Texas Instruments | Reduce DC Pwr Consump in CDMA RF Pwr Amps Thru the LMV225 & Eff Switcher (Rev. A) | Application notes | Texas Instruments Reduce DC Pwr Consump in CDMA RF Pwr Amps Thru the LMV225 & Eff Switcher (Rev. A) Application notes

Texas Instruments Reduce DC Pwr Consump in CDMA RF Pwr Amps Thru the LMV225 & Eff Switcher (Rev. A) Application notes
Application Report
SNOA466A – January 2006 – Revised May 2013
AN-1438 A Simple Method to Reduce DC Power
Consumption in CDMA RF Power Amplifiers Through the
LMV225 and an Efficient Switcher
.....................................................................................................................................................
ABSTRACT
This application report presents a simple power tracking technique for efficiency enhancement in CDMA
RF power amplifiers. This technique involves the use of a linear-in-dB RF power detector and a DC-DC
converter switch. This enhancement scheme switches the DC supply voltage, VCC, of an RF power
amplifier into two different levels through a DC-DC converter. Texas Instruments RF power detector
LMV225 determines the supply voltage of the RF power amplifier. An off-the-shelf CDMA2000 RF power
amplifier can be used in this technique to improve the energy efficiency of the mobile phone.
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Contents
Introduction ..................................................................................................................
RF Power Amplifier .........................................................................................................
Power Added Efficiency ....................................................................................................
Adjacent Channel Power Rejection .......................................................................................
Power Detector ..............................................................................................................
Switcher or DC/DC Converter .............................................................................................
Design Considerations .....................................................................................................
Application Circuit ...........................................................................................................
Power Saving at 10 dBm ...................................................................................................
Conclusion ...................................................................................................................
1
CDMA RF Power Amplifier
2
2
3
4
4
4
5
6
6
6
List of Figures
2
3
4
5
6
................................................................................................
Linearity of CDMA RF Power Amplifier ..................................................................................
RF Output Power Distribution .............................................................................................
LMV225 Detected Voltage vs. POUT ......................................................................................
Efficiency Enhancement Circuit Diagram ................................................................................
POUT and PDC vs. PIN ........................................................................................................
2
2
3
3
5
6
List of Tables
1
Adjacent Channel Power Rejection ....................................................................................... 4
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SNOA466A – January 2006 – Revised May 2013
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AN-1438 A Simple Method to Reduce DC Power Consumption in CDMA RF
Power Amplifiers Through the LMV225 and an Efficient Switcher
Copyright © 2006–2013, Texas Instruments Incorporated
1
Introduction
1
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Introduction
The need for higher wireless data rates is driving the migration of 2G to 3G mobile communication
systems. The higher data rates in these systems impose additional performance constraints on the radio
design of mobile phones.
In order to achieve the highest bandwidth efficiency of the allocated spectrum, these 3rd generation
mobile communication systems use spectrum efficient linear modulation schemes, such as Quadrature
Phase Shift Keying, 8-Phase Shift Keying and Quadrature Amplitude Modulation.
In IS-95 and CDMA2000 systems, the RF power amplifier typically operates at 6 dB to 40 dB back-off
from the peak power or 1 dB compression point. (This means that it operates from 6 dB to 40 dBbelow the
1 dB compression point.) Consequently, the RF power amplifier operates with very low efficiency most of
the time and is one of the most power consuming components in a handset. Studies show that the RF
power amplifier consumes as much as 20% to 40% of the battery energy in regular phone operation.
Now, we can see that it is supremely important to reduce the power consumption of RF power amplifiers
in order to achieve a long battery life or ‘talk time’ in a mobile phone.
2
RF Power Amplifier
An RF power amplifier is the centerpiece of this application. An off-the-shelf CDMA2000 RF power
amplifier, such as the SKY77152, is used in the evaluation. It can have more than 40% power added
efficiency near the 1 dB compression point as specified in the datasheet.
In a CDMA RF power amplifier there are usually two supply voltage pins, VCC and VBIAS, as shown in
Figure 1. There is also one reference voltage pin, which is usually called VREF. The VREF has to be at 2.85V
in all conditions. The power amplifier can be turned off by setting VREF equal to ground level. Since most of
the CDMA RF power amplifiers have two operation modes, High Power Mode and Low Power Mode, a
VCONT pin is used to set the operation mode of the power amplifier. When the RF output power is in the
high level, the CDMA RF power amplifier needs to operate in High Power Mode to keep the right distortion
performance. The CDMA RF power amplifier can be switched to Low Power Mode if the output signal
level is relatively low. However, an undesired side effect is that the signal path phase shifts have too much
difference between the two paths. This may cause problems in base-band processing and correction.
Figure 2 shows the typical POUT vs. PIN performance of a CDMA RF power amplifier when the DC supply
voltages, VCC and VBIAS, are lowered. It shows that output RF power can still be obtainable by reducing the
DC supply voltage of the RF power amplifier.
+
VCC And VBIAS
RFIN
RFOUT
VREF = 2.85V for On;
GND for ShutDown
VCONT = H for High Power Mode;
L for Low Power Mode
OUTPUT RF POWER (dBm)
-
40
35
+28 dBm
30
25
20
15
10
5
0 VCC = 3.4V
-5
-10
C/3IM =
-15
-30 dBc
-20
C/3IM =
VCC = 1.4V
-25
-28 dBc
-30
-35
IMD3 @
IMD3 @
-40
VCC = 1.4V
V
CC = 3.4V
-45
-50
-50
-40
-30
-20
-10
0
10
INPUT RF POWER (dBm)
Figure 1. CDMA RF Power Amplifier
2
Figure 2. Linearity of CDMA RF Power Amplifier
AN-1438 A Simple Method to Reduce DC Power Consumption in CDMA RF
Power Amplifiers Through the LMV225 and an Efficient Switcher
SNOA466A – January 2006 – Revised May 2013
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Power Added Efficiency
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3
Power Added Efficiency
The DC-to-RF efficiency or Power Added Efficiency, PAE, is defined by
PAE =
POUT - PIN
PDC
100%
(1)
The DC power consumption is defined by
PDC = VCC • ICC
(2)
Although the peak DC-to-RF efficiency of the PA occurs at the peak output power level as specified by all
RF power amplifier manufacturers, the RF power amplifier itself rarely operates at this peak power level.
Nevertheless, the peak power added efficiency contributes significantly to minimizing power dissipation for
heat constraints in the handset. On the other hand, the PAE of the RF power amplifier goes downhill when
the output RF power is lower.
In battery powered cellular phones, the output RF power probability distribution, shown in Figure 3, should
be considered to estimate the average efficiency of the mobile system.
As Figure 3 shows, most of the time the RF power amplifier in a handset is operating at POUT = +15 dBm
and below for an IS-95 handset. Therefore, it makes sense to improve the PAE of RF power amplifiers at
small signal levels.
Equation 1 and Equation 2 reveal the idea that the DC power consumption PDC can be reduced by
lowering the supply voltage of the RF power amplifier.
It may sound very simple to improve the PAE of an RF power amplifier; however, there are a few major
specifications that need to be considered while reducing the supply voltage of the RF power amplifier.
These include the ACPR, the EVM and the switching time from one supply voltage level to another.
2.250
2.000
3
1.750
VDET = 1.45V @
2.5
1.500
VDET (V)
PDF OF PA OUTPUT POWER (%)
3.5
2
1.5
POUT = +15 dBm
1.250
1.000
0.750
1
0.500
0.5
0.250
0
-50
-40
-30
-20
-10
0
10
20
30
OUTPUT RF POWER (dBm)
Figure 3. RF Output Power Distribution
SNOA466A – January 2006 – Revised May 2013
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0.000
-25.00 -15.00 -5.00
5.00
15.00 25.00 35.00
OUTPUT POWER FROM CDMA PA (dBm)
Figure 4. LMV225 Detected Voltage vs. POUT
AN-1438 A Simple Method to Reduce DC Power Consumption in CDMA RF
Power Amplifiers Through the LMV225 and an Efficient Switcher
Copyright © 2006–2013, Texas Instruments Incorporated
3
Adjacent Channel Power Rejection
4
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Adjacent Channel Power Rejection
The Adjacent Channel Power Rejection, known as ACPR, is defined as the ratio of the average power in a
specific offset frequency to the average power in the transmitted frequency. Table 1 shows the
performance requirements from the CDMA2000. Although ACPR is not officially required by the IS-95 or
the IS-98 air interface standards as it is in the CDMA2000, it is still suggested that the handset RF
designer verifies to see if the components meet the specifications of Table 1.
Table 1. Adjacent Channel Power Rejection
Air Interface
Frequency
Channel
Bandwidth
Offset
Frequency
@ ACPR1
Offset
Frequency
@ ACPR2
Measurement
Resolution
Bandwidth
IS-95
824–849 MHz
1.25 MHz
±885 KHz
±1.98 MHz
30 KHz
PCS
1850–1910 MHz
1.25 MHz
±1.25 MHz
±1.98 MHz
30 KHz
ACPR1 = –42 dBc and ACPR2 = –54 dBc
5
Power Detector
The RF power detector, which uses the RF output signal, generates a rectified DC voltage that determines
the output voltage of a DC-DC converter or switcher.
In this application, Texas Instruments LMV225 is chosen as an example since it provides 40 dB linear-indB detection range from 0 dBm down to –40 dBm.
RF power control in handsets is essential to ensure that the CDMA system operates smoothly. Since all
users share the same radio frequency band, 1.25 MHz in IS-95, then each user appears to others as
random noise. The power of an individual user must, therefore, be carefully controlled to prevent any one
user from unnecessarily interfering with the others who share the same radio frequency band.
The LMV225, as used in the suggested application block diagram in Figure 5, provides two different
functions. The first function is related to the output RF power control as mentioned previously. The second
function is to determine the supply voltage of the RF power amplifier. The next section of this article is
going to deal with the second function of the LMV225.
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Switcher or DC/DC Converter
In general, a switcher used for this application has a Pulse Width Modulation (PWM) Mode and a Bypass
Mode. The switcher normally operates in PWM mode to improve the efficiency of the handset. In PWM
mode, the programmable output voltage is a function of VCON. Equation 3 shows the relationship between
the programmable output (SW) and control voltage (VCON) of LM3200.
VOUT = 3 • VCON
(3)
Texas Instruments has switchers for RF Power Amplifiers that are perfect for this application. The LM3200
is capable of generating a dynamically variable output voltage between 0.8V and 3.6V with load currents
up to 300 mA in PWM mode and 500 mA in Bypass Mode.
4
AN-1438 A Simple Method to Reduce DC Power Consumption in CDMA RF
Power Amplifiers Through the LMV225 and an Efficient Switcher
SNOA466A – January 2006 – Revised May 2013
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Design Considerations
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VDD = 2.8V
A2
Enable Logic
B2
Analog Baseband/DSP
I/I
0.467V To VCON
Enable
Bypass Mode
23.7 k:
50:
PGND
C
D
D
S
SW
PWM PMOS
FB
L
VOUT
VCC & VBIAS
S
Bypass PMOS
VBattery = 2.8V to 4.2V
-
+
G
R1
FB
PWM NMOS
S
G
G
PVIN
B1
VCON
SGND
D
LMV225
VCON = 0.467V
Logic
And
Control
VDD
A1
PVIN
POUT
VREF = 2.85V or GND
VCONT = High
for
High Power Mode
Figure 5. Efficiency Enhancement Circuit Diagram
7
Design Considerations
After a brief discussion of each building block in this application, we are ready to move to an explanation
of the design procedure.
Assume that we are requested to design simple efficiency enhancement circuitry for an IS-95 RF power
amplifier. The maximum output RF power level is +28 dBm and the LMV225 is used as an RF power
detector. The switcher’s Programmable Output Voltage equation would be Equation 3.
Figure 3 is the handset PA’s probability graph and will be used as the efficiency optimization guideline.
This probability graph reveals that the CDMA RF power amplifier operates at +15 dBm output power and
below most of the time. If we can reduce the DC power consumption of the CDMA RF power amplifier in
this operating range, the handset will save significant battery energy and then talk time will be longer.
The simplest solution is to set the supply voltage, VCC, of the CDMA RF power amplifier to be the lowest
level possible when the output RF power is +15 dBm and below.
Figure 2 shows the CDMA RF power amplifier performance at two different supply voltages, VCC = 3.4V
and VCC = 1.4V. The 1 dB compression point at VCC = 3.4V is about +28 dBm and at VCC = 1.4V it is about
+20 dBm. The graph includes plots of the 3rd order intermodulation distortions for both cases.
A typical CDMA RF power amplifier can pass the ACPR requirements for a small output power level all
the way to a +28 dBm power level with VCC = 3.4V as specified in its datasheet. In the case of VCC = 3.4V,
the 3rd order intermodulation distortion level is 28 dBc below the fundamental, C/3IM = –28 dBc, at POUT =
+28 dBm. In the case of VCC = 1.4V, the 3rd order intermodulation distortion is 30 dBc below the
fundamental, C/3IM = –30 dBc, at POUT = +15 dBm.
SNOA466A – January 2006 – Revised May 2013
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AN-1438 A Simple Method to Reduce DC Power Consumption in CDMA RF
Power Amplifiers Through the LMV225 and an Efficient Switcher
Copyright © 2006–2013, Texas Instruments Incorporated
5
Application Circuit
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Since ACPR is a function of intermodulation distortion, we can predict that the ACPR at POUT = +15 dBm
with VCC = 1.4V should be as good as that at POUT = +28 dBm with VCC = 3.4V. Based on this information
and the statistics in Figure 3, we can reduce the use of the battery of the CDMA RF power amplifier by
setting its VCC = 1.4 for power levels from +15 dBm and below.
Figure 6 shows the DC power consumption of the supply voltage at VCC = 3.4V and VCC = 1.4V and it
demonstrates the saving of battery energy. The operating point ‘A’ is POUT = +15 dBm when VCC = 3.4V; its
PDC can be found to be +27 dBm from the secondary Y-axis. When the supply voltage is changed to VCC =
1.4V, the operating point for POUT = +15 dBm is ‘AA.’ Its PDC is +22.5 dBm.
Therefore, the power saving from VCC = 3.4 to VCC = 1.4V is 27 − 22.5 = 4.5 dB. This 4.5 dB power saving
corresponds to more than 50% saving in power.
OUTPUT RF POWER (dBm)
33
A
30
31
25
VCC = 1.4V
20
29
B
27
15
10
25
AA
PDC @ 3.4V
5
0
23
BB
VCC = 3.4V
-5 dB
21
-5
-6 dB
-10
19
PDC @ 1.4V
-15
17
3rd Order
-20
-50
-40
-30
-20
-10
0
DC POWER CONSUMPTION (dBm)
35
40
35
15
10
INPUT RF POWER (dBm)
Figure 6. POUT and PDC vs. PIN
8
Application Circuit
Figure 5 is the proposed application circuit for reducing the use of battery energy in a CDMA RF power
amplifier. We set the control voltage of the switcher to be VCON = 0.467V. This 0.467V can be obtained
from a voltage divider in the supply voltage of VDD = 2.8V. This 0.467V will generate a VOUT = 3 * 0.467 =
1.4V according to Equation 3. This VOUT = 1.4V is then supplied to the VCC of the RF power amplifier.
When POUT = +15 dBm and below, we need to set the switcher in PWM mode by setting BYPASS = Low.
The LMV225 is used to determine if the switcher needs to be in Bypass mode. We use R1 = 1.8kΩ as a
tapping resistor to achieve 31 dB coupling between the output of the RF power amplifier and the input of
the LMV225. Figure 4 is the LMV225 response vs. POUT of an RF power amplifier. At POUT = +15 dBm, the
detected voltage VDET = 1.45V.
In this application circuit, the base-band chip needs to check the value of VDET. When VDET is above 1.45V,
the base-band chip will set the switcher in Bypass mode by sending a logic high signal to BYPASS.
9
Power Saving at 10 dBm
Here is another illustration of battery saving. The operating point at ‘B’ is POUT = +10 dBm with VCC = 3.4V.
At this supply voltage level, the PDC for POUT = +15 dBm is about 26 dBm. If we lower the supply voltage to
VCC = 1.4V, the operating point becomes ‘B’ and the PDC for POUT = +15 dBm is about 20 dBm. This shows
a 6 dB saving in power or 75% less power in watts.
10
Conclusion
We have demonstrated the flexibility and benefits of using Texas Instruments LMV225 together with a
switcher in reducing battery energy consumption in a CDMA RF power amplifier. By adding this simple
circuitry, we can save 50% of DC power consumption of the CDMA RF power amplifier at the most
common operating points of IS-95 and CDMA2000 handsets.
6
AN-1438 A Simple Method to Reduce DC Power Consumption in CDMA RF
Power Amplifiers Through the LMV225 and an Efficient Switcher
SNOA466A – January 2006 – Revised May 2013
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