Power Management Solutions for W-CDMA Transceivers

The usage time of several 3G mobile phones currently on the market is only 2 to 3.5 hours, which is not really attractive to end users. The actual talk or connection time actually depends on the quality of the connection between the mobile phone and the base station, as well as the density of data content during the connection.

In order to attract more users, the global 3G network infrastructure has been deployed at an accelerated pace in 2008, especially in the United States and Europe. Several operators have launched relatively cheap unlimited data plans. Although these plans have certain restrictions on connection capabilities, they have increased support for network application software, such as video conferencing, Internet voice (VoIP), easy email and Internet browsing, thereby attracting users.

On the hardware side, for low-power mode operation and urban call service, mobile phone designers have managed to optimize the power consumption of the RF transceiver, especially the power consumption of the RF Power Amplifier (RFPA). Statistics provided by operators show that in this case, the transmission power of the mobile phone is less than 1mW. Most of the RFPAs currently in use have a low-power mode, and their power consumption is 10mA or less when the RF power is less than 1mW. In addition, they are optimized to obtain the best power added efficiency (Power Added Efficiency, PAE) (about 33%) at around 500mW (maximum RF operating power level). The problem at this time is that the RFPA power consumption is about 1W, which will generate too much heat and affect the performance of surrounding components. Figure 1 shows the relationship between the typical change of PAE and power consumption and the RF operating power level.

Figure 1: Graphical representation of the power added efficiency (PAE) of a dual-mode W-CDMA RFPA

According to power distribution statistics provided by 3G network operators, in urban areas, voice-focused phones operate at power consumption below 1mW 90% to 95% of the time, which should allow talk time of up to 5 hours under these conditions.

However, when the data capacity of the connection is large or the user is in a suburban or low-coverage area, the 3G mobile phone must increase the transmit power to more than 50mW to achieve a good signal-to-noise ratio. In these cases, an RFPA that has not been re-optimized will consume all the energy of the battery in 2.5 hours or less.

The best solution is to use a voltage-controlled DC-DC converter to dynamically adjust the RFPA supply voltage to achieve the highest possible power efficiency at every RF power level. This technology is called dynamic voltage scaling (DVS) technology (Figure 2).

Figure 2: Illustration of a 3G RF power amplifier powered dynamically using a DC-DC converter

Text in the picture (above): VBAT – DVS DCDC – VCC – WCDMA RFPA

(Middle): VBAT or battery voltage – VCC or RFPA voltage from DCDC

(Below): Transmitted RF power level

Figure 3 shows the improvement in power added efficiency of an RFPA (blue curve) using a DVS power management solution compared to an RFPA (black curve) powered directly by a battery. As can be seen in the figure, the latter saves 100mA of battery current in the power range of 16 to 24dBm, while it can save 10mA of battery current in the power range of 0 to 16dBm. In other words, a data-based 3G mobile phone using a DVS solution can save up to 20% of battery energy, thereby extending the data connection time accordingly.

Figure 3: Dual-mode W-CDMA RFPA (black curve) and a dual-mode W-CDMA RFPA using dynamic voltage scaling technology

Comparison of power added efficiency of single-mode RFPA (blue curve)

Text in the picture (left): DCDC using DVS

Another major advantage of using DVS technology is that when the battery is charged to 4.2V, the RFPA voltage can be clamped to 3.4V (Note 1), thereby reducing the heat generation at high battery levels by another 20%. This can reduce the size of the heat sink and/or shorten the spacing of integrated components on the PCB.

In addition, using the DVS power management solution, RF engineers can also replace complex multi-power mode RFPAs with single power mode amplifiers, improving power efficiency, reducing heat generation, and lowering bill of materials costs.

The requirement for DC-DC power device manufacturers is to provide compact solutions suitable for installation inside RF front-end modules and minimize the impact on baseband or RF spectrum. The real challenge is how to replace relatively large inductors (less than about 10mm2) with sub micro-henry inductors (3.2 mm2) to enable switching frequencies and switching noise to exceed baseband frequencies (> 5MHz).