LTC1957-1/LTC1957-2 Single/Dual Band RF Power Controllers with 40dB Dynamic Range U DESCRIPTIO FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Dual Band RF Power Amplifier Control (LTC1957-2) Internal Schottky Diode Detector with Improved Dynamic Range vs the LTC1757A Wide Input Frequency Range: 850MHz to 2GHz Autozero Loop Cancels Offset Errors and Temperature Dependent Offsets Wide VIN Range: 2.7V to 6V Allows Direct Connection to Battery RF Output Power Set by External DAC Fast Acquire After Transmit Enable Internal Frequency Compensation Rail-to-Rail Power Control Outputs Power Control Signal Overvoltage Protection Low Operating Current: 1mA Very Low Shutdown Current: < 1µA Available in a 8-Pin MSOP Package (LTC1957-1) and 10-Pin MSOP (LTC1957-2) Pin Compatible with the LTC1757A-X Improved Start Voltage Accuracy and Range Improved PCTL Input Filtering U APPLICATIO S ■ ■ ■ Single and Dual Band GSM/GPRS Cellular Telephones PCS Devices Wireless Data Modems U.S. TDMA Cellular Phones The LTC1957-1 is a single output RF power controller that is identical in performance to the LTC1957-2 except that one output (VPCA) is provided. The LTC1957-1 can be used to drive a single RF or dual channel module with integral multiplexer. This part is available in an 8-pin MSOP package. RF power is controlled by driving the RF amplifier power control pins and sensing the resultant RF output power via a directional coupler. The RF sense voltage is peak detected using an on-chip Schottky diode. This detected voltage is compared to the DAC voltage at the PCTL pin to control the output power. The RF power amplifier is protected against high supply current and high power control pin voltages. Internal and external offsets are cancelled over temperature by an autozero control loop, allowing accurate low power programming. The shutdown feature disables the part and reduces the supply current to < 1µA. , LTC and LT are registered trademarks of Linear Technology Corporation. U ■ The LTC®1957-2 is a dual band RF power controller for power amplifiers operating in the 850MHz to 2GHz range. The input voltage range is optimized for operation from a single lithium-ion cell or 3× NiMH. Several functions required for RF power control and protection are integrated in one small 10-pin MSOP package, thereby minimizing PCB area. TYPICAL APPLICATIO LTC1957-2 Dual Band Cellular Telephone Transmitter 68Ω VIN 33pF LTC1957-2 1 2 Li-Ion SHDN BSEL 3 4 5 VIN VCC RF VPCA SHDN VPCB BSEL TXEN GND PCTL 10 DIRECTIONAL COUPLER 9 8 7 900MHz DIPLEXER RF PA TXEN 6 50Ω DAC 1.8GHz /1.9GHz RF PA 1957 TA01 1 LTC1957-1/LTC1957-2 W W W AXI U U ABSOLUTE RATI GS (Note 1) VIN to GND ............................................... – 0.3V to 6.5V VPCA, VPCB Voltage ..................................... – 0.3V to 3V PCTL Voltage ............................... – 0.3V to (VIN + 0.3V) RF Voltage ........................................ (VIN – 2.2V) to 7V IVCC, Continuous ....................................................... 1A IVCC, 12.5% Duty Cycle .......................................... 2.5A SHDN, TXEN, BSEL Voltage to GND ............................ – 0.3V to (VIN + 0.3V) IVPCA/B, 25% Duty Cycle ...................................... 20mA Operating Temperature Range (Note 2) . – 30°C to 85°C Storage Temperature Range ................ – 65°C to 150°C Maximum Junction Temperature ........................ 125°C Lead Temperature (Soldering, 10 sec)................ 300°C U U W PACKAGE/ORDER I FOR ATIO ORDER PART NUMBER TOP VIEW VIN RF SHDN GND 1 2 3 4 8 7 6 5 VCC VPCA TXEN PCTL LTC1957-1EMS8 ORDER PART NUMBER TOP VIEW VIN RF SHDN BSEL GND 1 2 3 4 5 10 9 8 7 6 VCC VPCA VPCB TXEN PCTL LTC1957-2EMS MS8 PACKAGE 8-LEAD PLASTIC MSOP MS8 PART MARKING MS10 PACKAGE 10-LEAD PLASTIC MSOP MS10 PART MARKING TJMAX = 125°C, θJA = 160°C/W LTRH TJMAX = 125°C, θJA = 160°C/W LTRJ Consult LTC Marketing for parts specified with wider operating temperature ranges. ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 3.6V, SHDN = TXEN = HI, unless otherwise noted. PARAMETER CONDITIONS VIN Operating Voltage IVIN Shutdown Current MIN ● SHDN = LO, TXEN = LO, BSEL = LO ● IVIN Autozero Current SHDN = HI, TXEN = LO ● IVIN Operating Current SHDN = HI, TXEN = HI, IVPCA = IVPCB = 0mA, VPCA/B = HI TYP 2.7 IVCC Current Limit MAX UNITS 6 V 1 µA 1 1.6 mA 1.1 1.7 mA 150 mΩ 2.2 SHDN = LO, TXEN = LO VPCA/B VOL TXEN = HI, Open Loop, PCTL = –100mV ● VPCA/B Dropout Voltage ILOAD = 5.5mA, VIN = 2.7V ● VPCA/B Voltage Clamp RLOAD = 400Ω, PCTL = 2V, External Gain = 0.417 ● 2.7 2.85 VPCA/B Output Current VPCA/B = 2.4V, VIN = 2.7V VPCA/B = 2.6V, VIN = 3V ● ● 5.5 6.0 9 10 VPCA/B Enable Time VPCTL = 2V Step, CLOAD = 100pF (Note 5) ● 440 650 ns VPCA/B Bandwidth CLOAD = 100pF, RLOAD = 400Ω (Note 8) ● 280 370 500 kHz ● 1.2 2.2 V/µs ±1 µV/ms VPCA/B Load Capacitance (Note 6) VPCA/B Slew Rate VPCTL = 2V Step, CLOAD = 100pF (Note 3) VPCA/B Droop VIN = 2.7V, VPCTL = 2V Step VPCA/B TXEN Start Voltage Open Loop, TXEN Low to High, CLOAD = 100pF (Note 9) 2 90 A VIN to VCC Resistance 0 0.1 V VIN – 0.28 V 3.0 V mA mA 100 500 600 700 pF mV LTC1957-1/LTC1957-2 ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 3.6V, SHDN = TXEN = VIN, unless otherwise noted. PARAMETER CONDITIONS SHDN Input Threshold VIN = 2.7V to 6V, TXEN = LO ● 0.35 MIN TXEN, BSEL Input Threshold VIN = 2.7V to 6V ● 0.35 SHDN, TXEN, BSEL Input Current SHDN, TXEN or BSEL = 3.6V ● 10 PCTL Input Voltage Control Range VIN = 3V to 6V, RLOAD = 400Ω ● 0 PCTL Input Voltage Range VIN = 3V, RLOAD = 400Ω (Note 7) ● PCTL Input Resistance SHDN = LO, TXEN = LO ● VIN = 2.7V, RLOAD = 400Ω (Note 4) ● Autozero Settling Time (tS) tS, Shutdown to Enable (Autozero), VIN = 2.7V (Note 10) ● RF Input Frequency Range (Note 6) ● RF Input Power Range 900MHz (Note 6) 1800MHz (Note 6) 50 PCTL Input Filter Autozero Range TYP 25 90 MAX UNITS 1.4 V 1.4 V 50 µA 2 V 2.4 V 140 kΩ 350 RF Input Impedance Referenced to VIN, SHDN = LO, TXEN = LO BSEL Timing t1, Setup Time Prior to TXEN Asserted High t2, Hold Time After TXEN is Asserted Low Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: The LTC1957-1 and LTC1957-2 are guaranteed to meet performance specifications from 0°C to 70°C. Specifications over the – 30°C to 85°C operating temperature range are assured by design, characterization and correlation with statistical process controls. Note 3: Slew rate is measured open loop. The slew time at VPCA or VPCB is measured between 1V and 2V. Note 4: Maximum DAC zero-scale offset voltage that can be applied to PCTL. ● kHz 400 mV µs 50 850 2000 MHz – 26 –24 16 16 dBm dBm 100 200 Ω 350 200 200 ns ns Note 5: This is the time from TXEN rising edge 50% switch point to VPCA/B = 1V. Note 6: Guaranteed by design. This parameter is not production tested. Note 7: Includes maximum DAC offset voltage and maximum control voltage. Note 8: Bandwidth is calculated using the 10% to 90% rise time: BW = 0.35/rise time Note 9: Measured 1µs after TXEN = HI. Note 10: 50% switch point, SHDN HI = VIN, TXEN HI = VIN. 3 LTC1957-1/LTC1957-2 U W Detector Characteristics at 900MHz 10000 VIN = 3V TO 4.4V 1000 100 –30°C 10 75°C 25°C 1 –26 –20 –14 –8 –2 4 RF INPUT POWER (dBm) 10 16 1957 G01 U U U PI FU CTIO S Detector Characteristics at 1800MHz 10000 VIN = 3V TO 4.4V 1000 100 –30°C 10 25°C 75°C 1 –24 –20 –16 –12 –8 –4 0 4 8 RF INPUT POWER (dBm) 12 16 1957 G02 (LTC1957-2/LTC1957-1) VIN (Pin 1): Input Supply Voltage, 2.7V to 6V. VIN should be bypassed with 0.1µF and 100pF ceramic capacitors. Used as return for RF 200Ω termination. RF (Pin 2): RF Feedback Voltage from the Directional Coupler. Referenced to VIN. A coupling capacitor of 33pF must be used to connect to the ground referenced directional coupler. The frequency range is 850MHz to 2000MHz. This pin has an internal 200Ω termination, an internal Schottky diode detector and peak detector capacitor. SHDN (Pin 3): Shutdown Input. A logic low on the SHDN pin places the part in shutdown mode. A logic high places the part in autozero when TXEN is low. SHDN has an internal 150k pull-down resistor to ensure that the part is in shutdown when the drivers are in a three-state condition. BSEL (Pin 4): (LTC1957-2 Only) Selects VPCA when low and VPCB when high. This input has an internal 150k resistor to ground. GND (Pin 5/Pin 4): System Ground. PCTL (Pin 6/Pin 5): Analog Input. The external power control DAC drives this input. The amplifier servos the RF 4 PCTL REFERENCED DETECTOR OUTPUT VOLTAGE (mV) PCTL REFERENCED DETECTOR OUTPUT VOLTAGE (mV) TYPICAL PERFOR A CE CHARACTERISTICS power until the RF detected signal equals the DAC signal. The input impedance is typically 90kΩ. TXEN (Pin 7/Pin 6): Transmit Enable Input. A logic high enables the control amplifier. When TXEN is low and SHDN is high the part is in the autozero mode. This input has an internal 150k resistor to ground. VPCB (Pin 8): (LTC1957-2 Only) Power Control Voltage Output. This pin drives an external RF power amplifier power control pin. The maximum load capacitance is 100pF. The output is capable of rail-to-rail swings at low load currents. Selected when BSEL is high. VPCA (Pin 9/Pin 7): Power Control Voltage Output. This pin drives an external RF power amplifier power control pin. The maximum load capacitance is 100pF. The output is capable of rail-to-rail swings at low load currents. Selected when BSEL is low (LTC1957-2 only). VCC (Pin 10/Pin 8): RF Power Amplifier Supply. This pin has an internal 0.050Ω sense resistor between VIN and VCC that senses the RF power amplifier supply current to detect overcurrent conditions. LTC1957-1/LTC1957-2 W BLOCK DIAGRA (LTC1957-2) DIPLEXER 900MHz RF PA RF PA 1.8GHz/1.9GHz 50Ω Li-Ion 10 1 VCC VIN RSENSE 0.05Ω METAL 0.02Ω 0.02Ω TXENB 100Ω METAL 68Ω AUTOZERO – PA AZ OVERCURRENT – VPCA + ADJUSTABLE 9 CS + 33pF OFFSET TRIM 2 RF 200Ω + gm GAIN TRIM 50mV VIN –+ PB CAMP FILTER VPCB – 8 PROGRAMMABLE ICL 200Ω CC 400µA 33k 35k 28pF 35k 140k VPC + gm RFDET 110k – 22k 60µA 5 60µA 1.2V GND 33k COMPRESSION 1.2V BG1 1.2V BANDGAP 22k ADJUSTABLE 22k 12Ω BG1 THERMAL SHUTDOWN TSDB PB TSDB OPERATE SHDN TXENI XMT AUTOZERO 150k 150k 3 SHDN 7 TXEN MUX CONTROL 150k 6 PCTL 12Ω PA 4 100Ω 100Ω BSEL 1957 BD 5 LTC1957-1/LTC1957-2 U W U U APPLICATIONS INFORMATION Forward Control Amplifier The LTC1957 is an improved version of the LTC1757A. The Schottky diode detector dynamic range has been extended to over 40dB. The start voltage accuracy has been improved to ±17%. The autozero hold time has been increased for applications requiring transmit times of several hundred milliseconds. The PCTL input filter bandwidth has been reduced to 350kHz for improved rejection of DAC noise as well as smoother ramp shaping. The control amplifier supplies the power control voltage to the RF power amplifier. A portion (typically – 19dB for low frequencies and –14dB for high frequencies) of the RF output voltage is sampled, via a directional coupler, to close the gain control loop. When a DAC voltage is applied to PCTL, the amplifier quickly servos VPCA or VPCB positive until the detected feedback voltage applied to the RF pin matches the voltage at PCTL. This feedback loop provides accurate RF power control. VPCA or VPCB are capable of driving a 5.5mA load current and 100pF load capacitor. Operation The LTC1957-2 dual band RF power control amplifier integrates several functions to provide RF power control over frequencies ranging from 850MHz to 2GHz. The device also prevents damage to the RF power amplifier due to overvoltage or overcurrent conditions. These functions include an internally compensated power control, amplifier to control the RF output power, an autozero section to cancel internal and external voltage offsets, a sense amplifier with an internal sense resistor to limit the maximum RF power amplifier current, an RF Schottky diode peak detector and amplifier to convert the RF feedback signal to DC, a VPCA/B overvoltage clamp, compression, a bandgap reference, a thermal shutdown circuit and a multiplexer to switch the control amplifier output to either VPCA or VPCB. Band Selection The LTC1957-2 is designed for dual band operation. The BSEL pin will select output VPCA when low and output VPCB when high. For example, VPCA could be used to drive a 900MHz channel and VPCB a 1.8GHz/1.9GHz channel. BSEL must be established before the part is enabled. The LTC1957-1 can be used to drive a single RF channel or dual channel with integral multiplexer. 6 RF Detector The internal RF Schottky diode peak detector and amplifier converts the RF feedback voltage from the directional coupler to a low frequency voltage. This voltage is compared to the DAC voltage at the PCTL pin by the control amplifier to close the RF power control loop. The RF pin input resistance is typically 200Ω and the frequency range of this pin is 850MHz to 2000MHz. The detector demonstrates excellent efficiency and linearity over a wide range of input power. The Schottky detector is biased at about 60µA and drives an on-chip peak detector capacitor of 28pF. Autozero An autozero system is included to improve power programming accuracy over temperature. This section cancels internal offsets associated with the Schottky diode detector and control amplifier. External offsets associated with the DAC driving the PCTL pin are also cancelled. Offset drift due to temperature is cancelled between each burst. The maximum offset allowed at the DAC output is limited to 400mV. Autozeroing is performed when the part is in autozero mode (SHDN = high, TXEN = low). When the part is enabled (TXEN = high, SHDN = high) the autozero capacitors are held and the VPCA or VPCB pin is connected to the control amplifier output. The hold droop voltage of typically < 1µV/ms provides for accurate offset LTC1957-1/LTC1957-2 U U W U APPLICATIONS INFORMATION cancellation over the normal 1/8 duty cycle associated with the GSM protocol as well as multislot protocols. The part must be in the autozero mode for at least 50µs for autozero to settle to the correct value. Filter There is a 350kHz filter included in the PCTL path. Protection Features The RF power amplifier is overcurrent protected by an internal sense amplifier. The sense amplifier measures the voltage across an internal 0.050Ω resistor to determine the RF power amplifier current. VPCA or VPCB is lowered as this supply current exceeds 2.2A, thereby regulating the current to about 2.25A. The regulated current limit is temperature compensated. The 0.050Ω resistor and the current limit feature can be removed by connecting the PA directly to VIN. The RF power amplifier control voltage pins are overvoltage protected. The VPC overvoltage clamp regulates VPCA or VPCB to 2.85V when the gain and PCTL input combination attempts to exceed this voltage. The internal thermal shutdown circuit will disable the LTC1957-2 if the junction temperature exceeds approximately 150°C. The part will be enabled when the temperature falls below 140°C. Modes of Operation The LTC1957-2 supports three operating modes: shutdown, autozero and enable. In shutdown mode (SHDN = Low) the part is disabled and supply currents will be reduced to <1µA. VPCA and VPCB will be connected to ground via 100Ω switches. In autozero mode (SHDN = High, TXEN = Low) VPCA and VPCB will remain connected to ground and the part will be in the autozero mode. The part must remain in autozero for at least 50µs to allow for the autozero circuit to settle. In enable mode (SHDN = High, TXEN = High) the control loop and protection functions will be operational. When TXEN is switched high, acquisition will begin. The control amplifier will start to ramp the control voltage to the RF power amplifier. The RF amplifier will then start to turn on. The feedback signal from the directional coupler and the output power will be detected by the LTC1957-2 at the RF pin. The loop closes and the amplifier output tracks the DAC voltage ramping at PCTL. The RF power output will then follow the programmed power profile from the DAC. MODE Shutdown SHDN TXEN OPERATION Low Low Disabled Autozero High Low Autozero Enable High High Power Control LTC1957-2 Timing Diagram SHUTDOWN AUTOZERO ENABLE SHDN t2 t1 BSEL TXEN tS tS: AUTOZERO SETTLING TIME, 50µs MINIMUM t1: BSEL CHANGE PRIOR TO TXEN, 200ns TYPICAL t2: BSEL CHANGE AFTER TXEN, 200ns TYPICAL t3: START OF RAMP AFTER TXEN IS ASSERTED HIGH, 1µs MINIMUM, 10µs MAXIMUM NOTE 1 PCTL t3 VPCA VPCB START VOLTAGE START VOLTAGE 1957 TD NOTE 1: THE EXTERNAL DAC DRIVING THE PCTL PIN CAN BE ENABLED DURING AUTOZERO. THE AUTOZERO SYSTEM WILL CANCEL THE DAC TRANSIENT. THE DAC MUST BE SETTLED TO AN OFFSET ≤ 400mV BEFORE TXEN IS ASSERTED HIGH. 7 LTC1957-1/LTC1957-2 U W U U APPLICATIO S I FOR ATIO General Layout Considerations The LTC1957-1/LTC1957-2 should be placed near the directional coupler. The feedback signal line to the RF pin should be a 50Ω transmission line with optional termination or a short line. If short-circuit protection is used, bypass capacitors are required at VCC. External Termination The LTC1957 has an internal 200Ω termination resistor at the RF pin. If a directional coupler is used, it is recommended that an external 68Ω termination resistor be connected between the RF coupling capacitor (33pF), and ground at the side connected to the directional coupler. If the termination is placed at the LTC1957 RF pin, then the 68Ω resistor must be connected to VIN since the detector is referenced to VIN. Termination components should be placed adjacent to the LTC1957. Power Ramp Profiles The external voltage gain associated with the RF channel can vary significantly between RF power amplifier types. The LTC1957 frequency compensation has been optimized to be stable with several different power amplifiers and manufacturers. This frequency compensation generally defines the loop dynamics that impact the power/time response and possibly (slow loops) the power ramp sidebands. The LTC1957 operates open loop until an RF voltage appears at the RF pin, at which time the loop closes and the output power follows the DAC profile. The RF power amplifier will require a certain control voltage level (threshold) before an RF output signal is produced. The LTC1957 VPCA/B outputs must quickly rise to this threshold voltage in order to meet the power/time profile. To reduce this time, the LTC1957 starts at 600mV. However, at very low power levels the PCTL input signal is small, and 8 Power ramp sidebands and power/time are also a factor when ramping to zero power. For RF amplifiers requiring high control voltages, it may be necessary to further adjust the DAC ramp profile. When the power is ramped down the loop will eventually open at power levels below the LTC1957 detector threshold. The LTC1957 will then go open loop and the output voltage at VPCA or VPCB will stop falling. If this voltage is high enough to produce RF output power, the power/time or power ramp sidebands may not meet specification. This problem can be avoided by starting the DAC ramp from 100mV (Figure 1). At the end of the cycle, the DAC can be ramped down to 0mV. This applies a negative signal to the LTC1957 thereby ensuring that the VPCA/B outputs will ramp to 0V. The 100mV ramp step 10 0 –10 RFOUT (dBc) The LTC1957-1 is identical in performance to the LTC1957-2 except that only one control output (VPCA) is available. The LTC1957-1 can drive a single band (880MHz to 2000MHz) or a dual RF channel module with an internal mulitplexer. Several manufacturers offer dual RF channel modules with an internal mulitplexer. the VPCA/B outputs may take several microseconds to reach the RF power amplifier threshold voltage. To reduce this time, it may be necessary to apply a positive pulse at the start of the ramp to quickly bring the VPCA/B outputs to the threshold voltage. This can generally be achieved with DAC programming. The magnitude of the pulse is dependent on the RF amplifier characteristics. –20 –30 –40 –50 –60 –70 –80 –28 –18 –10 0 543 553 561 571 TIME (µs) DAC VOLTAGE LTC1957-1 Description START PULSE START CODE ZERO CODE 100mV TXEN SHDN 50µs MINIMUM, ALLOWS TIME FOR DAC AND AUTOZERO TO SETTLE Figure 1. LTC1957 Ramp Timing 1957 F01 LTC1957-1/LTC1957-2 U W U U APPLICATIO S I FOR ATIO must be applied at least 4µs before TXEN is asserted high to allow the autozero to cancel the step. Slow DAC rise times will extend this time by the additional RC time constants. Another factor that affects power ramp sidebands is the DAC signal to PCTL. The bandwidth of the LTC1957 may not be low enough to adequately filter out steps associated with the DAC. If the baseband chip does not have an internal filter, it is recommended that a 1-stage external filter be placed between the DAC output and the PCTL pin. Resistor values should be kept below 2k since the PCTL input resistance is 90k. A typical filter scheme is shown in Figure 2. The power control ramp should be started in the range of 1µs to 10µs after TXEN is asserted high. LTC1957 2k PTCL DAC 330pF 1957 F02 Figure 2 Demo Board The LTC1957 demo board is available upon request. The demo board has a 900MHz and an 1800MHz RF channel controlled by the LTC1957. Timing signals for TXEN are generated on the board using a 13MHz crystal reference. The PCTL power control pin is driven by a 10-bit DAC and the DAC profile can be loaded via a serial port. The serial port data is stored in a flash memory which is capable of storing eight ramp profiles. The board is supplied preloaded with four GSM power profiles and four DCS power profiles covering the entire power range. External timing signals can be used in place of the internal crystal controlled timing. A variety of RF power amplifiers are available. LTC1957 Control Loop Stability The LTC1957 provides a stable control loop for several RF power amplifier models from different manufacturers over a wide range of frequencies, output power levels and VSWR conditions. However, there are several factors that can improve or degrade loop frequency stability. 1) The additional voltage gain supplied by the RF power amplifier increases the loop gain raising poles normally below the 0dB axis. The extra voltage gain can vary significantly over input/output power ranges, frequency, power supply, temperature and manufacturer. RF power amplifier gain control transfer functions are often not available and must be generated by the user. Loop oscillations are most likely to occur in the midpower range where the external voltage gain associated with the RF power amplifier typically peaks. It is useful to measure the oscillation or ringing frequency to determine whether it corresponds to the expected loop bandwidth and thus is due to high gain bandwidth. 2) Loop voltage losses supplied by the directional coupler will improve phase margin. The larger the directional coupler loss the more stable the loop will become. However, larger losses reduce the RF signal to the LTC1957 and detector performance may be degraded at low power levels. (See RF Detector Characteristics.) 3) Additional poles within the loop due to filtering or the turn-on response of the RF power amplifier can degrade the phase margin if these pole frequencies are near the effective loop bandwidth frequency. Generally loops using RF power amplifiers with fast turn-on times have more phase margin. Extra filtering below 16MHz should never be placed within the control loop, as this will only degrade phase margin. 4) Control loop instability can also be due to open loop issues. RF power amplifiers should first be characterized in an open loop configuration to ensure self oscillation is not present. Self-oscillation is often related to poor power supply decoupling, ground loops, coupling due to poor layout and extreme VSWR conditions. The oscillation frequency is generally in the 100kHz to 10MHz range. Power supply related oscillation suppression requires large value ceramic decoupling capacitors placed close to the RF power amp supply pins. The range of decoupling capacitor values is typically 1nF to 3.3µF. 5) Poor layout techniques associated with the directional coupler area may result in high frequency signals bypassing the coupler. This could result in stability problems due to the reduction in the coupler loss. 9 LTC1957-1/LTC1957-2 U W U U APPLICATIO S I FOR ATIO Determining External Loop Gain and Bandwidth The external loop voltage gain contributed by the RF channel and directional coupler network should be measured in a closed loop configuration. A voltage step is applied to PCTL and the change in VPCA (or VPCB) is measured. The detected voltage is 0.6 • PCTL and the external voltage gain contributed by the RF power amplifier and directional coupler network is 0.6 • ∆VPCTL/∆VVPCA. Measuring voltage gain in the closed loop configuration accounts for the nonlinear detector gain that is dependent on RF input voltage and frequency. The LTC1957 unity gain bandwidth specified in the data sheet assumes that the net voltage gain contributed by the RF power amplifier and directional coupler is unity. The bandwidth is calculated by measuring the rise time between 10% and 90% of the voltage change at VPCA or VPCB for a small step in voltage applied to PCTL. BW1 = 0.35/rise time 180 160 140 120 100 80 60 40 20 0 –20 –40 –60 –80 –100 10M PHASE GAIN 1k 10k 100k FREQUENCY (Hz) 1M Figure 3. Measured Open Loop Gain and Phase, PCTL < 640mV VOLTAGE GAIN (dB) RLOAD = 2k CLOAD = 33pF 1957 F03 10 For example, to determine the external RF channel loop voltage gain with the loop closed, apply a 100mV step to PCTL from 300mV to 400mV. VPCA (or VPCB) will increase to supply enough feedback voltage to the RF pin to cancel this 100mV step which would be the required detected voltage of 60mV. VPCA changed from 1.498V to 1.528V to create the RF output power change required. The net external voltage gain contributed by the RF power amplifier and directional coupler network can be calculated by dividing the 60mV change at the RF pin by the 30mV change at the VPCA pin. The net external voltage gain would then be approximately 2. The loop bandwidth extends to 2 • BW1. If BW1 is 370kHz, the loop bandwidth increases to approximately 740kHz. The phase margin can be determined from Figures 3 and 4. Repeat the above voltage gain measurement over the full power and frequency range. 80 70 60 50 40 30 20 10 0 –10 –20 –30 –40 –50 –60 100 180 160 140 120 100 80 60 40 20 0 –20 –40 –60 –80 –100 10M RLOAD = 2k CLOAD = 33pF PHASE GAIN 1k 10k 100k FREQUENCY (Hz) 1M PHASE (DEG) 80 70 60 50 40 30 20 10 0 –10 –20 –30 –40 –50 –60 100 PHASE (DEG) VOLTAGE GAIN (dB) The LTC1957 control amplifier unity gain bandwidth (BW1) is typically 370kHz. The phase margin of the control amplifier is typically 86°. For PCTL voltages <650mV, the RF detected voltage is 0.6PCTL. For PCTL voltages >650mV, RF detected voltage is 1.18PCTL – 0.38. This change in gain is due to an internal compression circuit designed to extend the detector range. 1957 F04 Figure 4. Measured Open Loop Gain and Phase, PCTL > 640mV LTC1957-1/LTC1957-2 U W U U CONTROL AMPLIFER BW1 ≅ 370kHz RF POWER AMP VPCA/B G1 G2 + PCTL LTC1957 H1 RF H2 1957 F05 RF DETECTOR DIRECTIONAL COUPLER 14dB to 20dB LOSS Figure 5. Closed Loop Block Diagram External pole frequencies within the loop will further reduce phase margin. The phase margin degradation, due to external and internal pole combinations, is difficult to determine since complex poles are present. Gain peaking may occur, resulting in higher bandwidth and lower phase margin than predicted from the open loop Bode plot. A low frequency AC SPICE model of the LTC1957 power controller is included to better determine pole and zero interactions. The user can apply external gains and poles to determine bandwidth and phase margin. DC, transient and RF information cannot be extracted from the present model. The model is suitable for external gain evaluations up to 6 ×. The 350kHz PCTL input filter limits the bandwidth, therefore, use the RF input as demonstrated in the model. This model (Figure 7) is being supplied to LTC users as an aid to circuit designs. While the model reflects reasonably close similarity to corresponding devices in low frequency AC performance terms, its use is not suggested as a replacement for breadboarding. Simulation should be used as a forerunner or a supplement to traditional lab testing. Users should note very carefully the following factors regarding this model: Model performance in general will reflect typical baseline specs for a given device, and certain aspects of performance may not be modeled fully. While reasonable care has been taken in the preparation, we cannot be responsible for correct application on any and all computer systems. Model users are hereby notified that these models are supplied “as is”, with no direct or implied responsibility on the part of LTC for their operation 80 70 60 50 40 30 20 10 0 –10 –20 –30 –40 –50 –60 100 1k 180 RLOAD = 2k 160 CLOAD = 33pF 140 120 PHASE 100 80 GAIN 60 40 20 0 –20 –40 –60 –80 –100 10k 100k 1M 10M FREQUENCY (Hz) PHASE (DEG) – IFB CONTROLLED RF OUTPUT POWER VOLTAGE GAIN (dB) APPLICATIO S I FOR ATIO 1957 F06 Figure 6. SPICE Model Open Loop Gain and Phase Characteristics from RF to VPCA, PCTL < 640mV within a customer circuit or system. Further, Linear Technology Corporation reserves the right to change these models without prior notice. In all cases, the current data sheet information is your final design guideline, and is the only performance guarantee. For further technical information, refer to individual device data sheets. Your feedback and suggestions on this model is appreciated. Linear Technology Corporation hereby grants the users of this model a nonexclusive, nontransferable license to use this model under the following conditions: The user agrees that this model is licensed from Linear Technology and agrees that the model may be used, loaned, given away or included in other model libraries as long as this notice and the model in its entirety and unchanged is included. No right to make derivative works or modifications to the model is granted hereby. All such rights are reserved. This model is provided as is. Linear Technology makes no warranty, either expressed or implied about the suitability or fitness of this model for any particular purpose. In no event will Linear Technology be liable for special, collateral, incidental or consequential damages in connection with or arising out of the use of this model. It should be remembered that models are a simplification of the actual circuit. 11 LTC1957-1/LTC1957-2 U W U U APPLICATIO S I FOR ATIO *LTC1957 Low Frequency AC Spice Model* GIN1 ND2 0 ND1A IFB 100E-6 GX3 ND6 0 0 ND4 1E-6 GX4 ND7 0 0 ND6 1E-6 GX1 ND3 0 0 ND2 1E-6 GX2 ND4 0 0 ND3 1E-6 GX5 ND10 0 0 ND9 1E-6 GX8 ND14 0 0 ND12 1E-6 GX7 ND12 0 0 ND11 1E-6 GX6 ND11 0 0 ND10 1E-6 GXFB IFB 0 0 ND14 28.8E-6 EX1 ND8 0 0 ND7 1 RPCTL2 ND1 0 33E3 RFILT ND1 ND1A 50E3 RO1 ND2 0 70E6 RX3 ND6 0 1E6 RX4 ND7 0 1E6 RPCTL1 PCTL ND1 53E3 RX1 ND3 0 1E6 RX2 ND4 ND5 1E6 RSD RF ND9 500 RX5 ND10 0 1E6 RT RF 0 250 RX8 ND14 0 1E6 RX7 ND12 ND13 1E6 RX6 ND11 0 1E6 R9 ND8 ND8A 100 R9A ND8A VPCA 20 RLOAD VPCA 0 2E3 RFB1 IFB 0 22E3 CPCTL1 ND1A 0 7E-12 CX3 ND6 0 8E-15 CX4 ND7 0 12E-15 CC1 ND2 0 24E-12 CX1 ND3 0 2E-15 CX5 ND10 0 10E-15 CX6 ND11 0 1.2E-15 CLOAD VPCA 0 33E-12 CLINT ND8A 0 37E-12 CLINTA VPCA 0 18E-12 CFB1 IFB 0 300E-15 CP ND9 0 28E-12 LX2 ND5 0 34E-3 LX7 ND13 0 7E-3 **Closed loop connections, comment-out VPCTLO, VRF, Adjust EFB gain to reflect external gain, currently set at 3X** *EFB RF 0 VPCA VIN 3 VIN VIN 0 DC 0 AC 1 *VPCTLO PCTL 0 DC 0 **Open loop connections, comment-out EFB, VIN and VPCTLO** VPCTLO PCTL 0 DC 0 VRF RF 0 DC 0 AC 1 **Add AC statement and print statement as required** .AC DEC 50 100 1E7 .END Figure 7. LTC1957 Low Frequency AC SPICE Model 12 RPCTL2 33E3 RPCTL1 53E3 ND1 RF CPCTL1 7E-12 RFILT 50E3 ND1A CP 28E-12 RSD 500Ω RT 200Ω – + ND9 – + 100E-6 IFB GM GIN1 GM – + 1E-6 GX5 CX5 10E-15 RX5 1E6 – – + GM 28.8E-6 GXFB RFB1 22E3 – 1E-6 GX6 1E-6 CFB1 300E-15 ND11 GM CX6 1.2E-15 RX6 1E6 130MHz POLE 24MHz POLE CX1 2E-15 + GX2 – + ND4 GM – + VAMP – + 1E-6 GX7 GM LX7 7E-3 RX7 1E6 ND13 – + GM 1E-6 GX8 CLINT 37E-12 ND8A CX3 8E-15 R9 ND8 100Ω 1E-6 RX3 1E6 ND6 20MHz POLE GX3 EX1 ND12 23MHz ZERO LX2 34E-3 RX2 1E6 ND5 5MHz ZERO Figure 8. LTC1957 Low Frequency AC Model GM 1E-6 RX1 1E6 ND3 80MHz POLE GX1 + GM ND10 16MHz POLE CC1 24E-12 RO1 70E6 ND2 GM ND14 1957 F08 RX8 1E6 CLINTA 18E-12 R9A 20Ω – + RLOAD 2E3 1E-6 GX4 CX4 12E-15 RX4 1E6 13MHz POLE CLOAD 33E-12 VPCA ND7 U U W 100Hz POLE APPLICATIO S I FOR ATIO U PCTL LTC1957-1/LTC1957-2 13 LTC1957-1/LTC1957-2 U PACKAGE DESCRIPTIO MS8 Package 8-Lead Plastic MSOP (Reference LTC DWG # 05-08-1660) 0.118 ± 0.004* (3.00 ± 0.102) 8 7 6 5 0.118 ± 0.004** (3.00 ± 0.102) 0.193 ± 0.006 (4.90 ± 0.15) 1 2 3 4 0.043 (1.10) MAX 0.007 (0.18) 0° – 6° TYP 0.021 ± 0.006 (0.53 ± 0.015) SEATING PLANE 0.009 – 0.015 (0.22 – 0.38) 0.0256 (0.65) BSC * DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE ** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE 14 0.034 (0.86) REF 0.005 ± 0.002 (0.13 ± 0.05) MSOP (MS8) 1100 LTC1957-1/LTC1957-2 U PACKAGE DESCRIPTIO MS10 Package 10-Lead Plastic MSOP (Reference LTC DWG # 05-08-1661) 0.118 ± 0.004* (3.00 ± 0.102) 10 9 8 7 6 0.118 ± 0.004** (3.00 ± 0.102) 0.193 ± 0.006 (4.90 ± 0.15) 1 2 3 4 5 0.034 (0.86) REF 0.043 (1.10) MAX 0.007 (0.18) 0° – 6° TYP 0.021 ± 0.006 (0.53 ± 0.015) SEATING PLANE 0.007 – 0.011 (0.17 – 0.27) 0.0197 (0.50) BSC 0.005 ± 0.002 (0.13 ± 0.05) MSOP (MS10) 1100 * DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE ** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 15 LTC1957-1/LTC1957-2 U TYPICAL APPLICATION Single Band Cellular Telephone Transmitter 68Ω LTC1957-1 VIN 33pF 1 2 Li-Ion 3 SHDN 4 VIN VCC RF VPCA SHDN TXEN GND PCTL 8 DIRECTIONAL COUPLER 7 6 5 TXEN RF IN RF PA DAC 1957 TA02 Dual Band Cellular Telephone Transmitter Without Current Limiting 68Ω VIN 33pF LTC1957-1 1 2 Li-Ion SHDN 3 4 VIN VCC RF VPCA SHDN TXEN GND PCTL RF POWER MODULE WITH MUX 8 VCC 7 6 PWRCTRL TXEN DIRECTIONAL COUPLER DIPLEXER RFOUT1 900MHz BANDSELECT RFOUT2 1800MHz 5 RF1 IN RF2 IN 50Ω 1957 TA03 900MHz 1800MHz DAC RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC1261 Regulated Inductorless Voltage Inverter Regulated –5V from 3V, REG Pin Indicates Regulation, Up to 15mA, Micropower LTC1550/LTC1551 Low Noise Inductorless Voltage Inverter Regulated Output, <1mVP-P Ripple, 900kHz LTC1730 Li-Ion Pulse Charger Complete Pulse Charger for 1-Cell Li-Ion Battery LTC1732 Li-Ion Linear Charger Complete Linear Charger for 1- and 2-Cell Li-Ion Battery LTC1734 ThinSOTTM Li-Ion Linear Charger Only Two External Components, Allows Charge Current Monitoring for Termination LTC1758-1/LTC1758-2 RF Power Controllers Single/Dual Channel RF Power Controllers (Lower Bandwidth Version of LTC1957-1/LTC1957-2) LT®1761 ThinSOT LDO IOUT = 100mA, Low Noise: 20µVRMS LTC3200/LTC3200-5 Low Noise, Regulated Charge Pump 2MHz Constant Frequency, IOUT = 100mA, 2.7V ≤ VIN ≤ 4.5V, ThinSOT and MSOP Packages ThinSOT is a trademark of Linear Technology Corporation. 16 Linear Technology Corporation 1957f LT/TP 0601 2K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408)432-1900 ● FAX: (408) 434-0507 ● www.linear-tech.com LINEAR TECHNOLOGY CORPORATION 2001