LINER LTC1957-2 Single/dual band rf power controllers with 40db dynamic range Datasheet

LTC1957-1/LTC1957-2
Single/Dual Band RF Power
Controllers with 40dB Dynamic Range
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DESCRIPTIO
FEATURES
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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
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APPLICATIO S
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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.
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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
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LTC1957-1/LTC1957-2
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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
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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.
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LTC1957-1/LTC1957-2
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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
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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
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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
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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
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LTC1957-1/LTC1957-2
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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.
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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
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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.
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LTC1957-1/LTC1957-2
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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
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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
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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
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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
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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
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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
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