Powering Digital Set-Top Boxes

By Reno Rossetti

The digital set-top box (DSTB) market is bigger and growing faster than the notebook PC market. There are many architectures and implementations of set-top boxes, but they all have one thing in common: they need power. Application specific power ICs for set-top boxes have yet to appear, but power ICs originally developed for the desktop and notebook PC market can serve almost as efficiently.

The function of digital set-top boxes (DSTBs) is to control and decode compressed digital television signals from satellites, cable, and terrestrial broadcast systems. Many also have recording capability and may further serve as Internet access devices. Units typically comprise a tuner, audio and video signal processors, a modem, system memory, and a control CPU. High-end units may have additional memory and solid-state or hard disk drives for mass storage.

The architecture of DSTB designs ranges from a classic PC-like configuration based on Intel® Pentium® processors with associated chipsets to varying degrees of integration going as high as Systems on Chip (SoC) that include all but tuner, modem and memory functions. However, they share common underlying digital technologies with applications such as PCs and hand held computers. Such commonalties allow DSTB designers to draw from a rich portfolio of application-specific-standard-product integrated circuits (ASSP ICs) originally developed for notebook and desktop motherboards based on Intel architectures.

Strategies for powering set-top boxes are as diverse as their architectures but in general break down into two cases. The first is the high-performance, high power box consuming 50W to 240W. Also in this category are devices that must use line power factor correction, such as products meeting European EMC regulations. The second case is the low-power box, needing less than 50W without power factor correction. This low power level generally implies a less sophisticated system, for example one having less memory on board and without hard disk drives.

High Power Needs Synergy

A high-level view of the power system for DSTBs, shown in Figure 1, has three components. The first is rectification of the AC line power to produce raw direct current. The second stage is regulation to produce a stable intermediate DC voltage followed by a DC-DC converter stage to produce the final supply voltages. High-power and low-power designs differ considerably in the first two stages while the DC-DC converter stage is quite similar for both cases.

Figure 1 - Power for digital set-top boxes has three elements: rectification, conversion to DC, and low-voltage regulation. The first two elements differ considerably for low and high power applications.

The first two stages form an AC/DC conversion chain that produces an intermediate DC voltage (Vout) low enough (12 to 28V DC) to be safely distributed on the box motherboard. The chain for a high-power design or one that needs power factor correction (PFC) is shown in Figure 2. A full bridge diode rectifier converts the AC line voltage into a continuous, but poorly regulated, intermediate voltage. This is followed by a “forward” converter to reduce the voltage to something useable by the electronics on the motherboard.

Figure 2 - A high-power AC/DC converter chain can lower cost by combining the power factor and switching regulator control activities with a specialized power IC developed for desktop computing.

Because the best conversion efficiency is obtained when voltage and current drawn from the line are “in phase”, and to help meet EMC requirements, this stage uses a PFC block. The PFC block forces the correct phasing by modulating the drawn current according to the shape of the input voltage. The inductor L1, MOSFET switch Q1, and the diode D1, controlled by one half of the FAN4803 IC, form the block.

The FAN4803 IC is an example of the ASSP devices developed for the PC market that can now serve DSTB design. When a synergistic mode of operation between the PFC and forward conversion sections is implemented, a power supply can be built with a minimum bill of materials (BOM). The IC combines a PFC section with a pulse width modulator (PWM) used to control the forward converter.

The converter stage includes switches Q2 and Q3, diodes D1-D5, and passives L2 and C2. The PWM half of the FAN4803 provides primary side control and the RC431A diode provides secondary side control. For safety reasons, this stage requires electrical isolation between the high input and the low output voltages. A transformer (T) in the forward conversion path and an opto-coupler in the feedback path provide this isolation.

Low Power Allows Simplicity

Below 50W the architecture of the AC/DC converter chain becomes considerably simpler. In these applications a PFC section is no longer needed and the lower power rating allows a more basic converter. As shown in Figure 3, a diode bridge rectifier in conjunction with a simple fly-back controller such as the KA5x03xx family handles the entire chain with minimum number of external components. This level of simplicity is possible because of the multi-chip approach to integration in this controller family. The package houses two chips, a low voltage monolithic controller die and a high voltage discrete MOSFET die to provide the switching. Safety considerations, however, still require the isolation that the transformer and opto-coupler provide.

Figure 3 � Low-power conversion enjoys simpler design, although it still needs power isolation.

With an appropriate DC voltage (12V to 24V) delivered by the converter chain, all the low voltage electronics in the DSTB can be safely powered. This intermediate voltage is needed because the system elements have differing power needs. For example, the distribution of DC power on the motherboard, shown in Figure 4, involves the creation of as many as nine different power lines with regulated outputs. Creating all these voltages directly from AC would be expensive.

Figure 4 � The DC/DC conversion section of set-top box power needs to provide a range of voltages for different system elements. Low-power designs can use smaller passive components and eliminate the regulators for DDR memory

A series of PWM regulators handles this task in high-power systems. The FAN5236 dual PWM regulator powers the CPU Core and I/O. The regulators in this IC have output voltages adjustable down to 0.9V. This allows them to accommodate multiple generations of CPUs, from 0.18um lithography requiring 1.8V to 0.13 um requiring 1.2V to future 0.1um lithography requiring sub band-gap voltage rails.

A highly-integrated PWM controller such as the FAN5235 produces another five voltages: two ‘buck’ (step-down) regulators produce 3.3 and 5V for logic and conventional memory, one boost regulator creates 28V for the varactor bias in the tuner section, and two low power/low dropout regulators (LDOs) provide power for stand-by operation. A second dual PWM regulator provides power for DDR memory. One line, Vddq, supplies the memory device and another line supplies termination Vtt (equal to Vddq/2).

For a low-power system the same type of controllers can be used, but only two controllers are required to power the motherboard because there is no DDR memory. The design will also be able to employ smaller external discrete transistors and passive components, leading to a much more compact set-top box. At the lower power level it could also make sense to use LDO linear regulators instead of switching regulators. The trade off will be between low power dissipation and a more expensive bill of materials (including magnetics) for switching regulators and the higher power dissipation and need for heat sinks but simpler BOM without magnetics for LDOs.

As examining cases at the opposite end of the power spectrum shows, the current generation of set top boxes can be powered using of application specific standard products ASSPs developed for the PC and hand held market. As production volumes increase and architectures solidify around a few leading core logic chipsets, dedicated ASSP IC’s for set-top box will become necessary to allow increased performance at competitive cost. In the meantime, existing devices developed in support of the Intel architecture can fill the need.

Reno Rossetti is Director of Integrated Circuits Group Strategy at Fairchild Semiconductor. Reno has many years of experience in the analog and mixed signal semiconductor industry dealing with power IC’s and discrete semiconductors. He holds several patents and has a degree in Electrical Engineering from Politecnico di Torino and an MBA from the Bocconi University of Milan.