Under the Hood of a Tablet PC: The analog side of the story

In a previous article, we looked at the insides of a fresh approach to the portable PC market (see www.eetimes.com/sys/uth/OEG20031201S0076). Seeking a more natural format for on-the-go computing with a slate-like appliance, the HP/Compaq TC1000 still allows docking to a traditional keyboard for a standard a notebook use model.

Like most mobile computers, the discussion is often focused on CPUs, Memory, Graphics capability, and other digital attributes of the system. In a power-hungry environment however, these islands of digital capability must be served by an array of analog content to both regulate locally and convert battery pack supply to a mix of voltage levels often unique to each major device.

Since power conversion efficiency is of the essence and high current levels are the norm, a host of DC-DC converters are used. Despite added costs, performance issues dictate the use of switch-mode supply design versus cheaper—but lossy— shunt or series regulators. DC-DC controllers from Maxim and Linear Technologies (#MAX1715, #MAX1718, and #LTC1628) are teamed with Vishay Siliconix power FETs to deliver the often high-current demands of the NeoMagic graphics processor, Transmeta CPU, and VIA Southbridge chip.

In modern mobile computing design, these DC-DC converters must also deal with an often-dynamic voltage level requirement as compute horsepower—and thus device supply voltage—are scaled back during times of low demand. Of course other more unavoidable system elements such as the LCD and hard disk contribute to the roughly 20W peak power demands of the TC1000, but designers must do what they can, where they can, to save every bit of juice.

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Price premiums may contribute to the uphill battle facing Tablet PCs, but the familiar "pad-of-paper" product format makes for a potentially important product category. As with all mobile computing products, the design wouldn't even get off the ground without help from the analog world.

David Carey is President of Portelligent. The Austin, Texas company produces teardown reports and related industry research on Wireless, Mobile, and Personal Electronics. (www.teardown.com)