At the last hour, on the last day of the recent Communications Design Conference (CDC)-where the attendance was already affected by industry doldrums and heightened security concerns-I had an audience. Their presence was a testimony to continuing interest in cell phone power management. My session dealt with the taxes on battery life applied by color displays, Internet access, MPEG audio and advanced RF modulation schemes.
Mobile product planner Jim Schuessler of National Semiconductor talked about the Japanese trend toward spiffing up phones with color LCDs lit by bright white LED backlights. They will use a 4-V forward voltage, which requires a charge pump or boost regulator from the battery. The problem is there is little manufacturing consistency, Schuessler noted, which means a constant current regulation is required to ensure that each LED in a backlight array gives off the same brightness.
StrongARM platform architect Dick Lawrence of Intel showed what his company had learned with the design of Pentiums-that power requirements will spike. But you can assign a priority and count the cycles required to perform some cell phone or pocket PDA task (like playing audio or cruising the Net), and then tailor both the processor core voltage and the clock frequency to perform just that task.
Using their company's experience with GSM phones, applications engineer Kin Shun and product line manager Thomas Szepesi of Analog Devices revisited the controversy over step-up and -down cell phone power regulation. If you use a 3.6-V lithium ion cell to power 3-V logic, the battery voltage will discharge down to a measurable 2.9 or 2.8 V.
Advocates of a buck-boost switching regulator topology believe it would be able to capture residual energy when the battery voltage dropped below 3 V. But this method adds only another 4 percent to cell phone talk time and 7 percent to standby, said Shun. ADI advocated linear low-dropout regulators for low cost and small component count.
Not surprisingly, the linear regulator topology was what Motorola used to power the RF sections of GSM. Applications specialist Curtiss Gong's presentation examined the power envelopes produced by Global System for Mobile Communications, code-division multiple access and time-division multiple access phones.
The session offered an excellent match for speakers and audience interests, and the extended conversations (and exchange of business cards) went on long after the conference had officially ended.
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