Designs in a variety of wall-powered systems have become increasingly challenging. New, strict efficiency standards have shaped the market, resulting in the reinvention of power architectures. Furthermore, system complexity has increased due to more demanding processing power, increased feature-sets and the addition of other supportive functions. Addressing such challenges, while keeping development time and system cost in mind, has become very difficult. Prioritizing and trading-off parameters and functions associated with power conversion and power management is also becoming a major headache.

This article discusses system design challenges in wall-powered equipment and introduces design solutions that can accomplish low-cost, intelligent power control, design flexibility and fast time-to-market. A single-output DC/DC controller with digital interface control will be introduced for addressing these challenges.

To maximize efficiency, every single power rail needs to be optimized–a major task given that the number of power rails in a typical "box" is not declining but increasing to accommodate new features brought by integrated circuits using a wide variety of supply voltages.

Careful decisions need to be made regarding switching frequency, FET parameters, and typical/maximum load requirements before the design can begin. In addition, real-time power on/off control, for allowing only the necessary power domains to be active at any point of time, as well as dynamic voltage control, are also becoming inevitable features for meeting the new efficiency goals.

Power delivery and power management are transitioning to intelligent implementations for addressing today's challenges. While product development is becoming more complex, the demand for fast time-to-market has also changed due to the arrival of new technologies. In the past, a given solution might have a 5-7 year life, new technologies are emerging today at a far faster rate.

Optimizing a high number of power rails often requires the utilization of a high number of different power-IC components (DC/DC regulators, DC/DC power managers/monitors, DC protection chips, and similar). Integrating various components with different characteristics is a daunting task for the system designer.

Furthermore, evaluating, qualifying and holding inventory of a high number of components introduces significant "hidden" costs for companies. Unfortunately, users are in many cases focusing solely on the IC price vs. analyzing the total solution cost, Figure 1. It can be shown that a controller with many integrated features can easily reduce the total system cost providing the cost analysis goes beyond a simple comparison of DC-DC controller costing.

Figure 1: System cost analysis
(Click on image to enlarge)

One of the most common power architectures is the intermediate step-down converter one, which generally reduces system cost without compromising many of the critical performance parameters (system efficiency, noise, and related factors.). As shown in Figure 2, this architecture uses more-readily available DC/DC regulation chips for the majority of the rails, and requires only a limited number of DC/DC conversion ICs that need to support high input-voltage rails (which almost always translates to higher cost and less availability).

Figure 2: Traditional intermediate power bus architecture
(Click on image to enlarge)

In a typical server or telecom equipment design, the number of rails can vary from 20 to more than 50. While some of these rails may require high power or have some special requirements, most of them are within a power range that could be addressed by single, scalable power-conversion ICs. Such a solution will significantly reduce the development effort of evaluating and integrating a high number of components. Digital power control can allow the use of single power-conversion ICs, thereby significantly simplifying the design.

In addition to faster time-to-market and reduced system cost, which are two main concerns for system design engineers, the use of a single DC/DC regulation IC satisfies the requirements of the other organizations and disciplines within a given company. From the procurement point of view, maintaining a smaller number of power-management ICs reduces the cost of supporting more suppliers and/or components. In terms of qualification, the cost and time to perform all reliability and qualification testing and to complete all necessary paperwork is greatly reduced. Last but not least, the manufacturing process becomes easier by addressing the same package footprint and similar IC layout vs. a variety of different components.

The big question is how can digital control address such diverse system requirements and enable a single IC to be used across very complex and diverse boards. Here are some of the key features and their benefits for multi-rail applications:

• Programmable power-on delay: allows different power rails to come up at different times. When the main power is applied on the board, all DC/DC regulators can "wake-up"; however, specific voltage regulators must be placed in a strict order to meet the system's stringent sequencing requirements.
• Programmable switching frequency: allows scalability of output currents, filtering components, power MOSFETs, and optimization of efficiency and therefore external component-size minimization. For rails with higher load requirements, a lower switching-frequency setting may be used to achieve high efficiency and high output-current capability. A higher switching-frequency selection may limit the maximum amount of current, but permits the use of smaller inductors and capacitors.
• Programmable over- and under-voltage monitoring: addresses various critical monitoring requirements circumventing the necessity for additional ICs traditionally used for this standalone purpose.
• Programmable current limit and over-current limit response: enables each rail to be optimized for reacting to over-current conditions. Higher loads will require higher current limit and high-speed over-current response. Certain rails may need to shut down after an over-current event, while others may need to keep retrying to "power up" after an over-current is detected.

Many of these power-management and control functions are often performed by dedicated power management or multi-purpose microcontroller chips. These duties can often be transferred to intelligent DC/DC converters, thereby further reducing system complexity and cost, Figure 3.

Figure 3: Intelligent intermediate power bus architecture
(Click on image to enlarge)

An additional value of digitally-controlled DC/DC converters, in particular those which incorporate non-volatile memory, is that they can be loaded with their default configuration during the manufacturing process. This, in combination with their load-current and output-voltage scalability, allows easier inventory control, since a single part number can be stored to be used (and programmed) in different portions of the board.

An example of a diverse DC/DC converter solution with digital power control is the SMB211, Figure 4. This IC takes advantage of the best of two worlds: analog power delivery with digital power-management and programmability. This combination allows design versatility and high analog performance at a low system cost and minimum power consumption (key for standby power requirements). A unique feature of the SMB211 device is its ability to accept different supply voltages for the device itself and the external MOSFETs, thereby allowing additional design flexibility.

Figure 4: Example of a digital power controller used in multi-rail system design
(Click on image to enlarge)

A typical system design may contain a housekeeping supply of 5 volts, which is used to power the controller itself, while a higher voltage of 9 to 12 V is used to power the MOSFETs. Lower-voltage supplies for the MOSFETs are also permissible, down to 3.0 V, ±5%. This scheme allows higher-voltage, lower-current supplies be routed to point-of-load (POL) converters using minimal copper trace widths. The SMB211 offers a selection of 8 different trip voltages used in conjunction with the current through the high side MOSFET times its RDS(ON) to determine the best possible over-current setting. This scheme is reasonably accurate and is the lowest cost method available for providing gross and cycle-by-cycle current limiting.

Up to four different switch frequencies may be selected from to optimize designs for PCB board space usage and efficiency, while providing the user the choice of operating in PFM/PWM mode or PWM mode only further increases the flexibility of the design to fit virtually all system board voltage and current requirements.

Furthermore, the SMB211 provides intelligence and enhanced protection to the system by communicating the fault type such as output under-voltage, over-current, IC over-temperature, if one occurs. This allows the system to also take certain actions based on which rail is "problematic" and what the fault condition is. This "intelligence" is further enhanced in the form of an output pin which is asserted when a fault occurs, allowing the pin be connected to a processor's interrupt input, for example.

DC/DC converters with integrated power-management intelligence and communications interface can significantly reduce system cost, complexity, and development time in multi-rail applications. The scaling and optimization of the output-power capability, combined with the versatility of power control functionality, allows single devices to be used for powering multiple loads in the same system. While such an approach may not fit every application, most multi-rail designs can benefit by such a distributed and intelligent power implementation. ♦

About the authors
George Paparrizos and George Hall are with Summit Microelectronics, Sunnyvale, CA.