LM3203,LM3204,LM3205 Optimizing RF Power Amplifier System Efficiency Using DC-DC Converters Literature Number: SNVA593 POWER designer Expert tips, tricks, and techniques for powerful designs No. 110 Feature Article............1-7 Optimizing RF Power Amplifier System Efficiency Using DC-DC Converters Step-Down Switching Regulators........................2 -- By Mathew Jacob, Applications Engineering Manager 150 mA CMOS LDO ........4 Old Method RF Detector Family........6 Standard PA Power Design Tools......8 New Method PA with Supply Regulator * Output power controlled by RFIN * VCC directly connected to battery * Output power controlled by RFIN * VCC connected to DC-DC converter * VOUT is optimized for given POUT VBATT VBATT Supply Regulator VCTRL VCC VOUT VCC RFIN RFIN RFOUT RFOUT Figure 1. Old Method vs New Method RF power amplifiers used in CDMA / WCDMA cellular standards have been traditionally powered directly from the battery. This makes system implementation easy but the requirement for linear 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 higher capacity batteries. This makes system design challenging. System designers have to be very cautious and perform a power survey of each and every component 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 amplitude-modulated signal that exhibits a non-constant amplitude envelope. In order to preserve signal integrity and further spectral re-growth, a linear NEXT ISSUE: Emulated Current Mode Control Dynamic Power Management of RF Power Amplifiers Feature-Rich LM320x Family Enhances Battery Life in Portable Applications Family Features LM3205 Application Circuit * Dynamically adjustable output voltage optimizes RF PA power levels for increased battery life VIN 2.7V to 5.5V * Bypass mode maintains maximum output power regardless of battery voltage SW EN 10 F VOUT = 2.5 x VCON LM3205 * 2 MHz Switching frequency minimizes external components and complies with spectral emission requirements VCON Ideal for powering RF power amplifiers in cell phones, smart PDA phones, GPS systems, two-way radios, and portable communications systems VOUT 0.8V to 3.6V 3.3 H VDD PVIN FB 4.7 F SGND PGND RF PA System Power Savings 90 % Savings in Power/IBATT 80 Family Highlight: DC-DC converters deliver up to 5X transmit time in RF PAs 70 60 50 40 30 20 10 0 0 5 10 15 20 25 30 POUT, dBm Step-Down Switching Regulators for RF Power Amplifiers Product ID Description VIN Min Max VOUT IOUT (mA) Bypass Modes Packaging LM3200 Dynamically adjustable output voltages, 2.2 H 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 2 POWER designer Optimizing RF Power Amplifier System Efficiency profile (see Figure 2) for the modulation methods to really understand the impact of savings in powering a PA with a supply regulator. The profiles are different for urban and rural regions. 300 VIN =3. 4 V 250 Current (mA) power amplifier is necessary. However, power efficiency is traded off because power amplifiers operate efficiently when operated in gain compression. To meet the required linearity, the operating transmit power is backed off from the power amplifier'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. Battery current with new method 200 Battery current with old method >2 4 dBm, Vo = 3.25V (VBATT =3.4 V) 150 = 48 mA 100 Vo = 2.4V = 40 mA 50 Vo= 1.5V 0 0 dBm, Vo = 0.6V 0 5 10 15 20 25 30 POUT, dBm Power-Added Efficiency (PAE) is a key performance metric of a power amplifier. Figure 3. Savings in battery current when the DC-DC converter is used for powering the PA PAE (%) = ( POUT - PIN ) / Pdc It is important to consider the power probability As shown in Figure 3, the output voltage of the DC-DC converter has to be varied as the transmitted 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. 90 % Savings in Power/IBATT The key in using a DC-DC converter (PA supply regulator) is to reduce the Pdc factor in the denominator. 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. 80 70 60 50 40 30 20 10 0 0 WCDMA PA Power Probability 10 15 20 25 30 POUT, dBm 5.00 Figure 4. Percentage savings in power when the PA is powered by a PA supply regulator 4.00 p [%] 5 3.00 Voice 2.00 Data 1.00 0.00 0 5 10 15 20 25 30 POUT [dBm] 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 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 power.national.com 3 Industry's Lowest Noise 150 mA CMOS LDO LP5900 Low Dropout Regulator Requires No Bypass Capacitor Features LP5900 Output Noise * Industry's lowest noise combined with 85 dB of Power Supply Ripple Rejection guarantees signal integrity 33 30 Industry Standard Benchmark 27 Noise (V) * 25 A IQ minimizes current drain when system operates in low-power mode 36 * Elimination of bypass capacitor reduces BOM to only two ceramic 0.47 F capacitors 24 21 LP5900 18 15 12 9 6 3 * Low 80 mV dropout voltage (typ.) 0.01 0.1 1 10 100 IOUT (mA) * Virtually zero (<1A) Iq when disabled * Available in a micro SMD-4 package LP5900 PSRR * LLP(R) packaging available soon -20 -30 -40 Gain (dB) Ideal for powering analog and RF signal path ICs, including low-noise amplifiers, voltage controlled oscillators, and RF receivers -50 -60 -70 -80 -90 Product Highlight: -100 10 100 Unique 150 mA RF LDO eliminates bypass capacitor and achieves low 6.5 VRMS noise 1000 10K 100K Frequency [Hz] LP5900 Circuit Diagram 0.47 F VIN VOUT LP5900 VEN Output 0.47 F Gnd Bypass Capacitor 4 POWER designer Optimizing RF Power Amplifier System Efficiency -20 1.0 1.2 1.5 1.8 2.2 2.8 VOUT = VDD F = 1950 MHz 2.5 3.0 -25 0.8 3.2 DROPOUT ACLR1 [dBc] -30 -35 - 38 dBc (WCDMA) -40 -45 NO DC-DC, VDD = 3.5V -50 -55 -15 -10 -5 0 5 10 15 20 25 30 35 POUT [dBm] Figure 5. How ACLR is affected with respect to supply voltage to the PA and POUT systems. It is specified as the ratio of the PowerSpectral 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 sufficient 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 5 Simplify System Calibration with RF Detector Family Accurate and Stable RF Power Detectors for Portable Devices Linear in dB Performance LMV225 Output Voltage (V) 2.25 2.00 1.75 31.4 dB 1.50 LMV225 RF input power 1.25 1.00 0.75 0.50 PA output power 0.25 0.00 -50 -40 -30 -20 -10 0 10 20 30 40 Power (dBm) NO DIODE 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 Family Highlight: Real-time transmitter power adjustments simplify system calibration in communications systems Product ID Application Detector Channel LMV227 CDMA 2000, WCDMA, UMTS Log amp LMV225/226/228 CDMA, WCDMA, UMTS LMV232 3G Mobile communications 6 Range Package 1 40 dB, 2.1 GHz Micro SMD, LLP(R) Log amp 1 40 dB, 2.1 GHz Micro SMD, LLP Mean square 2 20 dB, 2.2 GHz Micro SMD POWER designer Optimizing RF Power Amplifier System Efficiency * 30 s Output slew rate and settling (50 s window in beginning of every 667 s transmit cycle in which the Vcon adjustments must be completed) In WCDMA architecture, transmit power is adjusted by 1 dB in every 667 s as requested by the basestation. 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. V1 TX_ON TX DAC R1 * R 2 * Low dropout and low ripple near 100% duty cycle Example: Low RDSON PFET 140 m (LM3205) or Bypass FET (LM3204) gives low dropout voltage and pulse-skipping schemes gives low ripple near 100% duty cycle. VCON VOUT ANT SW 3.3 VEN SW 4.7 F PGND FB VCC PA SGND LM3205 VREF VDD PVIN RF_TX * 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. Figure 6. Baseband Controls Vo Directly * High switching frequency Example: 2 MHz switching frequency helps the use of smaller sized external components and meet spectral emission requirements. In this case, the power detector is part of a closed loop and sets the output voltage. * Fast turn on time to meet time mask for transmit ON/OFF Example: LM3203 has turn-on time of 50 s for Vo = 3.4V from EN = low to high. TX_ON LDO 10 F VBATT *R1, R2 only required if V1 to VOUT gain needs to be modified V1 LMV228 TX_ON R1 R1 Power Detector * R 2 VCON VOUT ANT SW 3.3 5 0 PGND FB VCC 100% Duty Cycle vs Bypass Mode PA Coupler When the buck converter is operating at 100% duty cycle the dropout voltage is VEN SW 4.7 F SGND LM3205 VREF LDO VDD PVIN RF_TX TX_ON 10 F Dropout Voltage = (RON,P + RL) * Io, where RON,P is the RDSON of the PFET and RL is the inductor DCR. For a PA supply regulator that has a bypass FET the dropout voltage in bypass mode is, VBATT *R1, R2 only required if V1 to VOUT gain needs to be modified Figure 7. Using a Power Detector to Set Vo Dropout Voltage = (RON,BYP) * Io, Conclusion 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. 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 For more information on Powering RF Power Amplifiers, visit www.national.com/onlineseminars to watch Mathew Jacobs' online seminar! power.national.com 7 Power Design Tools WEBENCH(R) 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 Analog University(R) Expand your knowledge and understanding of analog with our free online educational training tool. analogU.national.com Don't miss a single issue! 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 Subscribe now to receive email alerts when new issues of Power Designer are available: power.national.com/designer Introducing our new Signal Path Designer ! View online today at: SM signalpath.national.com/designer (c)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. 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