Ready or Not, Here Comes Digital

Good Hybrid: A reference design for powering a fluorescent light using Fairchild Semiconductor's FMS7401 requires only three ICs and provides medium to high performance with programmability and a high degree of flexibility.

The first Digital Power Forum (DPF), Sept. 13-15 in San Jose, CA, created a compelling event to announce new products for companies that had been working on digital power control. At the DPF, Primarion, a mixed signal IC company that is addressing the transition from analog power to digital power, announced a digital control chipset for core power regulation in high-performance computing, graphics, and networking applications. The processors in these systems have operating voltages between 1.1 and 1.3V, and require supply regulation of 40 mV. With increasingly lower voltages and higher currents forecast for high-performance Intel and AMD processors, new design approaches using digital technology are expected to play a critical role for providing power for these logic chips.

Versatile integration

Primarion's PX3535 digital multiphase controller manages two to six phases, and the PX3536 handles from two to four phases. The multiphase approach is also used in analog designs to distribute the total current and minimize stress on individual components, such as MOSFETs and inductors. Integrated analog-to-digital converters (ADC) in the PX3535 and PX3536 transition voltage, current, and temperature to the digital domain for improved control. All of the internal registers for the digital controller are accessible through an industry standard I2C serial bus for customizing the protection features.

Operating at frequencies up to 1 MHz, the PX3520 power stage portion of the chipset integrates the low- and high-side MOSFET drivers and a control (P-Channel) MOSFET into a chip scale package delivering up to 30A per phase. Process and temperature independent loss less current sensing designed in the chip allows accurate loadlines and overcurrent protection. Integrated temperature sensing works in conjunction with the PX3535 controller, providing accurate programmable thermal shutdown protection.

The chipset has demonstrated 85 percent efficiency at 120A load current (four-phase) in a 1U (1 × 4 inches) voltage regulator module using a 2,000 µF capacitor. At 105A, the peak-to-peak ripple voltage was only 14.4 mV. The combination reportedly produces the highest power density for server and workstation buck regulators.

"These are some of the first announcements," says Jeff Shepard, president of Darnell Group and organizer of the DPF. "Those companies that did not see digital power coming are starting to feel they could be left behind."

The digital multiphase power ICs that are made by Primarion are second-sourced by Intersil to accelerate acceptance and adoption of the digital multiphase architecture. Intersil's products are the ISL6591 digital multiphase controller and ISL6597 integrated power stage.

More digital power solutions

Other companies showing digital power solutions at the DPF included Texas Instruments (TI), ST Microelectronics (STM), Fairchild Semiconductor, and Power-One.

TI showed several digital solutions including a digitally assisted power supply design made possible by an advanced analog controller that is not yet introduced. This approach, dubbed a DPPA interface for digital power partitioning architecture, combines the microcontroller (MCU) and a digital-compatible analog controller that adds enhanced control features. The analog controller drives the power stage and receives bias feedback information from the power stage for the MCU. The MCU receives temperature voltage and current limit flag information, and communicates with external circuitry while clocking the analog controller. Using an SMBus for communication, digital aspects that can be implemented include programming, configuration, adjustments, test and calibration, and remote status reporting and polling.

STM presented a monolithic IC design using asynchronous pulse width modulated (PWM) logic for digitally controlling voltage regulator modules. The design avoids the digital PWM approach that typically requires high sampling frequency and high resolution in both analog-to-digital and digital-to-analog converters to reduce quantization effects that include noise and higher output ripple. Instead, a PWM signal is generated by comparing the load current, Il, and output voltage, Vout, with two reference levels. The reference levels are generated by the digital controller and managed by a control algorithm. Integral digital to analog converters (DACs) provide analog voltage and current levels for the integrated comparators.

Some companies took advantage of the digital situation to reiterate previously introduced digital power solutions.

Multi-Approach: Primarion's digital power chip set that consists of a PX3535 digital multiphase controller and PX3520 power stage (top and back) can provide 30A per phase in a small form factor.

Fairchild discussed their FMS7401L, a hybrid digital power controller that integrates several analog functions with an 8-bit microcontroller core. The combination provides ease of analog interface with digital programmability. The FMS7401 has typical MCU peripherals including 1 kbyte of on-board code EEPROM, 64-byte data EEPROM, and 64-byte SRAM. Application-specific features for digital power control are a fast 12-bit PWM timer with dead time control and half-bridge output drive, and a 5-Ch 8-bit analog to digital converter. The analog portion of the hybrid includes an uncommitted amplifier, temperature sensors, auto-zero amplifier, and current source for interfacing to optocouplers for isolated power supplies. Fairchild's comparison of analog, digital (based on DSP technology), and the hybrid approach shows the hybrid to be a good compromise.

Power-One displayed and discussed their Z-One Digital Intermediate Bus Architecture (IBA) that provides a 50-percent reduction in printed circuit board (PCB) space, 20-percent cost savings, a 90-percent decrease in components, and reduced power-system development time.

The logistic and performance improvements from the digital IBA result from two companion chips: a Digital Power Manager (DPM) and a Z-POL converter. A Z-One Digital System has a special graphical user interface (GUI) to easily set Z-POL parameters including output voltage, sequencing, tracking, and protection type.

For those who want to see what happens next, the second Digital Power Forum is already scheduled for Oct. 3-5, 2005, in Boston.