There’s an ever-expanding list of power efficient microprocessors designed by Intel, Freescale, Marvell, ARM and many others that provide low power consumption and high performance processing for a wide range of wireless, embedded and networking applications. The original intent of these processors was to enable consumer product OEMs to develop smaller, more cost-effective portable handheld devices with long battery life, while simultaneously offering enhanced processing performance to run feature-rich multimedia consumer applications.

Achieving higher processing performance without increasing system power consumption requires lower voltage operation at ever increasing currents. Both portable and embedded systems contain various components optimized for operation at different voltages due to either the needs of the application or the line widths of the processing technology. The end result is that systems employing the latest “portable” processors require a large number of high-current, low-voltage rails—typically at or below 1.8 V. In addition to numerous low-voltage rails, many of these applications also require 3- or 3.3-V rails for powering large portable hard drives, memory, I/O supplies for external logic circuitry, etc. In embedded applications, all supply voltages that directly interface to the processor can be generated by high-efficiency buck dc-to-dc converters or low-dropout regulators (LDOs), depending on the current requirements.

Recently, demand for this combination of high-power efficiency and processing performance has spread to portable medical applications. Blood analyzers, patient-monitoring equipment, exploratory cameras, tissue and bone stimulators, ultrasound probes, fluid pumps, portable EKG units, and other portable medical devices demand similar or even greater levels of power efficiency and processing horsepower than the latest feature-rich high end consumer portable devices. Plus, they must be rugged, reliable, and lightweight enough to be considered “portable.” In all cases, however, a highly specialized, high-performance power-management companion IC is necessary to properly control and monitor the microprocessor’s power system and ensure that all of the efficiency benefits possible with these processors can be realized, regardless of the application.

In the case of portable applications, the main power source is typically a large single-cell Li-Ion/Polymer battery which may have a voltage above or below the 3.3-V system supply in the product. Applications such as these (e.g., handy terminals, bar-code scanners, RFID readers, etc.) require a buck-boost supply to generate the 3.3-V rail. Additional complications with these “portable” processor systems, whether they’re battery powered or not, need to sequence all supplies on and off in a specific order. They also must have the ability to be adjusted up and down dynamically depending on the system’s processing needs. For the system designer, a single integrated solution that addresses all of the microprocessor and associated application power supply needs is a huge advantage. Handling these needs across a wide range of applications requires a highly flexible, programmable and efficient multi-output power-supply solution.

Design challenges
Most of today’s modern feature-rich electronic systems still require voltage rails in the +3-V range, to power, for example, I/O or a peripheral rail in an automotive infotainment system. Integrating synchronous buck-boost switching capability into the power-management IC (PMIC) allows 3.3-V regulation across the entire input voltage range, 2.7 to 5.5 V, with high efficiency, resulting in increased operating margin. However, achieving high efficiency with a buck-boost design is more challenging than a simple step-down dc-to-dc converter, particularly if low noise and good load-step transient response are required.

Reducing heat, optimizing system efficiency
Many industry-standard PMICs come with a variety of onboard linear regulators. However, linear regulators, if not managed properly with sufficient copper trace routing, heat sinks, or well-designed I/O voltage and output current levels, can generate localized thermal “hot spots” on the pc board itself. Alternatively, a switching regulator provides a more efficient way to step down voltages when the difference between input and output voltage is high, and/or if the output current is large. Their use is common in today’s feature-rich devices designed with low-voltage processors. As a result, implementing switch-mode-based power supplies for the majority of voltage rails is increasingly necessary. LDOs, however, provide low-noise outputs and great PSRR performance, so tradeoffs must be assessed. In many cases, the correct IC partitioning includes both types of regulators.

Virtually all applications today are sensitive to heat. As the processing performance and associated operating currents go up, it’s more important to use switching regulators in place of LDOs. This is particularly true in highly integrated power supplies since single ICs are limited in their ability to dissipate power. Furthermore, achieving optimal power dissipation requires many of the core processing rails to dynamically adjust depending on the processing operations being performed. Higher supply voltages are necessary to achieve higher clock rate operation. Similarly, very low voltages are adequate for less processing-intensive modes of operation. Because the corresponding supply currents tend to track the input supply voltage, it’s desirable to operate the processor at the lowest possible supply voltage. Dynamically adjusting the processor voltage supplies requires a serial port, such as I²C, to communicate the changes. Nearly all of today’s high-end portable processors support this functionality. However, taking advantage of it requires an equally flexible and programmable power solution.

As with many other applications, low-power precision components have enabled rapid growth of portable medical instruments. However, unlike many other applications, portable medical products typically have higher standards for reliability, run time, and robustness, in addition to being lightweight for portability. Much of this burden falls on the power system and its components. Medical products must operate properly and often must switch seamlessly between different power sources, depending on the design and input source requirements. Great lengths must be taken to protect against and tolerate faults, maximize operating time when powered from batteries, and ensure that operation is reliable whenever a valid power source is present. Further, power levels are increasing as are the number of features and associated voltage rails.

Many of the past industry PMICs have not had sufficient power to handle these modern systems and microprocessors. Any solution to satisfy the power-management IC design constraints outlined above must combine a high level of integration, including high-current switching regulators and LDOs, dynamic I²C control of key parameters with hard-to-do functional blocks such as buck-boost regulators. Further, a device with high switching frequency reduces the size of external components, and ceramic capacitors reduce output ripple.

The LTC3589 is a complete power management solution for ARM-based processors and advanced portable microprocessor systems that accomplishes these goals (see the figure). The device contains three synchronous step-down dc-to-dc converters for core, memory and SoC rails, a synchronous buck-boost regulator for I/O at 2.5 to 5 V, and three 250-mA LDOs for low-noise analog supplies. An I²C serial port controls regulator enables, output voltage levels, dynamic voltage scaling and slew rate, operating modes, and status reporting. Regulator start-up is sequenced by connecting regulator outputs to enable pins in the desired order or via the I²C port. System power-on, power-off, and reset functions are controlled by a pushbutton interface, pin inputs, or I²C interface. Voltage monitors and active discharge circuits guarantee a clean power-down before the next enable sequence. In addition, selected regulators can be exempt from pushbutton control for supplies, such as memory, when it must be kept alive during a standby mode.

The LTC3589 power-management solution provides the features required for portable medical equipment.

The LTC3589 generates up to eight voltage rails for supplying power to the processor core, SDRAM, system memory, PC cards, always-on real time clock, and hard-drive functions. The single inductor, four-switch buck-boost dc-to-dc voltage mode converter generates a user-programmable output voltage rail from 2.5 to 5 V. By taking advantage of a proprietary switching algorithm, the buck-boost converter maintains high efficiency and low noise with input voltages that are above, below, or equal to the required output rail. The buck-boost error amplifier uses a fixed 0.8-V reference and the output voltage is set by an external resistor divider.

By replacing discrete power ICs or traditional large overly integrated PMICs (i.e., with audio codecs, touchscreen interfaces, etc.), a system designer can use a new generation of compact PMICs that integrate key power-management functions for a new level of performance with smaller and simpler solutions. Today’s high performance processors typically have a unique set of power supply requirements, including multiple high-current and low-noise rails, programmable sequencing and dynamic I²C adjustment. These high-end processors were originally developed for handheld applications but are now being implemented in embedded systems in the medical market segment. The latest PMIC enable system designers to exploit the full power saving and performance benefits of the latest microprocessors.

Steve Knoth is a senior product marketing engineer for the Power Products Group at Linear Technology Corp.