Embedded system designs are becoming increasingly complex. The end-user expects a feature-rich product, typically powered by a small battery. Many designs require a compact form factor and perhaps most importantly low cost. The scope of an embedded product now extends to include power-hogging interfaces such as wireless LAN (WLAN), a hard disk drive, and larger LCDs. As a consequence, apart from the constraint for processing power, the performance of the power supply for portable systems needs to be improved on a continual basis.

This article presents a generic power supply design for an embedded system and examines the function of each module in the design. It also provides guidelines for selection of topologies and components, thermal and packaging aspects, battery management, and hooks that interface the circuit to intelligent power management software. The principles can prove to be useful for the power supply design of any embedded system with robust feature sets and must operate battery power. Portable media players and video-conferencing units are examples of this product space. Based on the building blocks described in this paper, the reader can choose appropriate components available from different IC manufactures for a specific design.

Building blocks of a power management unit

Specifying the exact functions and building blocks of the power circuit is usually a non-trivial job, especially because it directly affects the play time of the battery-powered system. The architecture may vary with various classes of embedded products and usage scenarios (consumer, industrial, military, etc.). Figure 1 shows the blocks of a power solution for a typical consumer application.

Figure 1. Block diagram of a power management solution for a typical embedded system.
(Click this image to view a larger, more detailed version)

Referring to Figure 1, we'll define the requirements for each of the blocks in the diagram.

The product is expected to work from a battery pack or from an external wall adapter. A power path controller performs the switching function to the correct power source when more than one sources are present. Emerging alternate sources of power such as USB and power over Ethernet (PoE) may need to be accommodated in some designs.

Battery protection circuits protect the battery from conditions such as over-voltage, under-voltage, excessive heating, excessive current drain, and others.

A dedicated battery charging circuit should charge the batter any time an alternate source of power is available.

A fuel gauge circuit monitors the battery condition on a continuous basis providing status to the user and to power management software.

The system may require multiple DS-DC power converters such as SMPS (Switch Mode Power Supplies), LDO regulators, charge pumps, and others. The different converters account for all of the possible input sources and the different voltages required inside the product design.

A digital interface or a hardware push-button controller switches the system on and off -- sometimes called soft start. In some recently introduced power converters, the digital interface can also be used to fine-tune the output voltage generated by the various converters. This fine tuning is essential in power-conscious designs where the output voltage needs constant adjustment based on the application scenario.

Features of an efficient power supply

Now let's consider the efficiency of the power system, In an embedded application, the efficiency of a power supply is not limited to the conventional definition of the ratio of output power delivered to the input power utilized by the system. In the context of embedded systems, an efficient power supply refers a solution that address the following system usage and design characteristics:

1. Maximizing the play time of the device when working on battery,
2. Extending the life of the battery (in terms of number of charge-discharge cycles),
3. Limiting the temperature rise of components and the battery itself, and
4. Offering integrated software intelligence to maximize efficiency.

System designers would agree that there is no single set of guidelines to optimize the efficiency of a power supply solution. However, a designer can consider the following points in developing a power system.

The life of the battery (number of cycles) is dependent on the charging profile used to top-up the battery. For lithium-ion batteries, the manufacturer generally suggests the optimum charge current (in constant current mode) and the termination/precharge currents and these specs should be followed strictly while designing the charger circuit.

Battery protection must be viewed as an essential feature in consumer electronics products, since it is deeply linked to user safety. Adequate provisions for detecting over-voltage, under-voltage and temperature of the battery must be made and suitable components like temperature-dependent resistors used to ensure that the current is limited automatically in case of any anomalous conditions.

The fuel gauge must be used for ensuring battery safety apart from its normal function as a gas gauge. Most fuel gauges are mounted on the battery pack itself and can be used to sense the battery temperature, discharge current, etc.

One often overlooked aspect of the power path controller is that when switching from one source of power to another, there must not be a conducting path between both the sources even for a short period of time. This may need addition of reverse connected switches or diodes. Also, when one of the sources is powering the system, the voltage from the same should not appear at the input of the alternate source.

With so many different topologies of power converters available, it is difficult to make the correct choice of power converter. In general, linear regulators must be avoided in situations where efficiency is at a premium and high output currents are involved.

In the case of SMPS, designers should ensure that a suitable topology (buck, boost, buck-boost, charge pump, SEPIC, etc.) be used so that the desired output voltage is available even when the battery voltage drops to its minimum operating value. This helps to enhance the play time of the product.

In case of buck converters, it may often appear that synchronous converters (where the diode is within the IC in the form of a switch) are more efficient than their non-synchronous counterparts. However, this choice depends heavily on the output current required and the nominal duty cycle at which the converter works. Hence, the slight increase in converter efficiency by having a synchronous converter may not always justify the increased cost of the converter.

The type of inductor used for filtering the output ripple of the SMPS usually changes the efficiency of the converter. Among various choices of inductor, the one with low DC resistance and low magnetic losses at the operating frequency is preferred for the filtering function.

Thermal design should go hand-in-hand with the electrical design. The package of every IC or passive must be capable of handling the power that it would normally dissipate. Thermal pads with stitched vias and large pads on the PCB are recommended by manufacturers of many ICs for better thermal fan out. There is usually no scope for adding fans for embedded products, but vents and adequate thermal relief on the PCB must be planned for.

Power supply design is often looked at, as a purely hardware design. However, to attain a highly efficient power solution, designers must add software intelligence to the power circuit. Some of the basic functions of the software control are detecting the power source selected by the power path switcher, and reducing power to unneeded circuits if the power is being sourced by a battery.

A more elaborate power management software implementation would take inputs from other parameters such as the type of application running on the system, its minimum peripheral requirement, the minimum clock rate and voltage required for running this application and control the state of the power supply output, clock generators, interfacing ICs, etc accordingly.

Following the above rules of thumb can improve the performance of a portable power supply to a significant extent. The overall efficiency for a typical 30W multi-output power supply solution can be as high as 85 to 90% by keeping the above points in mind. This has been successfully achieved in the custom power supply designs built at Ittiam Systems for powering some of their multimedia application systems.

There are a host of highly integrated ICs available today from various manufacturers, which provide various functionalities described above. Based on the power requirements, specific applications may use such monolithic solutions while others may require discrete building blocks. After all, in a competitive market for embedded products, battery life and play time are key factors that influence the buyer's choice.

Authors Information

Shantanu P Prabhudesai, Senior Engineer, Multimedia Systems, Ittiam Systems Pvt. Ltd., India has been working with Ittiam for the last three years in the domain of hardware and product design for various systems such as IP-based videophone, digital photo frame and in-flight entertainment systems. He holds a masters' degree in Electronics Design and Technology from Indian Institute of Science, Bangalore in 2005.

Ritesh R Parekh, Lead Engineer, Multimedia Systems, Ittiam Systems Pvt. Ltd., India, has 8 years of experience in hardware and system design in field of multimedia and power systems. He has worked on system design of products such as Personal Media Player and Recorder, FPGA based Audio/Video reference Platform, IP-based Video Phone, Network Media Players and Video Surveillance. His interest areas are DSP based systems, FPGA based reference platforms and Power supply design. Prior to joining Ittiam Systems he worked with Crompton Greaves Ltd., India for 4 years. He holds a Masters Degree from Indian Institute of Science in Electronics Design and Technology.

Pawan Kumar Gupta, Manager, Media Streaming, Application Specific Media Systems, Ittiam Systems Pvt Ltd, India, has 13 years of experience in hardware and system design in the fields of telecommunication and multimedia. His expertise and interest is in DSP based consumer electronics systems such as IP Video Phone, Media distribution, Surveillance, Wireless multimedia, Health-Care etc. Prior to joining Ittiam Systems, he worked with the Centre for Development of Telematics (C-DOT), India for 8 years. He holds a Masters Degree from the Indian Institute of Science, India.