interrupts the signals, so they do not return to their reference ground level. This bounce can result in false switching and disrupt the operation of the circuit.
- Power spikes and noise are caused by hundreds of power pins on larger devices pulling a lot of the available current.
The PDN of a mixed-signal PCB must also have a reference ground plane designed to support all the circuitry’s return signal paths. The trace of a high-speed signal creates an electric field that couples to the nearest ground reference. As this could end up being cabling, the system frame, or shielding, the reference plane must be designed correctly to couple with the high-speed signals. This coupling will induce the signal’s return through the reference plane, giving it a clear path back to the source. It is essential that the return paths not be broken with plane splits or cut-outs, which will cause the return paths to wander through the board, creating noise and interference as they go. A single solid ground plane is the best solution for this, as long as the circuitry is isolated from each other, which we will examine in greater detail below.
Some other requirements of mixed-signal boards for their reference plane are to shield sensitive circuits from incoming EMI. The plane will also prevent interference generated by the board from escaping and affecting external devices. The large area of the ground plane will also create a lower impedance for the ground net than routing it with traces, which will also help reduce power supply noise. And lastly, the ground plane will help with the thermal cooling of components that run hot, such as the power supply parts themselves. Next, we’ll look at some layout considerations for power supply circuitry on mixed-signal designs.
PCB Layout of Mixed-Signal Power Supply Designs
Here are some PCB layout considerations for power supply design on mixed-signal circuit boards:
Layer stackup
In a standard mixed-signal design with analog and digital circuitry that interacts with each other, you should use a ground plane that extends throughout the entire board on an internal layer. To do this requires designing your board layer stackup with one or more internal ground planes depending on the routing needs of your sensitive analog and digital traces. It is also important to configure the layer stackup with a power or ground plane between the board’s exterior with the power supply components and routing and the internal layer with sensitive routing. This configuration will help shield these sensitive nets from the power supply.
Component placement
You will want to spend time floorplanning your design to develop the best partitioning strategy for the board. Remember that you want to keep the analog, digital, and power supply circuitry separate from each other, so you don’t have to cut up the ground plane. This isolation should ensure that the circuitry of the different areas doesn’t end up routed between them, except for those nets that interconnect between partitions. When placing the power supply components themselves, keep the parts close so that the eventual routing will be as short and direct as possible. Power supply components should also be placed on the same side of the board to avoid the inductance created by the vias routing between the top and bottom layers.
Trace routing
In some cases, it may make sense to route the traces as you place the power supply components to ensure that you have the best placement solution. Remember to keep the main power traces as short and direct as possible, and use 45 degrees or rounded corners when routing. Use sufficiently wide traces for the current in your power supply. Remember that the thermal properties are different between internal and external traces due to their contact with the air. Short and wide traces will also help improve the power integrity of the layout. Do not route analog or digital traces through the power supply area or high-speed signals over breaks in the reference ground plane.
Power and ground planes
As we said initially, a single solid ground plane is typically the best solution to ensure the board’s best signal and power integrity. A solid ground plane means that any interconnecting nets between the analog and digital sections of the board will have a clear return path on the plane. Power planes, however, are often split depending on the location of the power supplies on the board and the components they service. Remember the following with your planes:
- Don’t route sensitive lines over areas where the ground plane is cut or broken to ensure good return paths.
- Thermal relief pads are essential for the solderability of thru-hole pins but must have enough metal tied to the plane for the current requirements of the power supplies.
- The planes can also help with heat dissipation of hot components, which may require thermal vias to conduct the heat of power supply components into the planes.
There are many considerations when laying out a mixed-signal power supply design, but fortunately, there are some excellent resources available to you for help.
Engineering Resources Available for PCB Layout Questions
One of the best resources you can go to for help with your PCB design is the contract manufacturer building your circuit board. You should be working with them already on layer stackups and other design-related questions before you even begin laying out the board. They can also help you with component selection choices and PCB layout tactics that will optimize the performance of your design and its manufacturability.
At VSE, our team of engineers has been helping board designers like you for over 35 years. Our component engineers can assist in navigating the problematic component supply chain, and our design engineers can answer PCB place and route questions. Additionally, our engineering team will optimize the manufacturing, test, and eventually full system integration of a design.
