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Final Electrical Specifications
LTC1757A-1/LTC1757A-2 Single/Dual Band RF Power Controllers
FEATURES
s s s s
DESCRIPTIO
April 2000
s
s s s s s s s s s s
Dual Band RF Power Amplifier Control (LTC1757A-2) Improved Internal Schottky Diode Detector Wide Input Frequency Range: 850MHz to 2GHz Autozero Cancels Initial Offsets and Temperature Dependent Offset Errors Wide VIN Range of 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 RF PA Supply Current Limiting Battery Overvoltage Protection Power Control Signal Overvoltage Protection Low Operating Current: 1mA Very Low Shutdown Current: < 1A Available in a 8-Pin MSOP Package (LTC1757A-1) and 10-Pin MSOP (LTC1757A-2)
The LTC(R)1757A-2 is a dual band RF power controller for RF power amplifiers operating in the 850MHz to 2GHz range. The LTC1757A is pin compatible with the LTC1757 but has improved RF detection range. The input voltage range is optimized for operation from a single lithium-ion cell or 3x NiMH. Several functions required for RF power control and protection are integrated in one small 10-pin MSOP package, thereby minimizing PCB area. The LTC1757A-1 is a single output RF power controller that is identical in performance to the LTC1757A-2 except that one output (VPCA) is provided. The LTC1757A-1 can be used to drive a single RF channel 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 voltage and 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 < 1A.
APPLICATIO S
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Single and Dual Band GSM Cellular Telephones PCS Devices Wireless Data Modems TDMA Cellular Telephones
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
68 VIN 33pF Li-Ion SHDN BSEL
LTC1757A-2 Dual Band Cellular Telephone Transmitter
LTC1757A-2 1 2 3 4 5 VIN RF SHDN BSEL GND VCC VPCA VPCB TXEN PCTL 10 9 8 7 6 900MHz TXEN RF PA DIRECTIONAL COUPLER DIPLEXER
DAC
1.8GHz /1.9GHz
RF PA
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.
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50
1757A TA01
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1
LTC1757A-1/LT1757A-2
ABSOLUTE
AXI U
RATI GS
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)
PACKAGE/ORDER I FOR ATIO
TOP VIEW VIN RF SHDN GND 1 2 3 4 8 7 6 5 VCC VPCA TXEN PCTL
ORDER PART NUMBER LTC1757A-1EMS8 MS8 PART MARKING LTPL
VIN RF SHDN BSEL GND 1 2 3 4 5
MS8 PACKAGE 8-LEAD PLASTIC MSOP
TJMAX = 150C, JA = 250C/W
Consult factory for Industrial and Military grade parts.
ELECTRICAL CHARACTERISTICS
PARAMETER VIN Operating Voltage IVIN Shutdown Current IVIN Autozero Current IVIN Operating Current IVCC Current Limit VIN to VCC Resistance VPCA/B VOL VPCA/B Dropout Voltage VPCA/B Voltage Clamp VPCA/B Output Current VPCA/B Enable Time VPCA/B Bandwidth VPCA/B Load Capacitance VPCA/B Slew Rate VPCA/B Droop VPCA/B TXEN Start Voltage SHDN Input Threshold TXEN, BSEL Input Threshold SHDN = LO, TXEN = LO CONDITIONS (Note 7)
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 3.6V, SHDN = TXEN = HI, unless otherwise noted.
MIN
q q q q
SHDN = LO, TXEN = LO, BSEL = LO SHDN = HI, TXEN = LO SHDN = HI, TXEN = HI, IVPCA = IVPCB = 0mA, VPCA/B = HI
TXEN = HI, Open Loop, PCTL = - 100mV ILOAD = 5.5mA, VIN = 2.7V RLOAD = 400 VPCA/B = 2.4V, VIN = 2.7V VPCTL = 2V Step, CLOAD = 100pF (Note 5) CLOAD = 100pF, RLOAD = 400 (Note 9) (Note 6) VPCTL = 2V Step, CLOAD = 100pF (Note 3) VIN = 2.7V, VPCTL = 2V Step Open Loop, TXEN Low to High, CLOAD = 100pF (Note 10) VIN = 2.7V to 6V, TXEN = LO VIN = 2.7V to 4.7V
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(Note 1)
IVPCA/B, 25% Duty Cycle ...................................... 20mA Operating Temperature Range (Note 2) ................................................. - 30C to 85C Storage Temperature Range ................ - 65C to 150C Maximum Junction Temperature ........................ 125C Lead Temperature (Soldering, 10 sec)................ 300C
TOP VIEW 10 9 8 7 6 VCC VPCA VPCB TXEN PCTL
ORDER PART NUMBER LTC1757A-2EMS MS10 PART MARKING LTPM
MS10 PACKAGE 10-LEAD PLASTIC MSOP TJMAX = 125C, JA = 250C/W
TYP
MAX 6 1
UNITS V A mA mA A m V V V mA ns
2.7 0.9 1 2.2 90
1.5 1.6 150 0.1 VIN - 0.28
q q q q
0 2.7 5.5 250 1.5 400 2.85 9 200 400 3 10 550
3.0
q
550 100
kHz pF V/s V/ms
700 1.4 1.4
mV V V
q q
0.35 0.35
LTC1757A-1/LTC1757A-2
ELECTRICAL CHARACTERISTICS
PARAMETER SHDN, TXEN, BSEL Input Current PCTL Input Voltage Control Range PCTL Input Voltage Range PCTL Input Resistance PCTL Input Filter Autozero Range Autozero Settling Time (tS) RF Input Frequency Range RF Input Power Range RF DC Input Resistance VIN Overvoltage Range BSEL Timing CONDITIONS
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 3.6V, SHDN = TXEN = HI, unless otherwise noted.
MIN
q q q q
TYP 30
MAX 50 2 2.4
UNITS A V V k MHz mV s MHz dBm dBm V ns ns
SHDN, TXEN or BSEL = 3.6V VIN = 2.7V to 4.7V, RLOAD = 400 VIN = 3V, RLOAD = 400 (Note 8) SHDN = LO, TXEN = LO VIN = 2.7V, RLOAD = 400 (Note 4) Shutdown to Enable (Autozero), VIN = 2.7V (Note 11) (Note 6) 900MHz (Note 6) 1800MHz (Note 6) Referenced to VIN, SHDN = LO, TXEN = LO VPCA/B < 0.5V, RLOAD = 400 t1, Setup Time Prior to TXEN Asserted High t2, Hold Time After TXEN is Asserted Low
10 0 50
100 1.25
150 400 50
q q q
850 - 24 -22
2000 16 16 175 5.0 200 200 250 5.4
q q
100 4.8
Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: The LTC1757A-1 and LTC1757A-2 are guaranteed to meet performance specifications from 0C to 70C. Specifications over the - 30C to 85C 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. 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: For VIN voltages greater than 4.7V, VPCA/VPCB are set low by the overvoltage shutdown. Note 8: Includes maximum DAC offset voltage and maximum control voltage. Note 9: Bandwidth is calculated using the 10% to 90% rise time equation: BW = 0.35/rise time Note 10: Measured 1s after TXEN = HI. Note 11: 50% switch point, SHDN HI = VIN, TXEN HI = VIN.
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LTC1757A-1/LT1757A-2 TYPICAL PERFOR A CE CHARACTERISTICS
PCTL REFERENCED DETECTOR OUTPUT VOLTAGE (mV)
PCTL REFERENCED DETECTOR OUTPUT VOLTAGE (mV)
RF Detector Characteristics at 900MHz
10000 VIN = 3V TO 4.4V
1000
100
10
-30C
25C 1 -22 -18 -14 -10 -6 -2 2 6 RF INPUT POWER (dBm)
PI FU CTIO S
(LTC1757A-2/LTC1757A-1)
VIN (Pin 1): Input Supply Voltage, 2.7V to 6V. VIN should be bypassed with 0.1F and 100pF ceramic capacitors. Used as return for RF 175 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 175 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): (LTC1757A-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 power until the RF detected signal equals the DAC signal. The input resistance is typically 100k.
4
UW
75C
RF Detector Characteristics at 1800MHz
10000 VIN = 3V TO 4.4V
1000
100
10
-30C
75C 25C
10 14
1757A G02
1 -24 -20 -16 -12 -8 -4 0 4 8 RF INPUT POWER (dBm)
12 16
1757A G01
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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): (LTC1757A-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 (LTC1757A-2 only). VCC (Pin 10/Pin 8): RF Power Amplifier Supply. This pin has an internal 0.050 sense resistor between VIN and VCC that senses the RF power amplifier supply current to detect overcurrent conditions.
LTC1757A-1/LTC1757A-2
BLOCK DIAGRA
10 VCC 0.02 RSENSE 0.05 METAL
68
-
CS 33pF
+
OFFSET TRIM VIN 600mV GAIN TRIM gm 50mV
2
RF
175
22pF 42k 60A 5 GND 60A
BG1 THERMAL SHUTDOWN
OPERATE SHDN 150k
W
42k 3
(LTC1757A-2)
DIPLEXER
900MHz
RF PA 50
RF PA
1.8GHz
Li-Ion
1 VIN 0.02 TXENB 100 METAL AUTOZERO
-
AZ
PA VPCA 9
OVERCURRENT
+ + - +
CAMP PB ADJ
-+
6pF ICL
-
CC 400A 140k
VPCB
8
+
RFDET 33k
VPC gm 110k
-
16.7k 33k
1.2V OVP gm BG1 1.2V BANDGAP 1.2V 600mV 33k 54.5k 12 TSDB TSDB TXENI 150k MUX CONTROL PA PB 100 12 100 173k VIN
XMT AUTOZERO 150k
SHDN
7
TXEN
6
PCTL
4
BSEL
1757A BD
5
LTC1757A-1/LT1757A-2
APPLICATIO S I FOR ATIO
Operation
The LTC1757A-2 dual band RF power control amplifier integrates several functions to provide RF power control over two 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, a VIN overvoltage detector, a bandgap reference, a thermal shutdown circuit and a multiplexer to switch the control amplifier output to either VPCA or VPCB. Band Selection The LTC1757A-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 LTC1757A-1 can be used to drive a single RF channel or dual channel module with integral multiplexer. Control Amplifier 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 signal is sampled, via a directional coupler, to close the gain control loop. When a DAC signal is applied to PCTL, the amplifier quickly servos VPCA or VPCB positive until the detected feedback signal applied to the RF pin matches the signal 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. RF Detector The internal RF Schottky diode peak detector and amplifier converts the RF feedback signal from the directional coupler to a low frequency signal. This signal is compared to the DAC signal at the PCTL pin by the control
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amplifier to close the RF power control loop. The RF pin input resistance is typically 175 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 60A and drives an on-chip peak detector capacitor of 22pF. 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 by the autozero system. 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 10V/ms provides for accurate offset cancellation over the normal 1/8 duty cycle associated with the GSM protocol. The part must be in the autozero mode for at least 50s for autozero to settle to the correct value. Protection Features The RF power amplifier is overcurrent protected by an internal sense amplifier. The sense amplifier measures the voltage across an internal 0.050 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 0.050 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 RF power amplifier is protected against excessive input supply voltages. The VIN overvoltage detector starts
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LTC1757A-1/LTC1757A-2
APPLICATIO S I FOR ATIO
to reduce VPCA or VPCB when VIN exceeds 5V. VPCA or VPCB will be reduced to 0V as VIN continues to increase by about 200mV. This gain control voltage reduction lowers the RF output power eventually reducing it to zero. The internal thermal shutdown circuit will disable the LTC1757A-2 if the junction temperature exceeds approximately 150C. The part will be enabled when the temperature falls below 140C. Modes of Operation The LTC1757A-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 <1A. VPCA and VPCB will be connected to ground via 100 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 50s 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.
LTC1757A-2 Timing Diagram
SHUTDOWN SHDN t1 BSEL TXEN PCTL VPCA VPCB START VOLTAGE
1757A TD
AUTOZERO
tS NOTE 1 START VOLTAGE
tS: AUTOZERO SETTLING TIME, 50s MINIMUM t1: BSEL CHANGE PRIOR TO TXEN, 200ns TYPICAL t2: BSEL CHANGE AFTER TXEN, 200ns TYPICAL
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.
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The feedback signal from the directional coupler and the output power will be detected by the LTC1757A-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 Autozero Enable SHDN Low High High TXEN Low Low High OPERATION Disabled Autozero Power Control
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LTC1757A-1 Description The LTC1757A-1 is identical in performance to the LTC1757A-2 except that only one control output (VPCA) is available. The LTC1757A-1 can drive a single RF channel in the 850MHz to 2GHz range or a dual RF channel module with an internal multiplexer. Several manufacturers offer dual RF channel modules with an internal multiplexer. General Layout Considerations The LTC1757A-1/LTC1757A-2 should be placed near the directional coupler. The feedback signal line to the RF pin should be a 50 transmission line with a 68 termination. If short-circuit protection is used, bypass capacitors are required at VCC.
ENABLE
t2
7
LTC1757A-1/LT1757A-2
APPLICATIO S I FOR ATIO
External Termination
The LTC1757A has an internal 175 termination resistor at the RF pin. If a directional coupler is used, it is recommended that an external 68 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 LTC1757A RF pin, then the 68 resistor must be connected to VIN since the detector is referenced to VIN. Termination components should be placed adjacent to the LTC1757A. Power Ramp Profiles The external gain associated with the RF channel can vary significantly between RF power amplifier types. The LTC1757A 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 LTC1757A operates open loop until an RF signal appears at the RF pin, at which time the loop closes
10 0 -10
RFOUT (dBc)
-20 -30 -40 -50 -60 -70 -80 -28 -18 -10 0 TIME (s) 543 553 561 571
DAC VOLTAGE
START PULSE START CODE ZERO CODE
100mV TXEN SHDN 50s MINIMUM, ALLOWS TIME FOR DAC AND AUTOZERO TO SETTLE
1757A F01
Figure 1. LTC1757A Ramp Timing
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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 LTC1757A VPCA/B outputs must quickly rise to this threshold voltage in order to meet the power/time profile. To reduce this time, the LTC1757A starts at 550mV. However, at very low power levels the PCTL input signal is small, and 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. 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 LTC1757A detector threshold. The LTC1757A will then go open loop and the output voltage at VPCA or VPCB will stop
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LTC1757A-1/LTC1757A-2
APPLICATIO S I FOR ATIO
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 LTC1757A thereby ensuring that the VPCA/B outputs will ramp to 0V. Another factor that affects power ramp sidebands is the DAC signal to PCTL. The bandwidth of the LTC1757A is not 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 2-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 100k. A typical filter scheme is shown in Figure 2.
1k DAC
1k 1nF 1nF
LTC1757A PTCL
Figure 2
RF Input Voltage Levels The LTC1757A does not actually detect RF power levels, but detects peak RF voltage levels. The maximum peak RF voltage level is 2V corresponding to 16dBm in a 50 system. The RF signal is normally supplied via a directional coupler. The directional coupler loss for the low band is typically 19dB and for the high band 14dB. The high band generally requires a 5dBm lower minimum power level and to keep the minimum RF detector voltage levels similar between both bands, the directional coupler loss is adjusted accordingly. The maximum RF input voltage or power restriction must be considered when determining coupler loss requirements. If the RF power at the directional coupler is increased due to losses after the coupler, the increased power levels must not result in excessive RF voltages at the RF pin. If 2dBm is lost after the directional coupler, then the directional coupler loss should be increased by
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2dB. For example, if the maximum output requirement is 30dBm, but 32dBm is required at the directional coupler, then the coupler loss should be at least 16dB. Remember that excessive coupler loss will degrade low power performance due to lower Schottky detector efficiencies. If the directional coupler loss cannot be easily adjusted a resistor network can be used as shown in Figure 3.
3dB ATTENUATOR LTC1757A RF 33pF R2 30 R1 180 R3 180 DIRECTIONAL COUPLER BAND 1 BAND 2 50 PLACE NEAR LTC1757A
1757A F03
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Figure 3
Demo Board The LTC1757A has a demo board available upon request. The demo board has a 900MHz and an 1800MHz RF channel controlled by the LTC1757A. 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 amplifier channels are available. LTC1757A Control Loop Stability The LTC1757A 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 gain supplied by the RF power amplifier increases the loop gain raising poles normally below the 0dB axis. The extra gain can vary significantly over input/ output power ranges, frequency, power supply, temperature and manufacturer. RF power amplifier gain control
1757A F02
9
LTC1757A-1/LT1757A-2
APPLICATIO S I FOR ATIO
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 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 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 LTC1757A 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 3.3F. 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.
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Determining External Loop Gain The external loop 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 0.85 * PCTL and the external gain contributed by the RF power amplifier and directional coupler network is 0.85 * VPCTL/VVPCA. Measuring gain in the closed loop configuration accounts for the nonlinear detector gain that is dependent on RF input voltage and frequency. The LTC1757A unity gain bandwidth specified in the data sheet assumes that the net 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 The LTC1757A control amplifier unity gain bandwidth (BW1) is typically 400kHz. The phase margin of the control amplifier is typically 86. For example to determine the external RF channel loop 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 85mV. VPCA changed from 1.498V to 1.540V to create the RF output power change required. The net external gain contributed by the RF power amplifier and directional coupler network can be calculated by dividing the 85mV change at the RF pin by the 42mV change at the VPCA pin. The net external gain would then be approximately 2. The loop bandwidth extends to 2 * BW1. If BW1 is 400kHz, the loop bandwidth increases to approximately 800kHz. External gains exceeding 6 may cause loop frequency stability problems.
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LTC1757A-1/LTC1757A-2
PACKAGE DESCRIPTIO
0.007 (0.18) 0.021 0.006 (0.53 0.015)
* 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
0.007 (0.18) 0.021 0.006 (0.53 0.015)
* 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
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Dimensions in inches (millimeters) unless otherwise noted. MS8 Package 8-Lead Plastic MSOP
(LTC DWG # 05-08-1660)
0.118 0.004* (3.00 0.102)
8
76
5
0.193 0.006 (4.90 0.15)
0.118 0.004** (3.00 0.102)
1 0.040 0.006 (1.02 0.15) 0 - 6 TYP SEATING PLANE 0.012 (0.30) 0.0256 REF (0.65) BSC
23
4 0.034 0.004 (0.86 0.102)
0.006 0.004 (0.15 0.102)
MSOP (MS8) 1098
MS10 Package 10-Lead Plastic MSOP
(LTC DWG # 05-08-1661)
0.118 0.004* (3.00 0.102)
10 9 8 7 6
0.193 0.006 (4.90 0.15)
0.118 0.004** (3.00 0.102)
12345 0.040 0.006 (1.02 0.15) 0 - 6 TYP SEATING PLANE 0.009 (0.228) REF 0.034 0.004 (0.86 0.102)
0.0197 (0.50) BSC
0.006 0.004 (0.15 0.102)
MSOP (MS10) 1098
11
LTC1757A-1/LT1757A-2
TYPICAL APPLICATIO S
Single Band Cellular Telephone Transmitter
68 VIN 33pF Li-Ion SHDN LTC1757A-1 1 2 3 4 VIN RF SHDN GND VCC VPCA TXEN PCTL 8 7 6 5 TXEN RFIN RF PA 50 DIRECTIONAL COUPLER
Using the LTC1757A-1 in a Dual Band Cellular Telephone Transmitter Without Current Limiting
68 33pF DIRECTIONAL COUPLER DIPLEXER
VIN Li-Ion SHDN
LTC1757A-1 1 2 3 4 VIN RF SHDN GND VCC VPCA TXEN PCTL 8 7 6 5 TXEN
RELATED PARTS
PART NUMBER LTC1261 LTC1550/LTC1551 LTC1731 DESCRIPTION Regulated Inductorless Voltage Inverter Low Noise Inductorless Voltage Inverter Li-Ion Linear Charge Controller COMMENTS Regulated -5V from 3V, REG Pin Indicates Regulation, Up to 15mA, Micropower Regulated Output, <1mVP-P Ripple, 900kHz Small, Thin 8-Pin MSOP, Trickle Charge, EOC Indicator, 1% Accuracy
12
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 q FAX: (408) 434-0507 q www.linear-tech.com
U
DAC
1757A TA02
RF POWER MODULE WITH MUX VCC PWRCTRL RFOUT1 900MHz
BANDSELECT RFOUT2 1800MHz RF1 IN RF2 IN 50
1757A TA03
900MHz DAC
1800MHz
1757ai LT/TP 0400 4K * PRINTED IN THE USA
(c) LINEAR TECHNOLOGY CORPORATION 2000

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