[Design Application]
Buck Regulator Answers Portable Challenges
Multiple Voltage Levels Add To An Already Complex Puzzle Of Space, Heat, And Transient-Response Requirements
As portables, like notebook computers, PDAs, and barcode readers get slimmer, lighter, and sexier, the demands on the power electronics continue to rise. Along with the numerous low-voltage supply rails have come brutal, zero-to-maximum-CPU-load transients that characterize power-minimizing, clock-throttling techniques. All this is coming at a time when the shrinking size of notebooks makes waste-heat dissipation a serious problem for system, thermal, and mechanical designers.
For the moment, the classic solution—the standard buck topology—remains the design of choice. Armed with the latest MOSFETs and inductors from the desktop world, the buck topology can tame the waste-heat problem as well as any esoteric configuration. In addition, new controllers, tailored to the demanding CPU loads, allow the hot dc-dc circuit to be placed at least somewhat distant from the already-hot CPU. However, designers can employ a number of additional techniques to mitigate these electrical and thermal problems. Although the techniques used for managing future power needs currently apply to notebook computers, soon they will be used with a broader range of portable and handheld equipment.
Upcoming Power Needs As chipset supply voltages migrate downward, the main power distribution rails in a typical notebook are shifting from 5/3.3 V down to 2.5/1.8 V. These lower voltages, which don't exist in today's systems, power RAMbus, RDRAM, and chipset rails, are scheduled to appear in notebook systems in 1999.
Regardless of what the CPU supplies do, motherboard supplies are becoming more complicated as new voltages are introduced, without getting rid of old power-supply rails first. For example, the move from 3.3 V to 2.5 V for DRAM won't eliminate 3.3 V for a long, long time, due to the legacy of 3.3-V Cardbus cards. At some point, the 12-V rail might disappear, but even that won't happen soon.
Along with dropping voltages, the complexity of notebook power supplies has been evolving at a tremendous rate. Six years ago, we were seeing only one supply rail (5 V), sometimes obtained by powering the entire system directly off a naked stack of four NiCd cells. In 1999, there will be at least seven rails (12, 5, 3.3, 2.5, and 1.8 V, plus VI/O, and VCORE). Amazing!
Topology Choices As more supply rails appear, our job as power-supply designers is to make each dc-dc converter physically smaller while generating less heat. Accomplishing this mandates a complete reexamination of the fundamental power-supply design, products available, and specific tricks and trade-offs used—particularly for the demanding CPU core supply.
Choosing the right switching topology can be vexing. There are a surprising number of choices, each with merits and drawbacks. Sadly, many of the niftier choices from past designs are unusable today. For example, linear regulators, desirable due to their fast transient response, are now forbidden because their efficiency (equal to VOUT/VIN to a first order) has gone downhill as voltages have dropped. At sub-2-V levels, even the most minimal 200 to 300 mV of headroom needed for adequate transient response makes the waste-heat picture bleak. So, linear regulators are out, even when used as post-regulators to improve the transient response of switch-mode supplies.
Coupled-inductor buck regulators, which stack a flyback winding on top of the main buck output to generate a second output almost for free, are tempting as an easy way to make VCORE and VI/O simultaneously. However, the exact value of VCORE is a fast-moving target (a colleague jokingly calls it "voltage du jour"), which could force a painful transformer re-design with every processor upgrade. Most other multiple-output schemes must be rejected for this reason, or due to low efficiency or poor cross-regulation.
Some topology candidates do make it through the initial weeding-out process (see the table). Efficiency is listed two separate ways because of the importance of the impact worst-case waste-heat generation has on thermal design.
Three-Terminal Devices The three-terminal autotransformer buck topology is interesting because it can be employed to adjust the duty factor by changing the turns ratio. This eases the power-dissipation burden on the synchronous (low-side) switch, which otherwise sees greater than 90% duty. Equalizing the duty between high- and low-side switches also helps when designing the pulse-width-modulation (PWM) control loop, because extremely short control-switch on-times are avoided. However, the switch-stress voltage and switching losses are increased—and you have to add an extra termination to the inductor (an expensive redesign when the total series resistance needed for good efficiency is less than 4 mΩ).
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