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 RDS(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 RDS(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.