Power management solutions improve operating time in mobile handsets

With the proliferation of cellular mobile handsets, especially smartphones, in the world today where people are connected 24/7, consumers just can’t seem to get enough of the voice calls, emails, text messages and surfing the web. However, all of these activities greatly consume battery life in our handsets; as such, our handhelds lead to that one-bar battery power indicator very quickly. What can be done to prolong the batter life for these handsets so that we can use them longer?

Using a bigger battery is not an option as users demand smaller and thinner handsets. Design engineers now more than ever are constantly looking for ways to improve the power management performance in these devices, focusing on the three areas where these interventions will have the most impact—the power amplifiers (PAs), the display, and the applications/graphics processor. These areas consume the most power in handsets, next to the power-hungry baseband processor and RF transceiver. Now just why are these three areas the main focus? Consider this: nowadays, people talk and surf the Internet all at the same time. While doing this, the display is of course functioning as well. These activities drive the PA to transmit voice calls and data to the base station, while the applications processor is running so that you can bring up a website to watch a video or enable the applications.

The PAs for 3G networks consume a lot of power, especially when the reception is poor since it needs a higher output power to connect to the base station while maintaining linearity requirements to ensure 3G signal fidelity. A 3G PA will consume current based on the output power level; and the higher it is, the more current will be drawn from the battery. Transmitting data also requires higher output power and thus, consumes more current. There are two novel techniques used to reduce PA current consumption: DC/DC converters and envelope tracking. The use of DC/DC converters is increasingly being used in smartphones. The DC/DC converters operate by stepping down the supply voltage for the 3G PA to a level where it will meet the required output power level but minimize the amount of current consumed. Implementing such a solution provides a two-fold benefit—longer talk/data usage time and lower heat dissipation. Fairchild Semiconductor’s FAN5902, an 800mA, 6MHz step-down DC/DC converter with a bypass mode, was designed specifically for 3G PAs to lower power consumption and extend the connection time performance.

The FAN5902 works in conjunction with both the baseband processor and 3G PA to lower the current consumption. The baseband processor will set the output power level of the PA based on information it receives from the base station, and then translates it to a supply voltage for the FAN5902 to output to the PA. By dynamically adjusting the PA’s supply voltage and therefore the current, the FAN5902 can extend the talk and data usage times for mobile handsets by at least 15 percent. Figure 1 illustrates the performance under DG09 power distribution function between a standalone 3G PA and a 3G PA enabled by FAN5902.

In addition to extending talk times, a DC/DC converter-enabled solution also helps lower the heat dissipation so that handsets or USB dongles/data cards won’t get as hot. Figure 2 shows the thermal profile of 3G PA with FAN5902 and the other without it.

The display is the second major consumer of power after the PA since it is ON whether you are looking up contact information, surfing the web, reading emails or watching mobile TV/YouTube videos. Currently, the predominant display technology is TFT LCD, which requires white LEDs for backlight. Since the trend is for larger LCDs, it means more white LEDs are required to effectively backlight the display, which in turn means more current is needed to power the LEDs as well as the display itself. In high-end feature handsets and smartphones, dynamic backlight control (DBC) and auto luminous control (ALC) are used together not only to minimize current consumption but also enhance the visual experience for the user. ALC requires the use of an ambient light sensor, which detects the amount of light in the environment and then sets the LED current based upon an algorithm programmed in the LED driver or applications processor. So, depending on lighting conditions, the LED current will be set low or at maximum. DBC is a technique that adjusts the LED current based upon the image/video content on the display, such that a scene in a video with more dark content will have lower LED current vs. a brighter scene. The DBC programs the current based upon a pulse width modulation (PWM) signal from the graphics processor or LCD driver IC and is constantly changing with the video content that is being displayed. Figure 3 shows the difference between a display with and without ALC and DBC.

Fairchild’s 180mA FAN5702 charge pump LED driver with I2C interface can be configured to enable both ALC and DBC. The ambient light sensor is connected to the applications or baseband processor, which will take the input and determine the appropriate LED current level based on the algorithm for external lighting conditions. This data is then sent via the I2C interface to the FAN5702 and set the LED current. The PWM/EN pin of FAN5702 is programmed for PWM operation, which is in turn connected to the LCD driver IC, which will send the PWM signal to the FAN5702 based on the image/video content on the display. Figure 4 is a system block diagram of FAN5702 using both ALC and DBC. Adopting ALC and DBC for displays in mobile handsets will provide up to 50 percent savings in power consumption.

The third area where there is significant power consumption is the applications or graphics processor. However, this processor does not always run at full power all the time. To take advantage of instances when the chipset is running at lower power levels is to use a technique called dynamic voltage scaling (DVS). This is an ideal solution in mobile handsets and other portable electronics because the supply voltage can be scaled down to a lower core voltage and allows the chipset to operate at reduced clock frequency, helping to achieve lower current consumption. This is best illustrated by the following equation:



where the power (P) is proportional to the product of the clock frequency (f) and square of the core voltage (V). Therefore, the faster the processor clock frequency, the higher the power consumption. But when the core voltage is lowered, the power consumption can be decreased quadratically.

An applications processor can be powered by FAN5365, a 6MHz, 800mA/1A step-down DC/DC converter with I2C interface, for optimum power conservation. The I2C interface is used to dynamically program the voltage between 0.75V to 1.975V in 12.5mV steps to meet the processing power requirements of the chipset. The FAN5365 would supply a core voltage of 1.2V to the applications processor for maximum processing power when watching a video on a website, but once it is over it will drop down to 0.8V for lower level operation.

There are a variety of ways to improve overall power management performance of mobile handsets, in particular smartphones, with both simple and complex techniques. By implementing one or all three of these power management solutions for the PA, display and processor cores into mobile handsets, designers can provide significant power savings to prolong the battery power for these devices.


Author Information
Wayne Seto is the Technical Marketing Manager, Mobile Power Solutions, at Fairchild Semiconductor.

Captions

Figure 1: Talk time analysis of a 3G PA with and without FAN5902 under WCDMA signal modulation and 1Ah Li-ion battery. The power distribution function is DG09 and can be found in GSMA’s white paper “Battery Life Measurement Technique, v4.7”.


Figure 2: Thermal profile of 3G PA at POUT = 28dBm: (a) with FAN5902 at VPA = 2.97V; (b) PA only at VBAT = 3.7V; and (c) PA only during battery charging at VBAT = 4.2V.


Figure 3: Display that does not use ALC and DBC (a); and with ALC and DBC (b).


Figure 4: System block diagram of FAN5702 with ALC and DBC implemented.