Overview

Portable power management has drastically changed in the six months since we last explored it. The most influential players now talk about a package-level process, involving power management systems-on-chip, transferable cores and synthesizable intellectual property (IP)-so much so that parts of this issue will read more like EE Times' "Silicon Engineering" than "Signals."

We knew that adding miniaturized features and functions to the cell phone package-color screens, global-positioning systems, Internet browsers, MP3 players and, now, digital still cameras (DSCs) and video-would tax battery life. The answer to this problem, as our In Focus contributors make clear this week, is not more hardware.

For instance, while the digital still camera's voltage and current requirements will be drastically different from everything else in the cell phone, powering it doesn't mean simply adding another voltage regulator, unless you're intent on magnifying the size, cost and battery power consumption associated with this feature. Rather, the answer involves rethinking power management for the entire phone. This year's cell phone may have a separate regulator. Next year, that regulator will be integrated with other voltage regulators on the same chip-maybe even with the cell phone CPU itself.

So while each contributor will focus on such voltage and current requirements of the cell phone components as the CPU, the color screen, the audio, the camera, RF power section and the USB (or Bluetooth) connection, they are also asking what can be integrated, and how.

National Semiconductor Corp.'s Ravi Ambatipudi, for example, reviews some of the goals set by his company's partnership with ARM cores. The ARM9 core serves as the CPU for many cell phones and handheld devices. The best answer to saving battery life, Ambatipudi writes, is more than a matter of using efficient regulators and/or putting the ARM to sleep when you don't need it. It's more a matter of voltage scaling, in which you assign priorities to the tasks the CPU must perform-and then scale the voltage and clock frequency supplied to the ARM according to those priorities. This requires a voltage regulation-a power delivery system-that's intimately tied, cycle by cycle, to the ARM CPU core. Ultimately, said Ambatipudi, this "system monitor" will not be a separate chip, but rather, specialized IP that will follow the ARM through its various instantiations.

Microchip Technology Inc.'s Bonnie Baker points out that power savings for microcontrollers in battery-powered applications can be obtained by paying attention to the kinds of clocking systems utilized. An internal R/C oscillator will burn less power than an external crystal, though the trade-off is in timing accuracy. Sleep modes and low-power CMOS peripherals also help cut power. While her example uses an 8-bit data acquisition system, her observations can apply to cell phones and PDAs as well.

Of course, the CPU isn't the only device that uses a sleep mode to conserve battery life. In an exclusive online contribution, Motorola Inc.'s Bill Poole describes a "discontinuous-transmit" function that can be used to save RF transmitter power in GSM handsets. The discontinuous-transmit function turns the transmitter on and off during a voice call (that is, when the user is listening and not talking). Similar power savings can be applied to the audio amplifiers of the phone as well as the LCD backlight.

Up to 11 separate power-up/power-down functions can be initiated from a single IC, according to Analog Devices Inc.'s Gary Hurtz and Daryl Sugasawara. In their online piece, they describe the wide variety of sleep modes that could be utilized. "If the user is on an airplane, in a hospital, or in an automated factory environment," they suggested, "the use of a cell phone may cause interference, or may be prohibited. Thus, the phone will power up in the midload standby state, with the RF circuitry disabled." They recommend a battery of low-dropout regulators.

Noise concerns rise

Not only voltages and currents, but also noise requirements dictate the kind of regulators cell phone components will use. Consequently, each phone has many power regulators, says Texas Instruments Inc.'s Jeff Falin. "For example, the RF section requires a power rail with extremely low noise and high power-supply rejection to ensure the highest transmit and receive performance. Therefore, although rather inefficient, a linear regulator, with no output ripple, is the best choice for this rail," he said. "DSP/CPU core voltages, in contrast, have fallen to around 1 V. So, to improve efficiency for this rail, a high-efficiency inductor-based, switching step-down converter is appropriate."

Also online, Maxim Integrated Products' Karl Volk focuses on the consequences of using a separate switching regulator to power the RF power amplifier portion of the cell phone. Like the microprocessor, power amps can utilize power modes that correspond to the probability density function for cell phone use. That is, the probability the cell phone will be needed to take a call-and the distance to the basestation transceiver-correspond to places the user travels. These correspond to urban-voice, suburban-voice and suburban-data modes. Each will have a different power requirement that corresponds to its distance from the basestation and the data rate it utilizes. The cell phone, Volk said, can be adjusted for each location.

Finally, Eric Ogden of Advanced Analogic Technologies Inc. (Analogic-Tech) explains the power requirements for USB On-the-Go, the peripheral interconnect for battery-powered handhelds.

http://www.eet.com/