Power-Supply ICs Go Multiphase To Take On 100-A Loads

After a bumpy start, monolithic multiphase pulse-width-modulate (PWM) controllers have made significant strides in cost and performance. They're being rapidly accepted in fast-responding dc-dc converters that power new-generation multigigahertz microprocessors in desktops and workstations/servers. Innovative digital techniques are being implemented to leapfrog existing analog methods by several orders of magnitude in transient response.

As faster processors pervade notebooks, and CPU load requirements climb over 30 A, multiphase solutions also are being extended to mobile computing systems. A new breed of multiphase synchronous PWM controllers ensures that the core voltage of multigigahertz processors will remain within specified guidelines. This is even so when the processor performs a current-load step from a few milliamperes to over 50 A in less than a microsecond.

Multiphase converters equally distribute the total current across phase-shifted PWM channels and associated output MOSFETs and inductors. Consequently, they efficiently spread heat and lower stress on components, especially now that current requirements are surging toward 100 A at voltages below 1 V. Besides relaxing the power specifications, they also operate at higher frequencies to allow the use of miniature passives, resulting in higher volumetric efficiency.

As current requirements rise, the need for more phases per chip grows. A rule of thumb is that 15 to 20 A must be handled per phase. With current processor demands approaching 100 A at low voltages, suppliers have developed solutions with up to 4 phases on-chip. Dual-phase solutions have adequately served earlier-generation processors, but new and future breeds are asking for 3, 4, and more phases per chip.

In response, some suppliers are readying 6- and even 8-phase monolithic controller architectures for servers and workstations. Depending on power requirements, others have developed techniques to cascade single-chip dual-phase controllers to attain quad and even more phases in a converter.

Multiphase controllers also are incorporating clever feedback mechanisms to provide ultra-fast recovery times during steep step increases of load current. They can furnish slew rates as high as 80 A/ns to cope with the rigorous requirements set by Intel's 9.X specifications for its latest voltage regulator module (VRM) for desktops and workstations. And, Intel has created VR-Down specs for embedding voltage regulators on the same plane as the processor.

Additionally, controllers are helping notebook power-supply designers to comply with the latest Intel mobile voltage positioning (IMVP) standards. Version IV is the newest specification, while version V is in progress.

Like everything else in the IC world, greater integration is a key trend for controllers. Some are bringing drivers and MOSFETs on-chip, although not everyone agrees with more integration, especially as requirements surge beyond 30 A. Some argue that more integration means less design flexibility.

Novel Topologies: Realizing the limitations and drawbacks of earlier introductions, pioneer Semtech continues to modify its architecture and broaden its product portfolio (see "New-Generation Power Controllers Take Multiphase Route," electronic design, Oct. 28, 1999, p. 77). Semtech has made a smooth transition from asynchronous to synchronous techniques, and voltage-mode to current-mode sensing with active droop control, for multiphase PWM solutions.

Semtech has also developed multiphase solutions based on a hysteretic-mode for dc-dc converters deployed in the latest notebooks that comply with the IMVP specifications. The company has just released solutions for upcoming IMVP IV specifications and is ready to address the requirements of forthcoming IMVP V.

Semtech's charge integration method overcomes the shortcomings of existing input-current sensing and peak-current mode techniques employing a sense resistor (Fig. 1). While this patent-pending topology uses a sense resistor, it performs charge integration on internal capacitors, instead of looking at the peak current.

"It eliminates all noise and layout sensitivity of an input current-sensing scheme," says Ken Boyden, Semtech's director of computing systems. "Besides simplifying the layout, charge integration permits overlapping of phases, which was impossible with a peak-current mode technique."

This topology offers an important benefit. Overlapping can exist between the conduction time of different phases because the internal ramp generator can resolve the superimposed currents, explains Mehrzad Koohian, senior applications engineer for Semtech's multiphase products. "Consequently, it allows duty cycles of greater than 50% for 2-phase and over 33% for 3-phase converters using a single current-sense resistor. Thus, by extending the on time, this feature significantly improves transient response time," he explains.

As it implements filtering, charge integration eliminates external filtering or leading-edge blanking of the current signal. Noise and ringing are integrated over the duration of the on-time pulse. So this topology is inherently more immune than peak-current sensing to parasitic inductance introduced by imperfect layouts. It simplifies pc-board layout. Semtech's latest multiphase PWM controllers with charge integration include the SC2432 for Intel CPUs and the SC2436 for AMD CPUs.

For applications that don't require the cost and losses of a sense resistor, Semtech has crafted yet another clever technique. The company maintains that Combi-Sense, an accurate peak-current mode without a sense resistor, is the ultimate solution. Earlier attempts to implement this technique via inductor current or R_DS(ON) sensing weren't fruitful.

In inductor current sensing, the signal generated is extremely small, causing noise susceptibility and jittery operation. Plus, the loop gain and droop are subject to large changes over temperature and lot variations. "Meeting the load-line requirements with this topology is extremely difficult, and often requires additional compensation circuitry to make it work," asserts Boyden.

Likewise, the MOSFET's switching action complicates accurate measurement with R_DS(ON) sensing. According to Boyden, Combi-Sensing overcomes these problems. It offers four advantages over present methods. First, it provides an exact mirror image of the current in the output inductance. Combi-Sensing also senses current on the main three power devices, top and bottom MOSFETs and output inductance, as well as averages the information over the full period to curb noise. Lastly, it generates a high-level clean sawtooth that can be directly used in peak current-mode control.

Multiphase PWM controllers implementing Combi-Sensing are expected this summer. Aimed at desktop applications, initial introductions will offer two or three phases per chip.

Fairchild Semiconductor is evaluating newer topologies, like digital implementations, for future applications. Plus, it continues to tap the benefits of valley current-mode control based on leading-edge modulation (Fig. 2). An improvement over conventional peak current-mode control, "valley current-mode control offers superior regulation and transient response," states Nazzareno Rosetti, Fairchild's director of strategic product planning for the Analog and Mixed-Signal Products Group. Fairchild says its approach achieves a tenfold improvement in speed over conventional methods. It can stretch operating frequency up to 10 MHz/phase.

Valley current-mode control can meet and exceed VRM specifications. The latest member using this technique is the FAN5002 2- to 4-phase programmable interleaved buck controller that meets VRM 10 specifications for Pentium 4 processors. With an external MOSFET driver, it can drive in excess of 150 A and achieve a 100-ns loop response to transients. Fairchild also is looking at both a monolithic 6-phase controller and three dual-phase chips for future designs.

Scalable Master/Slave Approach: STMicroelectronics has developed a family of scalable interleaved multiphase controllers that comply with Intel's VRM 9.1 specifications. A soon-to-be-released L6919/L6919A IC handles up to eight phases from a single chip using a master/slave configuration. "A novel current-sharing bus provides active and accurate current sharing between different phases," says Edward H. Friedman, product marketing manager at STMicroelectronics. "It permits the use of additional controllers if needed."

The novel architecture has a number of other features. These include slave-IC auto detection, dynamic voltage identification, fully differential current and remote sensing with an integrated buffer, an externally programmable droop function, a "power good" output and enable function, and protection against over/undervoltage and current.

Using a similar master/slave configuration, STMicroelectronics offers another interleaved multiphase controller that complies with the VRM 9.0 standard and handles up to 100 A. With up to 6 phases per chip, the L6918A/L6918 controller comes with integrated drivers and provides complete control logic and protection for high-performance 4- or 6-phase synchronous step-down dc-dc converters.

Unlike others, STMicroelectronics has integrated drivers on-chip to enable compact VRMs. The chip comes with an integrated 5-bit digital-to-analog converter (DAC) for output voltage adjustment to within ±0.8% accuracy. The droop function guarantees that current sharing between the two phases of each device will remain within ±10% over static and dynamic conditions.

For processors requiring up to 50-A currents, STMicroelectronics has crafted dual-phase versions (the L6917 and L6917A) with integrated drivers. Each includes a 300-kHz free-running oscillator that's adjustable up to 1 MHz. Aimed at servers and workstations, the L6917 is compatible with the VRM 9.0 specifications. The L6917A is geared toward VRM 8.5 specifications.

All L691x devices employ current-mode control and are made on the company's smart-power BCD5 process. It has no plans at present to provide controllers for notebooks.

To serve new-generation Intel Pentium 4 and AMD Athlon processors, Analog Devices Inc. (ADI) and ON Semiconductor have also taken the quad-phase route. Both are investigating methods to further increase the number of phases per chip and drive the switching frequency upward.

ADI has prepared a highly efficient programmable 4-phase synchronous buck controller (the ADP3164) for converting a 12-V main supply into the core voltage required by the latest multigigahertz processors (Fig. 3). In this scheme, the four output phases share the dc output current to reduce overall output-voltage ripple.

To ensure equal sharing of the total load current among all phases, under all conditions, ADI's active current balancing uses a comparator and a single sense resistor. "It forces the peak current in each phase to be the same, and minimizes all peak current errors," says Doyle Slack, marketing manager for ADI's desktop power products.

To enhance the load transient response, the AD3164 employs active voltage positioning known as ADOPT. Unlike standard voltage-mode or current-mode designs, ADOPT adjusts the output voltage as a function of load current, ensuring that it's always optimally positioned for a system transient.

ADI also is pursuing improvements to its existing architecture, for a higher number of phases on-chip, and almost double the operating frequency per phase. Presently, the four-phase solution offers up to 500 kHz per phase.

For notebooks, the company is introducing a three-phase controller that complies with IMVP III specifications and is backward-compatible with IMVP II specifications. It combines hysteretic peak-to-peak current control with ADOPT optimal compensation to achieve excellent current sharing and very high load-transient response.

"A key feature of the 3-phase ADP3204 is pin-selectable 1-, 2-, or 3-phase operation," says Thomas Szepesi, ADI's product marketing manager. "When used in a 3-phase configuration, it can deliver up to 50 A." ADI has also released an accompanying MOSFET driver for its 3-phase part.

Inductor Current Sensing: Unlike many others, ON Semiconductor prefers inductor current sensing over a sense resistor and R_DS(ON) sensing. "Inductive sensing eliminates extra components. It's a loss-less technique that's less sensitive to temperature variations compared with R_DS(ON) sensing," notes Leslie Logan, analog marketing manager at ON Semiconductor. The company has combined inductive sensing with proprietary V² feedback control to move up the multiphase scale. Aiming at embedded voltage regulator applications complying with VR-Down specs for Pentium 4 processors, ON Semiconductor has released the CS5307, a 4-phase chip.

The CS5307 is made on a new high-speed bipolar process, on which 6- and 3-phase versions will be built. Higher integration also is on the drawing board with the next generation of multiphase controllers, including on-chip gate drivers and MOSFETs. Presently, ON Semiconductor offers 2-phase chips with integrated drivers.

While most companies have expanded into the notebook arena with multiphase solutions, Linear Technology set its sights on mobile applications from the start. It has developed a current-mode polyphase scheme that forms the basis of its biphase designs. For applications that need more than two phases, the polyphase controllers can be easily cascaded to achieve up to 12 phases.

"By operating at higher frequencies per phase and integrating the MOSFET drivers with the controller, the polyphase solution offers the smallest circuit profile and footprint, and reduced capacitance on both the input and output," says Tony Armstrong, Linear Tech's product marketing manager. "To improve thermal management, it offers accurate current sharing on each phase and faster transient response."

Linear Technology continues to refine its products by doubling the frequency/phase and tightening tolerances over predecessor products without compromising conversion ef- ficiency and load step response. For instance, one of the latest members, the LTC3728, offers 550 kHz/phase.

The rising demands of upcoming notebook processors and tighter IMVP III and IV specifications will soon force Linear Technology to go to 3-phase versions. It will release several parts in this range by month's end, starting with the LTC3732. This 3-phase synchronous step-down controller drives all N-channel external power MOSFETs in a phase-lockable fixed-frequency architecture, offering up to 600 kHz per phase. A unique feature is the choice of output stage shedding or Burst mode, to optimize light load efficiency.

More Companies Join In: International Rectifier, Microsemi, National Semiconductor, Maxim Integrated Products, and Texas Instruments all plan to introduce multiphase voltage controllers. Eying mobile systems, National Semiconductor is preparing both 4- and 2-phase hysteretic controllers.

"Current sharing or current balancing is a critical performance consideration," says Maria Laughlin, marketing manager for National's power-management ICs. "This is critical at the maximum current load for the system designer who wants to use optimal ratings for inductors, current-sense resistors, and FETs. Balanced current sharing avoids having to use larger components, which impact the total-solution size and cost."

Newcomer Microsemi, though, has developed a proprietary load-sharing scheme to accomplish accurate forced and programmable current sharing. In this patent-pending scheme, voltages on the primary side of each phase are matched to attain an accuracy of better than 10%. Its multiphase controller can realize dc-dc converters that conform to IMVP IV specs.

Intersil and Primarion are jointly developing controllers to meet the demands of advanced microprocessors, which will approach 10-GHz clock speeds in a few years. "Together, we will enable a new class of multigigahertz power-management solutions with digital control to provide new ways of delivering power to future processors," says Steve Rivet, Intersil's vice president of marketing.

The first fruit of this joint effort, a digitally programmable controller, can furnish up to eight phases per chip. Available from both Intersil and Primarion, it will target VRM 9.X specifications, with VRM 10.0 to follow.

While others ready digital control to multiphase solutions, Volterra Semiconductor has taken the lead. Implementing its proprietary digital technique, it has developed multiphase regulators for both VR-Down and VRM specifications.