Electronic Design

Power: Power Management ICs

Power Management Faces A Plate Full Of Tough Issues

A number of challenges lie ahead in the power-management arena. Most daunting may be those revolving around powering next-generation microprocessors. These CPUs will require voltage regulators that can supply extremely high peak currents at sub-1-V voltages with tightly controlled regulation, even in the face of super-fast load transients. Chip makers are already preparing complex multiphase controllers, gate drivers, and MOSFETs to satisfy the new requirements. But the CPU is just one of the tough power customers on the pc board.

New DSPs and ASICs also need more-efficient power-conversion circuits at low voltages. Ongoing innovation in silicon process development, circuit architecture, and packaging help to satisfy these requirements, while shrinking the size of the power supply.

However, in the face of these developments, the downward trend in voltages among DSPs and ASICs is raising the number of supply voltages required, which greatly complicates power-system design. System designers need to figure out the best way to generate the required voltages, either with embedded dc-dc converter designs or plug-in modules. Plus, they must manage a variety of system-level power-management issues, which include staying within the overall power budget, sequencing and tracking of supplies, power monitoring, current sharing, and battery management in portable systems. In some cases, the goal of optimizing individual elements within a power-system design may take a backseat to optimizing the power system as a whole.

For those developing power-management ICs—simple low-dropout linear regulators, complex dc-dc controllers, gate drivers, or any of the supervisory or control functions—the issue of power-system complexity is a key concern. It's right up there with requirements for high efficiency, effective thermal management, small size, and low cost. But for chip makers, all of these issues must be addressed more or less simultaneously.

>EXPECT LCD DISPLAY DRIVERS to move to higher levels of integration and to employ enhanced power management. Also, look for white LED drivers to undergo further development as their use in backlighting color LCDs and keypads increases. Not only will there be multiple displays to backlight in cell phones, there will also be "fun lights" — red-green-blue (RBG) LEDs that flash or scroll behind a keypad or logo. LED driver variety will grow. One chip vendor is integrating white LED drivers on the same chip with the RBG drivers for driving multiple LED backlights, a keypad backlight, and fun lighting.

>CONCERNS OVER POWER-SUPPLY-GENERATED noise will encourage further development of switching ICs characterized for low noise. These components will allow designers to control di/dt and dv/dt slew rates to reduce high-order harmonics. A newer approach to noise reduction is spread-spectrum switching, which was commercialized in Linear Technology's LTC3251 step-down regulator. Using a control scheme akin to direct sequencing in spread-spectrum communications, this regulator modulates the switching frequency with a pseudorandom number generator. That spreads the switching noise across a wide range of frequencies, cutting down on the pronounced spikes that are a source of EMI.

>LOW-DROPOUT REGULATORS (LDOs) developed in submicron CMOS processes will offer sub-100-mV dropout in the 150-mA current range. Compare that against existing LDOs with 300 mV of dropout. The improved performance will meet demands for simple but efficient voltage conversion in cases where designers require an additional low-current supply at a voltage just below that of another supply.

>IN THE WIRELESS HANDSET AREA, there may be a retreat from the trend to integrate power conversion circuitry onto the baseband processors and DSPs as these chips move to smaller process geometries. That trend will reflect the lower-voltage levels that are tolerable with the newer signal-processing chips. As a result, vendors may elect to implement functions like battery charging on a separate chip.

>NEXT-GENERATION MICROPROCESSORS will impose some daunting power-supply requirements. Anticipated specifications include load transients approaching 100 A, transient response on the order of 500 A/ms, and a dynamically changing supply voltage at or near 1 V with ±25 mV of regulation. Meeting these requirements in a synchronous rectified buck converter will take the development of multiphase converters with as many as six to eight phases. Such a complex converter design will probably spur on innovation in controllers, gate drivers, and MOSFETs. Changes in converter architecture and device packaging are likely.

>AS POWER SYSTEMS BECOME VERY COMPLEX with many voltages present in the application, the emphasis will shift from merely optimizing the performance of individual voltage regulators to optimizing the system performance as a whole. This shift will force a top-down approach to power-system design, in which various power-related issues must be addressed early on in system design. IC manufacturers will be challenged to craft power-management ICs and chip sets that help systems designers deal with these issues simply and with flexibility.

>SETPOINT ACCURACY for buck converters is improving thanks to better voltage references in dc-dc controllers. According to the Power Supply Manufacturers Association, the accuracy is now 1.6% or better after being at 2% in earlier standard buck converters. Expect further success in reducing this error, particularly in buck converters designed to meet next-generation CPU power requirements. To meet the ±25-mV regulation spec, voltage references will need 0.5% accuracy or better.

>BETTER GATE-DRIVE TECHNIQUES will up the efficiency of synchronous rectified dc-dc converters. For example, Texas Instruments' predictive gate drive more accurately adjusts the delays (deadtime) in the switching cycle than existing adaptive gate-drive techniques. With predictive gate drive, a controller like the TPS40000 uses information from the current switching cycle to adjust the switching delays of the next cycle. This reduces losses associated with body diode conduction, cutting losses in the MOSFET switches by 20% to 40%.

>SEMICONDUCTOR MANUFACTURERS will use ordinary and exotic approaches to create multiphase controllers with six phases or more. Because of the high level of complexity involved, single-chip analog-based control solutions (controller plus gate drivers on one die) seem unlikely. A more probable but still challenging approach will rely on a complex multiphase controller with separate gate driver ICs and MOSFETs. A novel approach from International Rectifier will repartition controller and gate driver functions into a scalable multiphase design that uses silicon efficiently. A digital control technique from Primarion and Intersil promises highly accurate load line control, a scalable design, and a flexible layout that exploits noise-immune digital communication between the controller and the power stages.

>MORE AND MORE POWER ICs are packaged with an exposed die attach pad to remove heat from the package. One such example, the QFN (quad flatpack no-leads), is becoming an increasingly popular alternative to the more traditional surface-mount packages like the TSOPs. Aside from the die attach pad on the underside, all of the chip's I/Os appear as pads underneath the package, rather than as protruding leads. This type of construction also allows the package to hold a slightly larger die than the TSOP. The QFN's sub-1-mm profile is another benefit. Expect more dc-dc controllers and other devices introduced in the QFN format.

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