The worldwide installed base of set-top boxes will increase significantly over the next few years, and so will the need for higher-efficiency and more sophisticated power management. Meeting new "Green" standards without compromising performance and cost will require improvements in three basic categories: new power architectures, higher-efficiency power supplies and optimized software. The utilization of switch-mode power solutions with digital control can address these requirements and simplify new set-top box system designs. Good designers pay close attention to layout to maximize the advantages of using a switch-mode supply.
Typical set-top box power requirements
Set-top boxes act as a gateway between your television or PC and your telephone, satellite, broadband over power line, terrestrial or cable feed. Their main function is to convert encoded/compressed digital signals from a signal source, decode/decompress and convert them into analog or digital signals that can be displayed on the television or PC monitor. The emerging of many new technologies over the last decade is enabling modern set-top boxes to go a step further, by also creating a user interface to the internet and by becoming a media for ecommerce, video-on-demand and two-way communication.
The set-top box power requirements vary significantly with the feature-set integrated in the individual box. For example adding video recording functionality requires the integration of a hard disk or a DVD drive, while multi-room service is enabled via additional tuners in the box. In a typical design the CPU is responsible for running the operating system and for parsing the MPEG transport stream, and is the most power-hungry component in the system. Naturally, the adoption of the high-definition and MPEG4 decoding standard introduced the need for higher computing power, thereby also increasing the power requirements of the CPU. The need for greater peripheral connectivity (IEEE1394, DVI, 802.11, ADSL) has also been a big factor in latest designs and consequently in adding design complexity. A typical set-top box also includes dynamic random access memory (DRAM), an MPEG decoder chip, and additional chips for audio decoding and processing. Figure 1 shows a typical power management block diagram for a set-top box system.
(Click on Image to Enlarge)
Figure 1: Typical power management system
In the United States alone the annual energy consumption of the installed set-top boxes is over 20 TW-h. The main processor, the power supply, and the RF front-end are the circuit blocks that have the highest effect in total system power consumption. The magnitude of this number, combined with the increasing popularity of set-top boxes, has driven many energy-efficiency initiatives over the last few years, both in the US and worldwide. The conversion from analog to digital broadcasting will help alleviate the issue, since the need for analog tuners and other associated components is eliminated, however more actions will be necessary to accomplish the industry's aggressive goals.
The state of California has already introduced appliance efficiency regulations, many of which have been adopted by set-top box oriented power supplies, thereby significantly increasing efficiency in existing designs. Mandatory California regulations also exist for digital-to-analog set-top boxes, requiring power dissipation to remain below 8 watts during operation and 1 watt during standby mode. Energy Star also introduced efficiency criteria for set-top boxes in 2001, targeting specifically low-power/standby modes. Under these original Energy Star guidelines, the maximum power allowed to meet the standard depends on the complexity of the set-top box (3 categories defined). More work is currently in progress for tightening these Energy Star standards and for allowing one specification to apply on all set-top box types.
Achieving maximum efficiency in system design
Power requirements of new set-top box chipsets are increasing due to the need for higher computing power (to meet new standards, enable multi-functionality, etc.) and the utilization of smaller geometries for new digital chips (lower voltage, higher current operation). This trend, combined with the new, strict efficiency standards, will greatly effect set-top box power architecture, since it will result in a higher number of switch-mode DC/DC converters being utilized in new designs.
Traditional power solutions rely heavily on linear regulators for DC/DC conversion, an approach that is attractive because of simplicity but with many shortcomings. The power losses in a linear regulator
are directly proportional to the voltage delta between input and output voltage and load current, making them impractical for modern designs that require high-current and low-voltage rails. Since the step down conversion in a linear regulator is completely dissipative, all power loss is in the form of heat. This complicates thermal design and results in the need for large heat sinks, which take up valuable real estate within the set-top box. On the other hand, inductor-based switching regulators are ideally suited for high-power applications, since the voltage conversion is accomplished by storing energy in the magnetic fields of an inductor, and therefore voltage conversion is not dissipative. This operation can achieve efficiencies in the 90 to 95 percent range and eliminate the need for bulky heat sinks.
Table 1 demonstrates the benefits of implementing switch-mode DC/DC conversion for driving some of the power-hungry components in a typical set-top box system: in a traditional power design the power dissipated, just for the four rails shown, is 7.9 watts, whereas a switch-mode power design can cut power dissipation to 2.45 watts. In addition to the direct environmental benefits associated with power consumption, higher efficiency also minimizes the need for a thermal design that can limit layout flexibility (critical components need to be far away from the "hot spots") and add unnecessary assembly and component cost to the system (heatsinks, etc.).
(Click on Image to Enlarge)
Table 1: Efficiency comparison, traditional versus switch-mode supplies
The new challenges in set-top box system designs have sparked additional developments for achieving the efficiency goals set by the industry. New system-on-chips (SOCs) and central processing units (CPUs) for the set-top box market incorporate a number of power modes that allow great power savings. These modes place certain power domains in shutdown or standby, thereby allowing only the absolute necessary functions to be activated. In addition, many of the new digital chips can operate at different clock frequencies by accepting different voltage levels by the power management ICs (dynamic voltage control). Such operation is again used when the maximum computation/processing speed is not required, and can result in significant power conservation given that power is proportional to the square of the voltage as given by:
P = C * f * V2
Last but not least, individual enable/disable control of each power rail allows system software to power off sections that are not needed to perform specific functions.
Other design considerations
To ensure the voltage at the load is as close to the desired value as possible, the output should be sensed at the point of load (POL). However, this does not mean that the inductor, FETs, and output capacitor should be placed next to the load, and away from the controller; rather, all components should be placed as closest to the PWM controller as possible— even if the load is several inches away from the PWM controller. By sensing the voltage with a remote sense pin, the designer can compensate for any voltage drops along the trace resistance by raising the output voltage at the beginning of the trace. This POL sensing is illustrated in Figure 2.
Figure 2: Good layout with remote POL sensing
The quality of the picture, and the ability to pass regulatory standards, can be greatly affected by noise present in, and emanating from the system. While linear regulators are impractical based on their power dissipation, they introduce virtually no noise into the system. Hence, the goal of designing any switching regulator is to minimize the noise that the switching converter creates in order to ensure that it does not interfere with sensitive circuit blocks. As today's wireless handsets are utilizing a number of inductor-based DC/DC converters it is clear that, through careful design and layout, even the most sensitive circuit blocks can run from switching regulators.
While some designs still use two-layer PCBs for cost savings, this has significant disadvantages due to the lack of a ground plane. A ground plane is essential because it provides a low-impedance path from all output voltages back to the supply voltage. For high-frequency signals the return path for the current through the ground plane tends to follow directly under the signal source. However, if the ground plane is heavily segmented or no ground plane is present, the high-frequency signals will radiate through the air as opposed to weaving across traces with high impedances. For this reason adding a ground plane significantly reduces radiated noise by keeping current loops as short as possible.
Buck regulators can both generate noise and are susceptible to noise. The noise generated is normally due to high-frequency ringing caused by fast rising edges, and current loops with rapidly changing currents and high return impedances. The noise susceptibility is due to capacitive coupling on high impedance nodes or inductive coupling on high impedance nodes due to large loop areas.