Digital power is advancing on a broad front, even as IC makers struggle to balance its purpose and scope against marketing hype to take a still ill-defined area "to the next level." With the defacto definition of digital power leaning towards more of a connection with power conversion, and less with power management (still its most widely used connotation), we're seeing more chips directly associated with digital feedback loop compensation in high-efficiency DC/DC power converters, versus traditional logic interfaces for controlling the sequencing and margining of a multiple-output supply.
As such, more chips are some combination of the two. Indeed, a slew of such chips, with wide gradation, have arrived to make a legitimate claim on expanded power conversion-with-power management/control functionality. And that's as it should be, as designers come to understand that digital power isn't bringing faster or more accurate response to power conversion or control over analog designs, as first touted; the digital process always adds some delay and quantization error. Much of digital power's value is tied in with doing away with the need to oversize components to account for parameter variation, eliminating concerns about component drift and temperature compensation, and minimizing the tweaking process in production. Thus, more digital power chips with a strong "logic" component will continue to reach deep into traditional control applications, with the most highly billed application being the universal control of both analog and "digital" supplies.
Conversion with management
At the systems level, Primarion (Torrance, Calif.) says its PX7520 is the industry's first dual-phase programmable digital-power conversion and power management IC. It touts a true digital control loop and active voltage positioning; real-time current, and input-voltage sense circuitry; internal voltage and temperature referencing; and sequencing and margining circuitry. Its I2C PMBus interface facilitates control and monitoring.
With mid-level system integration in mind, Zilker Labs' (Austin, Tex.) ZL2105 claims the most power management facility for 3-amp DC/DC designs. The state-machine based device combines a synchronous step-down controller with drivers and digital feedback compensation loop; voltage tracking, sequencing, and margining blocks; voltage, current, and temperature monitoring; and fault detection circuitry in a 6-by-6 mm package. "Customers can combine (our) multiple Digital DC devices on a given PCB and easily configure the parts to simplify a complicated power design," said Jim Templeton, vice president of marketing, stressing its ease of use as the key differentiator to competing products. "Ours are the only solutions to offer this 'plug and play' experience for board-level power."
Microcontrollers are also evolving to make a virtually seamless transition from analog to digital for systems of moderate complexity. Here, it's the DSP versus the microcontroller, with proponents on both sides. The dsPIC family of Digital Signal Controllers from Microchip (Chandler, Ariz.) is designed to maintain the generality and flexibility of a more expensive DSP solution (versus a dedicated controller) without the accompanying high cost. "Power supplies with higher power or complexity are now poised at a price threshold that is amenable to digital control," said Sumit Mitra, vice president of the Digital Signal Controller Division. "These devices accelerate innovation by giving early adopters newfound flexibility to create new topologies that were formerly impractical for analog approaches." With the aforementioned advantages of digital control in mind, the company's dsPICs blend the control advantages of a high-performance 16-bit microcontroller and the relatively high computation speed (in systems of moderate complexity) of a fully implemented DSP.
Indeed, cost is still a big design consideration, even for operations requiring a basic DSP solution. Targeted initially as a cost-effective solution for AC/DC rectifier and (DC/AC) inverter applications, Texas Instruments (Houston) has added to its TMS line of DSP controllers. TI also advanced a digital power development kit, consisting of a TMS320C200 software library and a series of hardware modules that provide off-the-shelf platforms. Yet, recognizing that the majority of today's digital power requirements fall far short of the capability of TI's DSP portfolio, the company may soon provide viable alternatives.
Chips at the circuit level for supporting the basic feedback loop include several with spot functionality as a viable alternative to optocouplers in basic power conversion. Some of these designs, which include substantial integration, include iWatt's (Los Gatos, Calif.) iW1688. It's designed to secure the benefits of digital control as applied to the primary side of low-power AC/DC adapters and chargers. Traditional solutions using analog (DC feedback) methods in the primary loop create problems with regulation, noted George Hill, director of marketing. Indeed, up to this time securing tight supply regulation required secondary feedback loops that add both components and create a concern as to the overall reliability of the optocoupler, he notes. Using what the company says is a unique combination of sampling techniques and an efficient digital engine, the iW1688 is designed to achieve tight regulation and a smaller, less expensive and more efficient implementation. Another component citing the "digital" connection, Silicon Labs' Si844x digital isolator serves as an alternative to competing optocoupler solutions for connecting the primary and secondary sides of an AC/DC or DC/DC supply. Working as such in the feedback loop with the company's Si8250 digital switchmode power controller (which has an on-chip hardware loop filter DSP engine), it's half again as fast as existing digital isolators, according to the company.
Power management and control
A variety of devices have also arrived that, for lack of a better descriptor, are aligned a bit more towards the system-control end of digital power, with some measure of digital power conversion circuitry. As appropriate to the control aspect, many tout their adaptability to analog and digital systems. The more notable products in the power-block integration category include Power-One's ZM7300 series of Z-one Digital IBA controllers, the first claimed to manage both analog-based systems (VRMs, linear regulator, POLs) as well as digital POLs. "The ZM7300 extends the open-architecture applications to include supporting analog devices, as well as industry-standard I2C digital interfaces," said Steve Goldman, chairman and CEO. These chips are suited to setting output voltage, tracking and sequencing, and under-voltage, over-voltage, and over-current limits.
Among the popular chip makers, National's (Santa Clara, Calif.) LP5550 is billed as the first digitally-controlled energy management unit (EMU) for handheld consumer products that implements the company's PowerWise architecture to facilitate adaptive voltage scaling (usually touted as a digital power conversion function) and boost power conversion efficiency. The chip, which works with a digital processor to adaptively adjust its supply voltage to the minimum required for the application, includes an adaptive buck regulator for the processor core, and three fixed voltage regulators. These regulators power the input/output ring, oscillator/phase locked loop and memory in the portable device's system on-chip (SoC) device. The PowerWise technology removes voltage margins associated with processor, temperature, IR-drop and regulator tolerances.
Maxim's (Sunnyvale, Calif.) MAX8688 fine-sets output voltage, sequencing, tracking, margining, and monitoring the status of a power supply. Here, the chip taps into the enable, feedback, and reference inputs of the existing analog power supply. The MAX6688 closes a slow loop around the power supply to fine tune the supply's output to ±0.2 percent of the desired output voltage. From the control standpoint, the chip also monitors supply current and on-board temperature using a 12-bit A/D converter to set multiple warning and fault thresholds.
One control chip that offers basic digital control of an analog power supply for production test environments is Linear Technology's LTC2970, a dual I2C power supply monitor and margining controller. The chip reads back output voltage and current, sets precise output voltage with appropriate margins, and can also set under/over voltage and under/over current limits.
The total power supply
At the modular level, "digital power supplies" are also starting to make their way to market, again with "digital" invoking various connotation. Emerson Network Power (Carlsbad, Calif.) touts its Astec DTX as the industry's first all-digital DC/DC converter. Its eighth-brick sized DTX42K48, the first product as part of the company's Digital Power Initiative that implies dynamic feedback loop compensation and configuration as well as self-diagnostics, efficiency optimization, and output impedance control, delivers up to 50 watts at 0.96 to 1.44 volts from a nominal 48-volt input. "Our devices will be able to connect to virtually any I2C-based power control and monitoring system," said Geof Potter, vice president of DC/DC advanced technology, "but the communication bus is not required for operation. These devices will operated in 'stand alone' as well." The supplies will be available in limited quantity in November.
Power-One's (Camarillo, Calif.) No-Bus point-of-load (POL) converters, which is said to integrate digital power conversion and intelligent power management, include the 7-amp ZY1207 , 15-amp ZY1115, and 20-amp ZY1120. Here, the POL eliminates external components for sequencing, tracking, protection, and monitoring; users set performance parameters by pin strapping and external resistors and capacitors and no bus is required. These converters are promoted as both more efficient as well as cost-effective alternatives to traditional analog POLs for low-to medium complexity applications.
Other supplies include Silicon Lab's single-phase Si825x POL reference design to facilitate quick implementation for digitally-controlled converters for servers, telecom, datacom and storage systems, medical equipment, and aeronautical systems. This 20-amp, 100-watt POL is based on the company's Si8252 digital power controller. The reference design comes with a GUI-based Application Builder that initializes the switch timing, loop compensation filter and processor set-up without the need for the designer to write applications software. A separate development kit product, the Si8250DK, is also available.