Cable and Satellite content providers are continually introducing new services into a fearlessly competitive market. These new features include internet access, video on demand, DVD/CD players and hard disk drives, which are causing simple set-top boxes (STB) to evolve into highly capable home gateways. These advanced boxes establish a local area network (LAN) within the home. Consequently, they typically have an Ethernet or FireWire port or some other means of communications-possibly even Bluetooth. In turn, these more sophisticated STB's require more power. As the power for these systems increases, so does the system architect's concern about temperature rise, time to market, efficiency, regulatory approval, cost, power sequencing and power factor correction (PFC).
One way to address these concerns is to change the power supply architecture from the traditional multiple output flyback power supply to a higher performance, more flexible distributed power architecture with point-of-load (POL) regulators. Most current STB's use offline multiple output flyback power supply architectures. These power supplies provide as much as 30W of power with conversion efficiencies in the 65% range. Advanced set-top box supplies provide up to 65W with a single distributed DC voltage such as 12V, and use downstream POL regulators yielding a combined conversion efficiency of up to 80%.
The services that content providers offer determine the features of the STB, such as processor power, modem speed, transmission medium, inclusion of DVD or hard-disk drive and the number of I/O ports. The standard architecture of a STB is peripheral computer interface (PCI) bus based, which superficially resembles to a notebook PC. Although the architecture of a PC is the same throughout the world, the architecture of a STB varies significantly from region to region and even manufacture to manufacture. Of course, due to the fickle nature of the consumer, the configuration of a STB is in a constant state of flux as the services demanded are constantly changing. Naturally, features determine the power supply requirements and the only obvious trend is that the system power is increasing. Competition among service providers and regional variations mean that there is considerable pressure to get STBs to market on time. Delays due to the power supply design are intolerable due to potential loss of market share.
It is not uncommon for STB power supply requirements to change late in the design cycle. Nevertheless, the last thing wanted is for the power supply to delay the STBs development, but that can happen if the use an offline multiple output flyback supply is needed. To design such a supply, and to obtain the required safety and EMI approvals, can take six to nine months-or even as long as a year. With a multiple output flyback design, late changes can necessitate lengthy magnetics design iteration. As a result, in some cases, desperate designers have added diode drops to accommodate last minute output voltage or current needs.
Standard off-the-shelf AC input (90-264VAC) single output power supplies offer the advantage of using a safety agency and EMI approved power supply. These supplies provide up to 90% efficient power conversion from a single output voltage ranging from 3.3V to 48V. Furthermore, standard off-the-shelf single output power supplies are inexpensive, are offered in several sizes and are in high-volume production for numerous applications. Using this scheme could cut six months from the development cycle. Even if the maximum power goes up, the designer can plan for a 20% to 30% buffer from the beginning of the development in anticipation of needing more power without much of a sacrifice in cost or real estate.
PFC is another issue that affects STB power supplies. The IEC/EN61000-3-2 specification limits the harmonic current emissions for line-powered systems having an input power of 75W or greater. In effect, this specification requires additional circuitry when the input power exceeds 75W. A 65% efficient multiple output flyback can supply a maximum of 49W output power without requiring extra PFC circuitry. A distributed power supply with 90% efficiency, combined with 90% to 95% downstream POL regulators, can make 60W to 64W available without requiring PFC. This higher efficiency has the obvious benefits of reduced heating, less ventilation and higher mean time between failures (MTBF).
Linear Technology offers a complete line-up of high efficiency synchronous POL DC/DC switching regulator's including modules, monolithics and controllers. The LTC3850 is a recently released POL dual output DC/DC switching regulator controller that can convert 12V to 5V/5A with 95% efficiency, or 12V to 3.3V/5A with 92% and 12V to 2.5V/5A with 91%.
The LTC3850 is a dual output synchronous step-down switching regulator controller that drives all N-channel power MOSFET stages. Its' 4V to 24V input range encompasses a wide variety of applications including most intermediate bus voltages. The strong on-board driver allows the use of high power external N-Channel MOSFETs to produce output currents up to 20A with output voltages ranging from 0.8V to 5.5V.
