Designing and Implementing COM Express Modules

Computer-on-Module Express (COM Express) has become a widely accepted way to implement embedded computing solutions in a very compact form factor. The main principle of the COM Express standard is to implement the processor and memory on a small module (basic module size for the type 2 specification is 125 × 95 mm) and then implement specific functions and interfaces for a particular application on the baseboard.

Module and Baseboard Power Supplies

The power supply implementation for a COM Express module and baseboard needs careful design attention to achieve reliable designs that operate correctly in all circumstances. Figure 1 shows a typical power architecture for a COM Express module and baseboard configuration.

The combined module and baseboard design must allow for the high power consumed by the module CPU at peak maximum loading, which can be as much as 50W or more. This high power requirement is due to the high currents flowing into the CPU on the module. Having such power dissipated in such a small space can also cause thermal design challenges. Regulators on the module require high-efficiency to avoid excessive thermal dissipation, which would lead to long term reliability issues or cause problems for extreme-rugged designs that need to operate at high temperatures. The power switching regulators on the module, therefore, need careful design and component selection. For multi-core processors, particular attention should be paid to the periods when the various cores activate because there are large instantaneous changes in the loading conditions. Processors using the Intel Mobile Voltage Positioning 6.5 specification also require careful control of the power environment.

Power regulator PCB layout design is very important on a COM Express module. This includes using tracks of appropriate widths and using copper layers of the necessary weight to handle the high current flows. It is important, also, that the power IC manufacturer’s recommendations are closely followed to avoid problems. If a system designer chooses a pre-existing COM express module, power supply design issues will have been addressed by the module manufacturer. However, the high power flowing in the module should always be kept in mind when designing the baseboard. The baseboard design for the high power 12V rail is often relatively straightforward and typically transfers the supply from an input power connector to the COM Express module connectors. Careful attention to inrush current at power-up is needed in the baseboard and module design to avoid stressing components excessively and to achieve reliable module boot-up under all circumstances.

The current COM.0 standard supports an external supply into the module of +12V ±5%, but for many real-world applications, particularly for rugged designs, there is a need to support a much wider input supply voltage, for example from +9V to +16V. There is, therefore, a design decision of whether to use a module that has this wider input supply rail tolerance built in, or to implement a power regulator on the baseboard that locally generates a true +12V supply to power the module. In general, using a module with a wide input supply range is the preferred option because the module design has then been extensively tested and proven under all operating conditions.

A significant complexity of the baseboard design power section is that it needs to implement a number of regulated voltages for the various interface devices that are located on the baseboard, with their corresponding supply rails. This generally requires a number of power regulators that must all be power sequenced correctly for consistent behavior of the baseboard solution. A common problem with module and baseboard compatibility is achieving the correct power sequencing of these baseboard regulators under all conditions. If this is not done correctly it can lead to the module having boot behaviors which are difficult to debug, and in the worst case, being unable to boot reliably at all.