The demand for portable electronic devices continues to increase with the popularity of smartphones, tablets and similar devices. These devices must be lightweight, rugged, responsive, and long battery life.

The key to designing portable devices is implementing a large amount of functionality packed into a small space while minimizing power consumption and enabling sufficient performance.

Circuit Optimization

Portable device design starts with the selection of components, including the processor, meant for embedded circuitry. These components typically support low power operation, low power modes, and are available for many years.

Engineers must know how to make the proper trade-offs to shrink the area of circuitry while not sacrificing performance. Numbers of pullup/pulldown resistors and bypass capacitors can be minimized through careful analysis of circuit functionality and power draw. Through the use of appropriate capacitor dielectric, equivalent series resistance (ESR), and current return path analysis, it is possible to reduce the number of bypass capacitors needed.

Size constraints placed on portable electronics often leads to dense printed circuit board (PCB) routing. This can lead to complex PCB features, including blind, buried, and split via structures that lead to multiple laminations. All of this translates to high PCB prices and longer lead times. Portable electronic design engineers understand how to define PCB routing rules, architecture power domains, and signal return paths, and thereby optimizing PCB signal and power integrity while not becoming excessively expensive.

Power Conservation

To maximize run time on a single battery charge, portable devices implement low power modes inherent in the processor, enable/disable peripherals as needed, and often provide closed loop power monitoring.

Current processors implement dynamic power management via dynamic voltage scaling, frequency modulation, and other techniques. The use of these modes involves both proper circuit design and tweaking operating system settings to enable these modes. For example, many Linux implementations include a library of performance governors that, when enabled, allow the OS to adjust processor frequency based on computational loading.

Peripheral power gating requires expertise in both electronic and operating system design. Point of load power sources or low impedance pass switches are used to gate power to peripherals when needed. The operating system software usually requires modification that often difficult to reliably implement, involving corner cases that must be anticipated and correctly implemented.

Adding power monitoring circuits allows closed loop power conservation techniques to be implemented. The use of inductors with higher Q-values results in less energy being wasted as heat.

It is important to know how the device is intended to be used, overall, in terms of power consumption and how much power will be used by each electronic sub-system.  This helps to size the battery, which has a major impact on physical size and weight of the portable device.

Temperature and Materials

Portable devices often must be completely sealed for protection from moisture and dust, thereby complicating heat management. When sealed, there is no path for air to escape and heat generated by components must be spread within. This has several implications on component selection and device design.

To accommodate the problem of extreme temperatures, components with industrial or automotive temperature ratings should be selected. Additionally, performance variation of internal components over temperature must be considered. For example, the actual capacitance of capacitors varies over temperature, which can impact power delivery at temperature extremes.

To cool components, such as the processor, that generate relatively large amounts of heat, thermal interface materials and heat spreaders or heat sinks are often added to the component package. These materials must exhibit adequate thermal transfer properties without adding excessive weight or cost to the device. Printed circuit board design can also enable heat spreading through higher copper weight power planes and thermal transfer vias.

Rugged Packaging

Portable electronic devices are often subjected to many forms of environmental stress, whether in extreme temperatures, vibration, shocks (often induced by being dropped or subjected to other types of impact), moisture (whether humidity, spray, or even submersion), dust, solar loading, and more. The device packaging design must utilize materials and secure all components that enable the device to survive these stresses. Additionally, the device must be easy to use, be intuitive to operate, and meet ergonomic standards.

Device Security

Since portable electronic devices are inherently small, mobile, and often have common external interfaces, they are vulnerable to compromise. Device security therefore becomes critical in protecting data and operations.

Access to portable electronic devices is typically provided through physical interfaces such as USB or through remote interfaces such as Wi-Fi or Bluetooth.  Access must be controlled via strong passwords, encryption, authentication, and other means as appropriate.

When a device falls into the wrong hands, opening it becomes an option, which can lead to theft of intellectual properly or data, and can even provide an opportunity for operation of the device in undesirable ways. Physical tamper detection techniques can sense whether a device has been opened, or is in the process of being opened, and react to this event by erasing data or intentionally locking its operation.

Summary

Portable electronic device design requires specialized knowledge and skill sets to simultaneously accommodate a number of tough and competing requirements. In addition to the technical considerations described above, the device must pass various regulatory compliance tests (such as FCC Part 15 for electromagnetic compliance), meet ergonomic standards and price targets, and be manufactured with high yields and low return rates.

When put together, the design of portable devices requires a high complex and niche skill set. Product design firms, such as InHand, have the expertise, processes, and experience to design and deliver custom designed portable electronic devices. InHand’s Modified COTS design processes leverage existing platforms to deliver a customized device with lower cost and risk, and within shorter schedules. Leveraging expertise is critical in meeting time-to-market and quality requirements.