Enhancing CDMA PA Power Using Linear-in-dB Log Amp RF Power Detector and High-Efficiency Switcher

JasonYoo

Enhancing CDMA PA Power Using Linear-in-dB Log Amp RF Power Detector and High-Efficiency Switcher [Copy link]

　　The demand for higher data rates is driving the migration of mobile communication systems from 2G to 3G. The higher data rates in these systems have added more performance and specification requirements to mobile phone RF designs.

　　In order to achieve the highest bandwidth efficiency in the occupied frequency band, the third-generation mobile communication system adopts linear modulation methods to improve spectrum utilization efficiency, such as orthogonal phase shift keying, 8-phase shift keying and orthogonal amplitude modulation.

　　These non-fixed envelope modulation methods require the use of linear RF power amplifiers in the transmission path to maintain good adjacent channel power rejection ratio (ACPR) and error vector modulation (EVM) characteristics. The typical linear RF power amplifier used by CDMA adopts a Class A structure to meet the linearity requirements. The efficiency of the textbook Class A amplifier is about 30% at the 1dB compression point. The power efficiency of the Class A amplifier decreases when it operates below the 1dB compression point.

　　In IS-95 and CDMA2000 systems, the RF power amplifier generally operates at a back-off point of 6dB to 40dB from the peak power or 1dB compression point. (This means that it operates 6dB to 40dB below the 1dB compression point.) Therefore, the RF power amplifier is in a very low efficiency state most of the time. However, the RF power amplifier is the largest power consumption component in the mobile phone. Studies have shown that in typical mobile phone applications, the RF power amplifier consumes 20% to 40% of the total power consumption.

　　So, we know the extreme importance of reducing RF power consumption, only in this way can we achieve longer mobile phone battery life or longer talk time.

　　This article presents a simple power tracking technique for improving the efficiency of an RF power amplifier. The technique uses a linear-in-dB RF power detector, LMV225, and a DC-DC converter switch. This improved method switches the DC supply voltage (VCC) of the RF power amplifier at two different output power levels through a DC-DC converter. The RF power detector LMV225 determines the supply voltage of the RF power amplifier. This technique can be used with an off-the-shelf CDMA2000 RF power amplifier to improve the energy efficiency of the mobile phone.

　　RF Power Amplifier

　　RF power amplifier is the core of such applications. RF power amplifier and DC-DC converter together form the efficiency improvement circuit of power amplifier. SKY77152 is a very common CDMA2000 RF power amplifier product on the market. According to its product specification, it can have more than 40% power increase efficiency, PAE, near the 1dB compression point.

　　CDMA RF power amplifiers usually have two supply voltage pins, VCC and VBIAS, as shown in Figure 1. There is also a reference voltage pin, usually called VREF. VREF must be 2.85V in any case. The power amplifier can be shut down by setting VREF to ground level. Most CDMA RF power amplifiers have two operating modes: high power mode and low power mode. The VCONT pin can be used to set the operating mode of the power amplifier. When the RF output signal is high, the CDMA RF power amplifier needs to work in high power mode to maintain proper distortion performance. If the output signal level is relatively low, the CDMA RF power amplifier can be switched to low power mode. However, this switching has a side effect that the phase shift of the two signal paths is different. This may cause problems with baseband processing and correction.

Figure 1: CDMA RF power amplifier

　　Figure 2 shows the typical POUT and PIN characteristics of a CDMA RF power amplifier when both the DC supply voltages VCC and VBIAS are reduced. The figure shows that the output RF power can be obtained by reducing the DC supply voltage of the RF power amplifier.

Figure 2: Linearity of a CDMA RF power amplifier

　　Power increase efficiency

　　The DC to RF efficiency (or power added efficiency, PAE) is defined as follows:

　　Formula 1:

　　The DC power consumption is defined as:

　　Formula 2:

　　Although all RF power amplifier manufacturers specify the peak DC to RF efficiency of the power amplifier at maximum output power, in reality the RF power amplifier itself rarely operates at this peak power level. In mobile phone applications, high peak power plays an important role in heat dissipation. On the other hand, when the output RF power is lower, the PAE of the RF power amplifier will also decrease.

　　In a battery-powered mobile phone, the probability distribution of the output RF power (as shown in Figure 3) should be used to estimate the average efficiency of the mobile system. This average efficiency represents the ability of the mobile system to convert battery energy into usable transmit power in actual operation.

　　Figure 3: RF output power distribution

　　As shown in Figure 3, most of the time, the output power of the RF power amplifier of an IS-95 mobile phone is less than POUT = +15dBm. Therefore, it is meaningful to improve the PAE of the RF power amplifier at small signal levels.

　　Formula 1 and Formula 2 show a concept: by reducing the supply voltage VCC of the RF power amplifier, the DC power consumption PDC can be reduced.

　　It may seem that improving the PAE of an RF PA is a simple matter, but there are several important specifications to consider when reducing the RF PA supply voltage. These include ACPR, EVM, and the transition time from one supply voltage to another.

　　Adjacent Channel Power Rejection Ratio

　　The adjacent channel power rejection ratio, or ACPR, is defined as the ratio of the average power at an offset frequency to the average power at the transmission frequency. Table 1 shows the performance requirements for CDMA2000. Although the air interfaces of IS-95 or IS-98 do not have formal requirements for ACPR like CDMA2000, it is still recommended that mobile RF designers check whether their designs meet the specifications in Table 1. Poor ACPR values indicate that the transmission path is not linear enough, so the RF signal is distorted before entering the base station's receiver.

　　Table 1: Adjacent Channel Power Rejection Ratio (ACPR)

　　Since CDMA2000 RF signals will show different peak-to-average envelopes in different channel structures, RF signals with high crest factors will cause more interference to adjacent channels. When testing RF power amplifiers, the worst-case measurement method should be used for high crest factor channel structures.

　　Power Detector

　　The RF power detector uses the RF output signal to produce a rectified DC voltage that is used to determine the output voltage of the DC-DC converter or switcher.

　　In this application, the LMV225 was chosen because it provides 40 dB of linear-in-dB detection from 0 dBm up to -40 dBm.

　　Mobile phone RF power control is very important to ensure the smooth operation of CDMA systems. Since all users share the same RF band, such as 1.25MHz in IS-95, each user appears as random noise to other users. Therefore, the transmit power of each user must be carefully controlled to prevent any user from interfering with other users in the same RF band.

　　When used according to the application block diagram, the LMV225 has two different functions:

　　The first function is the RF output power control mentioned earlier.
　　The second function is to determine the supply voltage of the RF power amplifier. The following section of this article will discuss the second function of the LVM225.

　　Switcher or DC-DC Converter

　　The switch enables the phone to use the battery's full voltage operating range, so the phone can maintain peak performance even when the battery is nearly fully discharged (below 3V).

　　Typically, the switchers in this type of application use pulse width modulation mode (PWM) and bypass mode (BYPASS). Usually the switcher operates in PWM mode to improve the efficiency of the mobile phone. In PWM mode, the programmable output voltage is a function of VCON. Equation 3 shows the relationship between the LM3200 programmable output voltage (SW) and the control voltage (VCON).

　　National Semiconductor's RF power amplifier switchers are perfectly suited for this type of application. One of the latest products is the LM3200. The LM3200 can generate a dynamically variable output voltage between 0.8V and 3.6V, with a load current of up to 300mA in PWM mode and 500mA in bypass mode.

　　Design considerations

　　After a brief discussion of each functional block in this application, we can turn to a description of the design steps.

　　Assume that we want to design a simple efficiency enhancement circuit for an IS-95 RF power amplifier. The maximum RF output power is +28dBm, and LMV225 is used as the RF power detector. Equation 3 is the programmable output voltage equation of the switcher.

　　Figure 3 is a probability diagram of the output of the mobile phone power amplifier, which can be used as a guide for efficiency optimization. This conceptual diagram shows that most of the time, the CDMA RF power amplifier operates at an output power of +15dBm or less. If we reduce the DC power consumption of the CDMA RF power amplifier within this operating range, the mobile phone can save considerable battery energy consumption and extend the use time.

　　The simplest way is to set the supply voltage of the CDMA RF power amplifier to the lowest value, when the output RF power is below +15dBm.

　　Figure 4 shows the performance of the CDMA RF power amplifier at two different supply voltages (VCC = 3.4 V and VCC = 1.4 V). The 1dB compression point is about +28 dBm at VCC = 3.4 V and about +20 dBm at VCC = 1.4 V. The figure also shows the third-order intermodulation distortion for both cases.

Figure 4: LMV225 sense voltage vs. POUT

　　According to the data sheet, when VCC=3.4V, the CDMA RF power amplifier can be used at all powers from low power to +28dBm. When VCC=3.4V, the third-order intermodulation distortion level is 28dBc lower than the basic level when POUT=+28dBm, C/3IM=-28dBc. When VCC=1.4V, the third-order intermodulation distortion level is 30dB lower than the basic level when POUT=+15dBm, C/3IM=-30dBc.

　　Since ACPR is a function of intermodulation distortion, we can predict that the ACPR of POUT=+15 when VCC=1.4V is as good as POUT=+28dBm when VCC=3.4V. Based on this and the statistics in Figure 3, we can set the supply voltage of the CDMA RF power amplifier to VCC=1.4 when the power is below 15dBm, thereby reducing battery consumption.

　　Figure 5 shows the DC power consumption when the supply voltage is VCC=3.4V and VCC=1.4V, confirming the power saving effect. Operating point A is POUT=+15dBm when VCC=3.4V, and its PDC can be read on the second Y-axis. Its PDC is +27dBm. When the supply voltage is changed to VCC=1.4V, the operating point of POUT=+15dBm becomes AA. Its PDC is +22.5dBm.

Figure 5: POUT and PDC compared to PIN

　　Therefore, the energy saving effect after the supply voltage is changed from VCC=3.4 to VCC=1.4V is: 27-22.5=4.5dB. The effect of 4.5dB is equivalent to saving 50% of the energy.

　　Application Circuit

　　Figure 6 is an application circuit for reducing battery power consumption of CDMA RF power amplifier. We set the control voltage of the switcher to VCON=0.467V. This 0.467V can be obtained from the power supply of VDD=2.8V using a voltage divider. According to formula 3, this 0.467V will generate a VOUT=3*0.467=1.4V. Then, VOUT=1.4V is supplied to the VCC of the RF power amplifier. When POUT=+15dBm or lower, we need to set BYPASS=Low to put the switcher in PWM mode.

Figure 6: Power boost circuit diagram

　　LM225 is used to detect whether the switch needs to be in Bypass mode. We use R1 = 1.8kW as the tap resistor to achieve 31dB coupling between the RF power amplifier output and the LMV225 input. Figure 4 is the relationship curve between the LMV225 response and the RF power amplifier POUT. When POUT = +15dBm, the detection voltage VDET = 1.45V.

　　In this application circuit, the baseband chip checks the value of VDET. When VDET is higher than 1.45V, the baseband chip sends a logic high signal to BYPASS to put the switch into Bypass mode.

　　Energy saving at 10dBm

　　This is another example of saving battery. Operating point B is POUT=+10dBm at VCC=3.4V. At this supply voltage, PDC for POUT=+15dBm is about 26dBm. If we reduce the supply voltage to VCC=1.4V, the operating point becomes BB, and PDC for POUT=+15dBm is about 20dBm. That is a 6dB power saving, or a 75% reduction in power consumption wattage.

　　Summarize

　　We have demonstrated the flexibility and benefits of using the LMV225 in conjunction with a switcher to reduce battery drain in CDMA RF PAs. By adding this simple circuit, we can save 50% of the DC power consumption of a CDMA RF PA at the operating points commonly used in most IS-95 and CDMA2000 cell phones.