Fully understand the system power budget. Make a matrix of the power supply voltages and the maximum current required of each supply. Multiply the maximum current by the supply voltage to obtain the maximum power for each supply. The sum of the power consumed by all supplies divided by the target efficiency is the total input power that must be delivered by the battery. A practical design requires an analysis of the total power demand of the system under all conditions. For example, a system may conceivably require maximum current of all supplies for an extended period of time, thereby narrowing the chances of relaxed specifications for certain components stressed under this condition. Conversely, an over-designed power supply; one that can safely deliver full power for extended periods of times in a system that doesn’t simultaneous power supply full power, may use excessive PCB real estate and require unnecessary costly components. Choose a battery from the Li-Ion class for small size, high float voltage and generous ampere-hour rating. The battery capacity depends on the targeted time of use before recharge and be certain to include the power supply inefficiency in your calculation. One last item to consider is the battery’s recharge time and ability to recharge quickly without affecting its reliability or life. Handheld users want to get back ‘on the air’ as soon as possible.
Identify power supply sequencing needs. It is quite common for complex integrated circuits to require multiple supply rails that must be brought up in a certain order. Failure to follow the manufacturer’s specifications can cause the device to malfunction or may result in permanent damage. The rate of rise of the individual voltages must also be considered when designing the power supply. Too slow a rise may draw excessive current from a supply voltage causing the entire system power-up event to fail. These supply sequencing and voltage slew rate considerations may also apply to subsections of the system with the same disastrous results if power supply voltages are not turned on in the correct order and slew rate. A supply sequence satisfying a microprocessor’s specifications may need minor modification to meet some other system sequence requirement.
Component selection must give consideration to allotted PCB real estate and height limitations both above and below the PCB. Although a component may meet design guidelines for efficiency, reliability and cost, it may fail to fit in the maximum dimensional outline for the power supply. Always give consideration to the bottom side of the PCB for mounting suitably sized components. Choose passive components, such as resistors, to be as small as possible without exceeding the power and voltage ratings. Check with manufacturing for any minimal size restrictions that would otherwise restrict or compromise the automation process. The magnetic components, such as the inductor, are offered in a variety of form factors to be used advantageously in a space-limited design. Some capacitors offer a similar variation in form factor and may also be used to optimize the design. All things considered, keep an eye out for increased cost when considering alternate form factors. Also make certain that lead times are acceptable when a single-sourced component is chosen.
Printed circuit board layout is critical to optimal electrical performance and the use of multiple layer boards is encouraged for a compact layout free of voltage drops, EMI radiation, noisy outputs, etc. Multiple layer boards allow for extensive ground and power planes and for the routing of difficult traces. An inner layer ground plane permits short connections from noise sensitive circuits and components.
Locate voltage reference bypass capacitors as nearby the power controller as is possible. Do the same for external oscillator components, when required. Loop stability compensation components must be placed nearby the feedback amplifier pins and away from high edge rate gate drive signals. MOSFET placement must be close to the controller to prevent gate overshoot and ringing when also minimizing the value of the series gate resistor.
Perform a practical worst case design analysis starting with the power train components. These components are generally the most expensive and the most likely to fail if undersized. While it is critical to know the absolute maximum requirements placed on the system power supply, it is more important to know whether these requirements will change over the life of the product. A supply designed to allow for some future increase in power is a reliable design and not necessarily an oversized design. The use of slightly oversized power components does not always imply physically larger components but will always ensure the system is reliable and has some room for growth. Handheld products are always the target of increased features and the inevitable increase in power.