RF "sleep modes" can improve battery life in wireless phones

Just a few years ago a cell phone had a few hours of standby time and maybe 60 minutes of talk time. Today we expect days of standby time and hours of talk time. It is through techniques collectively known as "power management" that we have attained such talk and standby times in today's products.

Today's digital cellular phone is highly integrated and highly complex. It has a microprocessor to control the function and the display, and a DSP to manipulate the signal after it is converted from RF. These two functions might be in the same IC. The phone will also have an RF circuit that contains receive and transmit sections. It has a transmit power amplifier, and audio amplifiers to drive the speaker and ringer. It can also have numerous other features such as lights and a camera. Linking all of that to the battery will be power management circuitry. In all likelihood most of the power management components will be in a dedicated IC. Other handheld devices such as PDAs will have a similarly complex array of internal components.

In older products, such as an early cell phone, the various circuit blocks would be turned on and left on continuously. Modern cell phones have even more complex circuitry. Simply leaving the receiver, audio and processor running while waiting for a call would quickly use up the battery's charge.

For many products, there will be times when the device is "on" but not in active use. The cell phone in "standby" is the most notable example of this. The user expects it to be "on" waiting to receive a call as it hangs on his belt. But in fact it is doing nothing most of the time.

The fact that the phone has nothing to do most of the time can be used to save battery life. The processor in the phone simply turns almost all the functionality of the phone off most of the time. It carefully times the instances when it comes to life, to check for paging signals from the base stations, to verify that it is still within range, to switch to a new cell if need be, and to check for incoming calls. Since the caller's patience may be limited, the notification of an incoming call won't be valid for long. While leaving the receiver off for a long time will save battery life, turning it on often to listen for incoming calls will decrease the likelihood of missing the incoming call.

The GSM system has such a scheme called DRx, Discontinuous Receive (DRx). The handset can sleep and only wake up every few "multiframes" — a time period of about an eighth of a second). The system can direct the phone to wake up and check for calls about 4 times per second, (DRx2) or about once every second (DRx9). The tradeoff is how long it will take to detect an incoming call. In DRx operation, the processor will shut off the receiver and put itself into a low power sleep mode. An internal timer will turn the processor back on after the proper sleep time.

The wake-up sequence can actually be fairly complex. After the processor has awakened, it must turn on the DC voltage to the RF circuitry. First it turns on and tunes the synthesizer to give it a chance to stabilize, then it turns on the analog amplifier portions of the receiver and directs them to perform their self calibration routines. The antenna switch is switched to "receive" and the RF front end is turned on. The DSP is enabled and starts converting the burst of data that is received. As soon as the data is received, the RF and analog portions of the receiver are shut down, while the DSP completes decoding the received data and the processor determines what to do with it. Unless it needs to act on that data, the processor will then put itself to sleep until the next time. This is shown in the timing diagram below.

One could simply turn everything on far enough in advance of the receive burst to allow the synthesizer to tune and settle and allow the DC adapt routines to occur, but by staggering the turn on times of the various circuits so that they come on just when they are needed, tens of milliamps are saved for a few microseconds. The net result may be as much as 10 percent savings on the average current drain in standby, which of course translates into 10 percent longer standby time, perhaps giving a competitive advantage for your product.

During a phone call with a GSM phone, the timing of the RF circuitry is similar, except the receive bursts occur far more often, about once every 4.6mS, and the receive bursts are interspersed with transmit bursts. With the time slots that GSM uses, the receiver and transmitter RF as well as analog sections are each turned on about one-eighth of the time. Obviously the power amplifier during transmit has the biggest current draw, this varies as RF output power is dynamically controlled to just enough signal to reach the base station Additionally, a GSM feature called Discontinuous Transmit, DTx allows the transmitter to skip transmit pulses when the user is not actually speaking. This further enhances battery life, since almost anytime someone is using a phone 50 percent of the call is listening and not talking. Periodic transmit pulses are still required so the network can recognize that the phone is still in range and in the call.

Balancing on/off functions

During an actual phone call, even though the RF portion can be turned on and off, most of the rest of the phone, notably the audio circuitry, is turned on throughout the call. The microphone bias, amplification and digitization path must be running continuously because the user could speak at any time. The receive audio to the speaker can be turned on and off since it is only needed when receive data contains a voice that must be fed to the user. The analog part of the audio amplifier will likely be left on continuously to avoid "pops" or "clicks" associated with turning the DC on and off, and to amplify the "comfort noise" in the background that gives the user a sense that he is still in a call. But the processing of received digital data into an analog voice need not occur when the party at the far end is not speaking, so that portion of the DSP can be slowed down or partially disabled to save power.

Other power consuming features such as the backlight or the flashing multicolored backlit keyboard that lights up the user's face will be on at the pleasure of the user, and while the LED drivers and voltage boosting DC-DC converter will be designed for maximum efficiency, the current to light the LEDs will typically not be included in current measurements used to estimate talk time.

Typically most of these features will be controlled on and off. Commands such as DC offset adapting and the frequency information to the synthesizer will be sent over a SPI bus at a very high rate. By integrating the power regulators, audio and other power management functions in a single IC, the number of chip-enable pins is minimized and multiple functions can be controlled with a single command.

Another popular battery powered device is the Personal Digital Assistant, or PDA. Like phones, PDAs are powered by a battery and long battery life is a desirable market feature. While not as obvious, like the phone, a PDA is really only working a fraction of the time. If the PDA has a wireless feature, then it will have standby wake-up and even transmit and receiving timing similar to a phone, and can use similar techniques in the RF section to enhance battery life. But even a PDA without wireless capability can use power management techniques to enhance battery life.

Typically a PDA is never completely powered down. Unlike a home or laptop computer, where the programs are stored on the hard disk and loaded into memory for use during the boot up routine lasting several minutes, the PDA must appear to be "off" to the user and come to life in a second or two. There is no time (and no hard drive) to load the programs.

When the PDA is "off", it is really still "on" but the user expects it to come on minutes, hours or days later with a full battery, so it must be extremely efficient in this mode. The programs and data are stored in memory which is always powered on. When not being accessed it draws very little power from the battery. The processor is put into such a deep sleep mode when the product is off that it draws almost no power. All other functions, such as display and sounds are turned off. The power management circuitry must still supply keep-alive voltage to the sleeping processor and memory, and it must do it at extremely low quiescent current. This mandates low power states in the regulators or buck switching regulators with topologies that allow quiescent current in the microamps range for a switcher that may have to provide half an amp when called upon.

In operation, while a PDA appears "on" and fully operational to the user, in fact most of the time it is only the display which is on. As soon as any action is completed, the processor reverts most of its function to sleep mode. Keeping alive all data and continuing to operate the display while waiting for the interrupts from the touchscreen.

Upon detecting a penstroke on the touchscreen the processor "wakes up," brings its clock to full speed, brings its dynamically scaled voltage to full value, performs whatever action it needs to take, and then immediately goes back into sleep mode. During a routine PDA session like looking up an address or writing a memo, the product might be operational for a few milliseconds about 2 times per second, and the entire computer (less display) is off for 99 percent of the time.

Through a combination of low current operation and low duty cycle operation, the average current consumed by a product can be just a fraction of what it consumes instantaneously when running; and high performance, long battery life and small product and battery size can be realized.