LM3203,LM3204,LM3205
Optimizing RF Power Amplifier System Efficiency Using DC-DC Converters
Literature Number: SNVA593
RFpower amplifiers used in CDMA / WCDMA cellular standards
have been traditionally powered directly from the battery. This
makes system implementation easy but the requirement for lin-
ear power amplifiers in such standards have intrinsic inefficiencies throughout
the transmit power spectrum.
Cellular standards have been evolving with transmission speeds that started
from 14.4 kbps in CDMA-1 to 2 Mbps in CDMA2000/WCDMA. Apart from
this, cellular providers have increased the services bundled with the 3G phones
in order to increase the average revenue per subscriber. At the same time, the talk
time and battery life is expected to be improved with the same or slightly high-
er capacity batteries. This makes system design challenging. System designers
have to be very cautious and perform a power survey of each and every compo-
nent on the phone board. The RF Power Amplifier (RF PA) powered directly
from the battery is a major concern from the power budget perspective.
The modulation schemes used in CDMA and WCDMA result in an ampli-
tude-modulated signal that exhibits a non-constant amplitude envelope. In
order to preserve signal integrity and further spectral re-growth, a linear
No. 110
NEXT ISSUE:
Emulated Current Mode Control
POWER designer
Expert tips, tricks, and techniques for powerful designs
Feature Article............1-7
Step-Down Switching
Regulators........................2
150 mA CMOS LDO ........4
RF Detector Family........6
Power Design Tools......8
Optimizing RF Power Amplifier System
Efficiency Using DC-DC Converters
— By Mathew Jacob, Applications Engineering Manager
Figure 1. Old Method vs New Method
RF
Supply
Regulator
Old Method New Method
Standard PA
• Output power controlled by RFIN
• VCC directly connected to battery
IN
PA with Supply Regulator
• Output power controlled by RFIN
• VCC connected to DC-DC converter
• VOUT is optimized for given POUT
RFIN RFOUT
VOUT
VCC
VCC
VBATT
VBATT VCTRL
RFOUT
2
Dynamic Power Management of RF Power Amplifiers
Feature-Rich LM320x Family Enhances Battery Life in Portable Applications
Step-Down Switching Regulators for RF Power Amplifiers
0
10
20
30
40
50
60
70
80
90
0 5 10 15 20 25 30
POUT, dBm
VDD
PVIN
EN
VCON
LM3205 FB
SW
PGND SGND
VOUT = 2.5 x VCON
VOUT
0.8V to 3.6V
VIN
2.7V to 5.5V
10 µF
4.7 µF
3.3 µH
% Savings in Power/IBATT
Family Features
• Dynamically adjustable output voltage
optimizes RF PA power levels for increased
battery life
• Bypass mode maintains maximum output
power regardless of battery voltage
• 2 MHz Switching frequency minimizes
external components and complies with
spectral emission requirements
Ideal for powering RF power amplifiers in cell
phones, smart PDA phones, GPS systems,
two-way radios, and portable communications
systems
Family Highlight:
DC-DC converters deliver up to 5X transmit
time in RF PAs
RF PA System Power Savings
Product ID Description Min Max VOUT IOUT (mA) Bypass Modes Packaging
LM3200 Dynamically adjustable output voltages, 2.2 µH
inductor 2.7 5.5 Adj (0.8 to 3.6V) 500 Forced and
automatic micro SMD-10
LM3202 Miniature, adjustable, step-down DC-DC converter 2.7 5.5 Adj (1.3 to 3.16) 650 None micro SMD-8
LM3203 Miniature, adjustable, step-down DC-DC converter 2.7 5.5 Adj (0.8 to 3.6) 500 Forced micro SMD-10
LM3204 Miniature, adjustable, step-down DC-DC converter 2.7 5.5 Adj (0.8 to 3.6) 355/500 Forced and
automatic micro SMD-10
LM3205 Miniature, adjustable, step-down DC-DC converter 2.7 5.5 Adj (0.8 to 3.6) 650 None micro SMD-8
VIN
LM3205 Application Circuit
power amplifier is necessary. However, power
efficiency is traded off because power amplifiers
operate efficiently when operated in gain compres-
sion. To meet the required linearity, the operating
transmit power is backed off from the power ampli-
fier’s compression point that causes an overall
reduction in efficiency. When the handset is
operating in transmit mode, the RF power section
consumes up to 65% of the overall power budget as
a result of the PA’s intrinsic inefficiencies.
For this reason, linear PAs are ideal candidates to be
powered with a magnetic buck converter which
will dramatically increase efficiency of the system.
Power-Added Efficiency (PAE) is a key perform-
ance metric of a power amplifier.
PAE (%) = (POUT – PIN ) / Pdc
The key in using a DC-DC converter (PA supply
regulator) is to reduce the Pdc factor in the denom-
inator. When the PA is connected directly to the
battery, Pdc=Vbatt*Ibatt and, when it is powered
by a PA supply regulator, Pdc=Vo*Io. Now it can
be seen that for increasing the PAE we have to have
a low Vo and Io compared to Vbatt and Ibatt. This
is achieved by lowering the output voltage of the
PA supply regulator at lower transmitted RF power
levels. This in turn reduces Io (current drawn by
the PA) and results in a much lower input current
drawn from the battery due to the inherent high
efficiency of the DC-DC converter.
It is important to consider the power probability
profile (see Figure 2) for the modulation methods
to really understand the impact of savings in pow-
ering a PA with a supply regulator. The profiles are
different for urban and rural regions.
As shown in Figure 3, the output voltage of the
DC-DC converter has to be varied as the transmit-
ted power levels are changed to maintain the
Adjacent Channel Power/leakage Ratio (ACPR)
specifications. The savings in battery current can be
as high as 50 mA in the 0 dBm to 20 dBm power
levels. Figure 2 shows that the PA is operating in
this band of power levels for a majority of its time.
So why do we have to change the voltage of the
DC-DC converter as the transmitted power level is
increased? The answer is that this change is needed
to maintain the ACPR ratios. ACPR is used to
characterize the distortion of power amplifiers and
other subsystems for their tendency to cause
interference with neighboring radio channels or
3
Optimizing RF Power Amplifier System Efficiency
POWER
designer
power.national.com
WCDMA PA Power Probability
0.00
1.00
2.00
3.00
4.00
5.00
0 5 10 15 20 25 30
POUT [dBm]
Voice
Data
p [%]
Figure 2. PA transmits low power levels for a high
percentage of time in a typical cellular phone which
reinforces the savings possible with a PA supply regulator
0
50
100
150
200
250
300
0 5 10 15 20 25 30
P
OUT
, dBm
Vo= 1.5V
∆= 40 mA
0 dBm, Vo = 0.6V
Vo = 2.4V
>2 4 dBm,
(V =3.4V)
Current (mA)
Battery current with new method
Battery current with old method
∆= 48 mA
BATT
IN
V=3.4V
Vo = 3.25V
Figure 3. Savings in battery current when the DC-DC
converter is used for powering the PA
0
10
20
30
40
50
60
70
80
90
0 5 10 15 20 25 30
P
OUT
, dBm
% Savings in Power/IBATT
Figure 4. Percentage savings in power when the PA is
powered by a PA supply regulator
4
Industry’s Lowest Noise 150 mA CMOS LDO
-20
-100
-90
-80
-70
-60
-50
-40
-30
Frequency [Hz]
100K10 100 1000 10K
VEN Gnd
0.47 µF
VOUT
0.47 µF
Output
VIN
LP5900
1001010.10.01
24
21
18
15
12
9
6
3
IOUT (mA)
Noise (µV)
LP5900 PSRR
LP5900 Circuit Diagram
LP5900 Output Noise
27
30
LP5900
33
36
Industry
Standard
Benchmark
Bypass
Capacitor
Gain (dB)
LP5900 Low Dropout Regulator Requires No Bypass Capacitor
Features
• Industry’s lowest noise combined with
85 dB of Power Supply Ripple Rejection
guarantees signal integrity
• 25 µA IQ minimizes current drain when
system operates in low-power mode
• Elimination of bypass capacitor reduces
BOM to only two ceramic 0.47 µF capacitors
• Low 80 mV dropout voltage (typ.)
• Virtually zero (<1µA) Iq when disabled
• Available in a micro SMD-4 package
• LLP®packaging available soon
Ideal for powering analog and RF signal path
ICs, including low-noise amplifiers, voltage
controlled oscillators, and RF receivers
Product Highlight:
Unique 150 mA RF LDO eliminates bypass
capacitor and achieves low 6.5 µVRMS
noise
5
POWER
designer
Optimizing RF Power Amplifier System Efficiency
systems. It is specified as the ratio of the Power-
Spectral Density (PSD) of the main channel to the
PSD measured at several offset frequencies.
In Figure 5 it can be seen that if the supply voltage
to the PA is not increased as POUT is increased, the
ACLR specifications cannot be met.
The system-level specification (3 GPP) for
WCDMA is -34 dBc and, in order to preserve suf-
ficient margin caused by temperature and device
variances, the ACLR value of -38 dBc is used.
Key Requirements of Buck Converters for
Powering RF Power Amplifiers
Buck converters that power RF PAs have specialized
functions and are quite different from buck converters
that power digital core processors. These differences
arise in operating characteristics and parameters
such as switching FET ON-resistances, current
limit, transient response, modes of operation such
as PFM/PWM, startup time, quiescent current,
and dropout behavior. The following examples
illustrate these differences:
• High efficiency over wide output voltage and
load range
Example: LM3205 has efficiency of 96% at
VIN = 4.2V, Vo = 3.4V, Io = 400 mA (high RF
power) and 87% at VIN=3.9V, Vo=1.5V, Io =
100 mA (low RF power).
• Dynamic output voltage adjustment
Example: In LM3205 the output voltage can be
adjusted between 0.8V to 3.6V using a Vcon pin.
The voltage gain from Vcon to Vo is 2.5.
power.national.com
-55
-50
-45
-40
-35
-30
-25
-20
-15 -10 -5 0 5 10 15 20 25 3035
P
OUT
[dBm]
ACLR1 [dBc]
0.8
1.0 1.2 1.5 1.8 2.2
2.5
2.8
3.0
3.2
DROPOUT
VOUT = VDD
NO DC-DC, VDD = 3.5V
-38dBc(WCDMA)
F = 1950 MHz
Figure 5. How ACLR is affected with respect to supply voltage to the PA and POUT
6
Simplify System Calibration with RF Detector Family
Family Features
• LMV227 100% RF tested for accuracy
• 30 dB Linear-in-dB power detection range
• Multi-band operation from 450 MHz to 2 GHz
• Accurate temperature compensation
• Logarithmic amplifier and mean square
RF detector technology
Ideal for use in handsets, wireless LAN, WiFi, PC
and PDA module cards, and GPS navigation
modules
Accurate and Stable RF Power Detectors for Portable Devices
Family Highlight:
Real-time transmitter power adjustments simplify
system calibration in communications systems
2.25
2.00
1.75
1.50
1.25
1.00
0.75
0.50
0.25
0.00
-50 -40 -30 -20 -10 0 10 20 3040
31.4 dB
PA output power
LMV225
RF input power
Power (dBm)
LMV225 Output Voltage (V)
NO DIODE
Product ID Application Detector Channel Range Package
LMV227 CDMA 2000, WCDMA, UMTS Log amp 140 dB, 2.1 GHz Micro SMD, LLP®
LMV225/226/228 CDMA, WCDMA, UMTS Log amp 140 dB, 2.1 GHz Micro SMD, LLP
LMV232 3G Mobile communications Mean square 220 dB, 2.2 GHz Micro SMD
Linear in dB Performance
• 30 µs Output slew rate and settling (50 µs window
in beginning of every 667 µs transmit cycle in
which the Vcon adjustments must be completed)
In WCDMA architecture, transmit power is
adjusted by ±1 dB in every 667 µs as requested by
the basestation.
• Low dropout and low ripple near 100% duty cycle
Example: Low RDSON PFET 140 mΩ(LM3205)
or Bypass FET (LM3204) gives low dropout volt-
age and pulse-skipping schemes gives low ripple
near 100% duty cycle.
• Low duty cycle operation for low output voltages
Example: Minimum on time, 50 ns facilitates 10%
duty cycle operation for output voltages of 0.8V
and lower depending on the VIN range.
• High switching frequency
Example: 2 MHz switching frequency helps the
use of smaller sized external components and meet
spectral emission requirements.
• Fast turn on time to meet time mask for transmit
ON/OFF
Example: LM3203 has turn-on time of 50 µs for
Vo = 3.4V from EN = low to high.
100% Duty Cycle vs Bypass Mode
When the buck converter is operating at 100% duty
cycle the dropout voltage is
Dropout Voltage = (RON,P+ RL) • Io,
where RON,Pis the RDSON of the PFET and RLis the
inductor DCR. For a PA supply regulator that has a
bypass FET the dropout voltage in bypass mode is,
Dropout Voltage = (RON,BYP) • Io,
where RON,BYP is the RDSON of the bypass FET. The
bypass FET can be turned on automatically or
manually. As shown, the key advantage in having a
bypass mode is lower dropout voltages; which
translates to longer talk times and lowering the low
battery shutdown point for the phone. The
alternative is to use low DCR inductors and a low
RDSON PFET.
Example Application Circuits
In this example, the baseband will have a lookup
table scheme where it sets the output voltage
depending on the output power levels required.
In this case, the power detector is part of a closed
loop and sets the output voltage.
Conclusion
DC-DC converters enhance the RF PA system
efficiency in portable communication devices and
support the addition of more features or functions
by improving battery life. I
7
POWER
designer
Optimizing RF Power Amplifier System Efficiency
power.national.com
3.3 SW
FB
PVIN VDD
VBATT
TX DAC
VCON
LDO
VEN
TX_ON
LM3205
ANT SW
R1
R
2
PA
VCC
4.7 µF
VREF
RF_TX
TX_ON
VOUT
PGND
SGND
10 µF
*
V1
*R1, R2 only required if V1 to VOUT gain needs to be modified
Figure 6. Baseband Controls Vo Directly
ANT SW
R1
Power
Detector
Coupler
LMV228
5
0
3.3 SW
FB
PVIN VDD
VBATT
VCON
LDO
VEN
TX_ON
LM3205
R1
R
2
PA
VCC
4.7 µF
VREF
RF_TX
TX_ON
VOUT
PGND
SGND
10 µF
*
V1
*R1, R2 only required if V1 to VOUT gain needs to be modified
Figure 7. Using a Power Detector to Set Vo
For more information on Powering RF Power
Amplifiers, visit www.national.com/onlineseminars
to watch Mathew Jacobs’ online seminar!
Analog University®
Expand your knowledge and understanding of analog
with our free online educational training tool.
analogU.national.com
National Semiconductor
2900 Semiconductor Drive
PO Box 58090
Santa Clara, CA 95052
1 800 272 9959
Visit our website at:
power.national.com
For more information,
send email to:
new.feedback@nsc.com
©2006, National Semiconductor Corporation. National Semiconductor, , WEBENCH, LLP, and Analog University are registered trademarks, LMV
is a trademark, and Signal Path Designer is a service mark of National Semiconductor. All other brand or product names are trademarks or
registered trademarks of their respective holders. All rights reserved.
550263-010
Don't miss a single issue!
WEBENCH®Online Design Environment
Our design and prototyping environment simplifies and
expedites the entire design process.
1. Choose a part
2. Create a design
3. Analyze a power supply design
– Perform electrical simulation
– Simulate thermal behavior
4. Build it
– Receive your custom prototype kit 24 hours later
webench.national.com
Power Design Tools
Subscribe now to receive email
alerts when new issues of Power
Designer are available:
power.national.com/designer
Introducing our new Signal Path
DesignerSM! View online today at:
signalpath.national.com/designer
IMPORTANT NOTICE
Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications, enhancements, improvements,
and other changes to its products and services at any time and to discontinue any product or service without notice. Customers should
obtain the latest relevant information before placing orders and should verify that such information is current and complete. All products are
sold subject to TI’s terms and conditions of sale supplied at the time of order acknowledgment.
TI warrants performance of its hardware products to the specifications applicable at the time of sale in accordance with TI’s standard
warranty. Testing and other quality control techniques are used to the extent TI deems necessary to support this warranty. Except where
mandated by government requirements, testing of all parameters of each product is not necessarily performed.
TI assumes no liability for applications assistance or customer product design. Customers are responsible for their products and
applications using TI components. To minimize the risks associated with customer products and applications, customers should provide
adequate design and operating safeguards.
TI does not warrant or represent that any license, either express or implied, is granted under any TI patent right, copyright, mask work right,
or other TI intellectual property right relating to any combination, machine, or process in which TI products or services are used. Information
published by TI regarding third-party products or services does not constitute a license from TI to use such products or services or a
warranty or endorsement thereof. Use of such information may require a license from a third party under the patents or other intellectual
property of the third party, or a license from TI under the patents or other intellectual property of TI.
Reproduction of TI information in TI data books or data sheets is permissible only if reproduction is without alteration and is accompanied
by all associated warranties, conditions, limitations, and notices. Reproduction of this information with alteration is an unfair and deceptive
business practice. TI is not responsible or liable for such altered documentation. Information of third parties may be subject to additional
restrictions.
Resale of TI products or services with statements different from or beyond the parameters stated by TI for that product or service voids all
express and any implied warranties for the associated TI product or service and is an unfair and deceptive business practice. TI is not
responsible or liable for any such statements.
TI products are not authorized for use in safety-critical applications (such as life support) where a failure of the TI product would reasonably
be expected to cause severe personal injury or death, unless officers of the parties have executed an agreement specifically governing
such use. Buyers represent that they have all necessary expertise in the safety and regulatory ramifications of their applications, and
acknowledge and agree that they are solely responsible for all legal, regulatory and safety-related requirements concerning their products
and any use of TI products in such safety-critical applications, notwithstanding any applications-related information or support that may be
provided by TI. Further, Buyers must fully indemnify TI and its representatives against any damages arising out of the use of TI products in
such safety-critical applications.
TI products are neither designed nor intended for use in military/aerospace applications or environments unless the TI products are
specifically designated by TI as military-grade or "enhanced plastic."Only products designated by TI as military-grade meet military
specifications. Buyers acknowledge and agree that any such use of TI products which TI has not designated as military-grade is solely at
the Buyer's risk, and that they are solely responsible for compliance with all legal and regulatory requirements in connection with such use.
TI products are neither designed nor intended for use in automotive applications or environments unless the specific TI products are
designated by TI as compliant with ISO/TS 16949 requirements. Buyers acknowledge and agree that, if they use any non-designated
products in automotive applications, TI will not be responsible for any failure to meet such requirements.
Following are URLs where you can obtain information on other Texas Instruments products and application solutions:
Products Applications
Audio www.ti.com/audio Communications and Telecom www.ti.com/communications
Amplifiers amplifier.ti.com Computers and Peripherals www.ti.com/computers
Data Converters dataconverter.ti.com Consumer Electronics www.ti.com/consumer-apps
DLP®Products www.dlp.com Energy and Lighting www.ti.com/energy
DSP dsp.ti.com Industrial www.ti.com/industrial
Clocks and Timers www.ti.com/clocks Medical www.ti.com/medical
Interface interface.ti.com Security www.ti.com/security
Logic logic.ti.com Space, Avionics and Defense www.ti.com/space-avionics-defense
Power Mgmt power.ti.com Transportation and Automotive www.ti.com/automotive
Microcontrollers microcontroller.ti.com Video and Imaging www.ti.com/video
RFID www.ti-rfid.com
OMAP Mobile Processors www.ti.com/omap
Wireless Connectivity www.ti.com/wirelessconnectivity
TI E2E Community Home Page e2e.ti.com
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright ©2011, Texas Instruments Incorporated
