Today's complex telecom, storage and server systems present the board level power designer with a host of different design challenges. This article reviews the most significant and discusses how configurable power controller ICs are making the task more manageable.
New voltages appear more quickly than the older ones disappear
Moore's law, which sees IC voltages driven down every year or so, simply doesn't govern how quickly technology leaves the market. While 90nm devices, with their sub-1volt requirements become more commonplace, many old legacy I/O's still exist, and ASICs developed on much older technologies are still being used. As a result, it's common to find circuit boards with ten or more voltages.
Hundreds of Watts of power can't be delivered from the edge of a board
It is feasible to design a system where hundreds of Watts at 5 Volts are delivered from an isolated DC/DC converter ("brick") at the edge of a board. Today's lower voltages mean that DC/DC converters must exist where the power is consumed to prevent enormous IR losses, and to respond to load transients effectively. So rather than a single 1 Volt supply, a large board may have several. The ten or more voltages required by the board might in fact be furnished by twenty or more DC/DC converters!
Sequencing requirements are complex and subject to change
Leading edge systems use leading edge chips. These now typically require at least three different power supplies for analog and digital supplies, and various I/O voltages, etc. There seems to be as many different ways to turn on the supplies to a chip as there are chip manufacturers, including proportional, simultaneous, and time sequenced (Figure 1). To further exacerbate the power designer's life, the adjacent chip requires a completely different sequence! To make matters worse, datasheets are known to be "tweaked" occasionally, in order to change a sequencing requirement or marginally change the recommended operating voltages in order to get silicon working properly.
Figure 1. Various sequencing scenarios
These factors, along with other perennial concerns like system uptime, thermal management, development time and expense combine to make the power designer's job a daunting challenge. A configurable architecture is required, where as much as possible of the power system's behaviour can be made flexible so that evolving and changing board level power requirements are easily handled.