Cost, Space, and Complexity Are All Increased With SMPS
Engineering is a field that deals with practical applications of science and technology. As engineers, we often find ourselves weighing the pros and cons of any design implementation. While using a power amplifier with SMPS reduces average current consumption, there are other considerations.
First, adding SMPS with a DC/DC converter requires additional external components. The cost of these extra components is roughly equal to the cost of the power amplifier. When all of the extra components are considered, the complete PA solution cost for the user when implementing a DC-DC converter is nearly twice that of a standalone single-band PA. Therefore, the cost of the bill of materials for the entire RF section will increase.
With the addition of the DC/DC converter and inductors, space is allocated not only for the physical devices, but also for routing. This can increase RF printed circuit board space requirements significantly (Figure 3). Additionally, it creates a challenge in layout. Not only does the implementation require additional board space, but it also adds complexity and engineer design time to achieve an optimized layout.
Figure 3 (Click graphic to download larger image.)
Lastly, mobile device calibration becomes significantly more challenging with a DC/DC converter regulating the voltage, because voltage must be considered along with frequency, gain, and temperature in the calibration algorithm. This does not affect the linearity of the calibrated device, but it does create a challenge at the factory calibration step.
Switched Mode Power Amplifiers
In recent years, some power amplifier manufacturers have developed unique designs that have eliminated the need for DC/DC converters. The driving force behind this effort has been a desire to overcome the drawbacks of SMPS, while achieving performance gains. Switched-mode designs provide multiple gain states within the power amplifier to select more efficient amplification paths in low power modes. The end result is a power amplifier that is able to achieve excellent efficiency across the entire dynamic range of operation. Innovation in power amplifier design has continued to drive down current consumption.
The recent development of power amplifiers with three gain states, such as ANADIGICS’ fourth-generation HELP™ power amplifier, HELP4™, offers designers low power mode (LPM), medium power mode (MPM), and high power mode (HPM). The current consumption using the DG09 WCDMA probability model indicates that these switch-mode power amplifiers offer a significant average current savings compared with a standard WCDMA power amplifier (Figure 4).
Switched mode power amplifiers such as ANADIGICS HELP technology products optimize current consumption at several output power modes through internal changes in the RF amplification circuits to deliver improved efficiency at backed-off power levels. Switched mode PA’s are designed to efficiently from a fixed supply voltage, but many of them can also operate from a variable voltage supplied by a DC-DC converter or switched-mode power supply (SMPS). A plot of the current consumption of ANADIGICS’ HELP4 power amplifier (Figure 5), demonstrates how the use of SMPS provides additional current consumption advantages in high power mode.
Figure 5 (Click graphic to download larger image.)
ANADIGICS set out to create a family of devices that combines the best of both worlds: A switch-mode power amplifier optimally designed for use with a DC / DC converter.
Introducing Another Option
In order to provide an additional option for reducing current consumption, ANADIGICS is introducing a new series of power amplifiers. The new HELP3DC power amplifiers offer two gain states and have been optimized for use with a DC/DC converter, allowing the two technologies to complement each other.
Utilizing the same DG09 WCDMA models, we can compare the current consumption of two gain state, SMPS-optimized two state mode, and three state mode power amplifiers with and without SMPS (Figure 6). The performance advantage of using SMPS with an optimized two state power amplifier, is equivalent to a three state power amplifier. This provides an additional path to greater battery life.
Figure 6 (Click graphic to download larger image.)
Designers now have multiple design paths when developing the RF section of a mobile device. There are a few questions to initially answer: Does my design need the lowest current consumption? Can the chipset support three gain states? Do the drawbacks of adding SMPS outweigh the performance advantage? Since each device has a specific set of requirements, the answer to these questions will change based upon the device that is being created. With two-state power amplifiers optimized for use with a DC/DC converter offering equivalent performance to a three-state device, one thing is certain -- RF engineers and designers have several choices to deliver the optimal device for each device.
1. Battery Life Measurement Technique ver 5.1” Document DG.09 by GSM Association
About the Author:
Bruce Webber currently serves as Director of Marketing for Wireless Products at ANADIGICS, leading the company’s product development and marketing strategy for the wireless business. Prior to joining ANADIGICS, Bruce has held marketing leadership positions at leading wireless companies, including Flarion, Agere, and Lucent Microelectronics. Bruce received a B.S. from Purdue University and a Ph.D. from Stanford University.