LTC1957-2EMS#PBF [Linear]
LTC1957 - Single/Dual Band RF Power Controllers with 40dB Dynamic Range; Package: MSOP; Pins: 10; Temperature Range: -40°C to 85°C;
| 型号： | LTC1957-2EMS#PBF |
| 厂家： | 
Linear |
| 描述： | LTC1957 - Single/Dual Band RF Power Controllers with 40dB Dynamic Range; Package: MSOP; Pins: 10; Temperature Range: -40°C to 85°C 电信 光电二极管 电信集成电路 |
| 文件： | 总16页 (文件大小：207K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LTC1957-1/LTC1957-2
Single/Dual Band RF Power
Controllers with 40dB Dynamic Range
U
DESCRIPTIO
FEATURES
The LTC®1957-2 is a dual band RF power controller for
poweramplifiersoperatinginthe850MHzto2GHzrange.
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 inte-
grated in one small 10-pin MSOP package, thereby
minimizing PCB area.
■
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
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.
Internal Frequency Compensation
Rail-to-Rail Power Control Outputs
Power Control Signal Overvoltage Protection
Low Operating Current: 1mA
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.
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
■
Internal and external offsets are cancelled over tempera-
ture by an autozero control loop, allowing accurate low
power programming. The shutdown feature disables the
part and reduces the supply current to <1µA.
Single and Dual Band GSM/GPRS Cellular Telephones
■
PCS Devices
■
Wireless Data Modems
■
U.S. TDMA Cellular Phones
, LTC and LT are registered trademarks of Linear Technology Corporation.
U
TYPICAL APPLICATIO
LTC1957-2 Dual Band Cellular Telephone Transmitter
68Ω
LTC1957-2
33pF
V
IN
1
2
3
4
5
10
9
V
V
IN
CC
PCA
PCB
DIRECTIONAL
COUPLER
Li-Ion
RF
V
V
DIPLEXER
8
SHDN
BSEL
SHDN
BSEL
GND
900MHz
RF PA
7
TXEN
PCTL
TXEN
6
50Ω
DAC
1.8GHz/1.9GHz
RF PA
1957 TA01
1
LTC1957-1/LTC1957-2
W W
U W
ABSOLUTE AXI U 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
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
I
VCC, 12.5% Duty Cycle.......................................... 2.5A
SHDN, TXEN, BSEL
Voltage to GND ............................ –0.3V to (VIN + 0.3V)
U W
U
PACKAGE/ORDER I FOR ATIO
ORDER PART
NUMBER
ORDER PART
NUMBER
TOP VIEW
TOP VIEW
V
1
2
3
4
5
10
9
V
V
V
TXEN
PCTL
IN
CC
PCA
PCB
V
1
2
3
4
8 V
7 V
6 TXEN
5 PCTL
IN
CC
PCA
RF
SHDN
BSEL
GND
LTC1957-1EMS8
LTC1957-2EMS
RF
SHDN
GND
8
7
6
MS8 PACKAGE
MS10 PACKAGE
10-LEAD PLASTIC MSOP
MS8 PART MARKING
LTRH
MS10 PART MARKING
LTRJ
8-LEAD PLASTIC MSOP
TJMAX = 125°C, θJA = 160°C/W
TJMAX = 125°C, θJA = 160°C/W
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
MIN
TYP
MAX
6
UNITS
V
V
Operating Voltage
Shutdown Current
Autozero Current
Operating Current
●
●
●
2.7
IN
I
I
I
I
SHDN = LO, TXEN = LO, BSEL = LO
SHDN = HI, TXEN = LO
1
µA
mA
mA
A
VIN
VIN
VIN
VCC
1
1.6
1.7
SHDN = HI, TXEN = HI, I
= I
= 0mA, V = HI
PCA/B
1.1
2.2
90
0
VPCA
VPCB
Current Limit
to V Resistance
V
V
V
V
V
SHDN = LO, TXEN = LO
150
0.1
mΩ
V
IN
CC
V
TXEN = HI, Open Loop, PCTL = –100mV
= 5.5mA, V = 2.7V
●
●
●
PCA/B OL
Dropout Voltage
I
V
IN
– 0.28
3.0
V
PCA/B
PCA/B
PCA/B
LOAD
IN
Voltage Clamp
Output Current
R
= 400Ω, PCTL = 2V, External Gain = 0.417
2.7
2.85
V
LOAD
V
V
= 2.4V, V = 2.7V
= 2.6V, V = 3V
●
●
5.5
6.0
9
10
mA
mA
PCA/B
PCA/B
IN
IN
V
V
V
V
V
V
Enable Time
Bandwidth
V
C
= 2V Step, C = 100pF (Note 5)
LOAD
●
●
440
370
650
500
100
ns
kHz
PCA/B
PCA/B
PCA/B
PCA/B
PCA/B
PCA/B
PCTL
LOAD
= 100pF, R
= 400Ω (Note 8)
280
1.2
LOAD
Load Capacitance
Slew Rate
(Note 6)
pF
V
V
= 2V Step, C
= 100pF (Note 3)
●
2.2
±1
V/µs
µV/ms
mV
PCTL
LOAD
Droop
= 2.7V, V
= 2V Step
IN
PCTL
TXEN Start Voltage
Open Loop, TXEN Low to High, C
= 100pF (Note 9)
500
600
700
LOAD
2
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
MIN
0.35
0.35
10
TYP
MAX
1.4
1.4
50
UNITS
V
SHDN Input Threshold
TXEN, BSEL Input Threshold
SHDN, TXEN, BSEL Input Current
PCTL Input Voltage Control Range
PCTL Input Voltage Range
PCTL Input Resistance
PCTL Input Filter
V
V
= 2.7V to 6V, TXEN = LO
= 2.7V to 6V
●
●
●
●
●
●
IN
IN
V
SHDN, TXEN or BSEL = 3.6V
25
µA
V
V
V
= 3V to 6V, R
= 400Ω
0
2
IN
IN
LOAD
= 3V, R
= 400Ω (Note 7)
2.4
140
V
LOAD
SHDN = LO, TXEN = LO
50
90
kΩ
kHz
mV
µs
350
Autozero Range
V
= 2.7V, R = 400Ω (Note 4)
●
●
●
400
50
IN
LOAD
Autozero Settling Time (t )
t , Shutdown to Enable (Autozero), V = 2.7V (Note 10)
S
S
IN
RF Input Frequency Range
RF Input Power Range
(Note 6)
850
2000
MHz
900MHz (Note 6)
1800MHz (Note 6)
–26
–24
16
16
dBm
dBm
RF Input Impedance
BSEL Timing
Referenced to V , SHDN = LO, TXEN = LO
●
100
200
350
Ω
IN
t , Setup Time Prior to TXEN Asserted High
200
200
ns
ns
1
t , Hold Time After TXEN is Asserted Low
2
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 5: This is the time from TXEN rising edge 50% switch point to
V = 1V.
PCA/B
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 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 3: Slew rate is measured open loop. The slew time at V
or V
is
PCA
PCB
measured between 1V and 2V.
Note 4: Maximum DAC zero-scale offset voltage that can be applied to
PCTL.
Note 10: 50% switch point, SHDN HI = V , TXEN HI = V .
IN
IN
3
LTC1957-1/LTC1957-2
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Detector Characteristics
at 900MHz
Detector Characteristics
at 1800MHz
10000
1000
100
10000
V
= 3V TO 4.4V
V
= 3V TO 4.4V
IN
IN
1000
100
10
–30°C
–30°C
75°C
75°C
25°C
25°C
10
1
1
–26 –20 –14 –8
–2
4
10
16
–24 –20 –16 –12 –8 –4
0
4
8
12 16
RF INPUT POWER (dBm)
RF INPUT POWER (dBm)
1957 G01
1957 G02
U
U
U
PI FU CTIO S
(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.
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.
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 direc-
tionalcoupler.Thefrequencyrangeis850MHzto2000MHz.
This pin has an internal 200Ω termination, an internal
Schottky diode detector and peak detector capacitor.
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.
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 inter-
nal150kpull-downresistortoensurethatthepartisinshut-
down when the drivers are in a three-state condition.
VPCA (Pin9/Pin7):PowerControlVoltageOutput.Thispin
drives an external RF power amplifier power control pin.
The maximum load capacitance is 100pF. The output is
capableofrail-to-railswingsatlowloadcurrents.Selected
when BSEL is low (LTC1957-2 only).
BSEL (Pin 4): (LTC1957-2 Only) Selects VPCA when low
and VPCB when high. This input has an internal 150k
resistor to ground.
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.
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
LTC1957-1/LTC1957-2
W
BLOCK DIAGRA
(LTC1957-2)
DIPLEXER
900MHz
RF PA
RF PA
1.8GHz/1.9GHz
50Ω
Li-Ion
10
1
V
V
IN
CC
R
SENSE
0.05Ω
0.02Ω
0.02Ω
METAL
TXENB
100Ω
AUTOZERO
METAL
68Ω
–
+
PA
PB
AZ
V
V
PCA
OVERCURRENT
9
8
ADJUSTABLE
–
+
CS
33pF
OFFSET
GAIN
TRIM
g
m
+
–
TRIM
CAMP
50mV
FILTER
V
IN
PCB
+
–
200Ω
RF
2
PROGRAMMABLE
ICL
200Ω
35k
C
C
400µA
33k
140k
110k
VPC
+
28pF
g
RFDET
m
35k
–
22k
60µA
60µA
1.2V
GND
33k
22k
5
1.2V
BG1
22k
COMPRESSION
1.2V BANDGAP
ADJUSTABLE
12Ω
BG1
TSDB
THERMAL
PB
SHUTDOWN
TSDB
TXENI
100Ω
MUX
CONTROL
OPERATE SHDN
150k
XMT AUTOZERO
150k
12Ω
100Ω
150k
PA
SHDN
TXEN
7
PCTL
BSEL
3
6
4
1957 BD
5
LTC1957-1/LTC1957-2
U
W U U
APPLICATIONS INFORMATION
Control Amplifier
Forward
Thecontrolamplifiersuppliesthepowercontrolvoltageto
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
toPCTL, theamplifierquicklyservosVPCA orVPCB 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.
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 band-
width has been reduced to 350kHz for improved rejection
of DAC noise as well as smoother ramp shaping.
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 func-
tions 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 feed-
back signal to DC, a VPCA/B overvoltage clamp, compres-
sion, a bandgap reference, a thermal shutdown circuit and
a multiplexer to switch the control amplifier output to
RF Detector
The internal RF Schottky diode peak detector and ampli-
fier converts the RF feedbackvoltage from the directional
coupler to a low frequency voltage. This voltage is com-
pared 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
widerangeofinputpower. TheSchottkydetectorisbiased
at about 60µA and drives an on-chip peak detector capaci-
tor of 28pF.
Autozero
either VPCA or VPCB
.
An autozero system is included to improve power pro-
gramming accuracy over temperature. This section can-
cels internal offsets associated with the Schottky diode
detectorandcontrolamplifier.Externaloffsetsassociated
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
Band Selection
The LTC1957-2 is designed for dual band operation. The
BSEL pin will select output VPCA when low and output
VPCB whenhigh.Forexample,VPCA couldbeusedtodrive
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
LTC1957-1/LTC1957-2
U
W U 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.
Modes of Operation
The LTC1957-2 supports three operating modes: shut-
down, 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.
Filter
There is a 350kHz filter included in the PCTL path.
In autozero mode (SHDN = High, TXEN = Low) VPCA and
Protection Features
V
PCB will remain connected to ground and the part will be
intheautozeromode.Thepartmustremaininautozerofor
The RF power amplifier is overcurrent protected by an
internalsenseamplifier. Thesenseamplifiermeasuresthe
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.
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
TXENisswitchedhigh, acquisitionwillbegin. Thecontrol
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
theoutputpowerwillbedetectedbytheLTC1957-2atthe
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.
The RF power amplifier control voltage pins are overvolt-
age protected. The VPC overvoltage clamp regulates VPCA
or VPCB to 2.85V when the gain and PCTL input combina-
tion attempts to exceed this voltage.
MODE
SHDN
Low
TXEN
Low
Low
High
OPERATION
Disabled
The internal thermal shutdown circuit will disable the
LTC1957-2 if the junction temperature exceeds approxi-
mately 150°C. The part will be enabled when the tempera-
ture falls below 140°C.
Shutdown
Autozero
Enable
High
High
Autozero
Power Control
LTC1957-2 Timing Diagram
SHUTDOWN AUTOZERO
ENABLE
SHDN
BSEL
TXEN
PCTL
t
t
2
1
t
S
t : AUTOZERO SETTLING TIME, 50µs MINIMUM
S
NOTE 1
t : BSEL CHANGE PRIOR TO TXEN, 200ns TYPICAL
1
t : BSEL CHANGE AFTER TXEN, 200ns TYPICAL
2
t
3
t : START OF RAMP AFTER TXEN IS ASSERTED HIGH,
3
START
VOLTAGE
1µs MINIMUM, 10µs MAXIMUM
V
V
PCA
PCB
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.
START
VOLTAGE
1957 TD
7
LTC1957-1/LTC1957-2
W U U
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APPLICATIO S I FOR ATIO
LTC1957-1 Description
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 depen-
dent on the RF amplifier characteristics.
The LTC1957-1 is identical in performance to the
LTC1957-2 except that only one control output (VPCA) is
available.TheLTC1957-1candriveasingleband(880MHz
to 2000MHz) or a dual RF channel module with an
internal mulitplexer. Several manufacturers offer dual RF
channel modules with an internal mulitplexer.
Power ramp sidebands and power/time are also a factor
when ramping to zero power. For RF amplifiers requiring
highcontrolvoltages,itmaybenecessarytofurtheradjust
theDACrampprofile.Whenthepowerisrampeddownthe
loopwilleventuallyopenatpowerlevelsbelowtheLTC1957
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
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 termina-
tion 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 recom-
mended 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.
10
0
–10
–20
–30
–40
–50
–60
–70
–80
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 opti-
mized to be stable with several different power amplifiers
and manufacturers. This frequency compensation gener-
ally 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
voltageappearsattheRFpin,atwhichtimetheloopcloses
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 thresh-
old voltage in order to meet the power/time profile. To
reduce this time, the LTC1957 starts at 600mV. However,
atverylowpowerlevelsthePCTLinputsignalissmall, and
–28
–18 –10
0
543
553 561
571
TIME (µs)
START
PULSE
START
CODE
ZERO
CODE
100mV
TXEN
SHDN
1957 F01
50µs MINIMUM, ALLOWS TIME FOR DAC
AND AUTOZERO TO SETTLE
Figure 1. LTC1957 Ramp Timing
8
LTC1957-1/LTC1957-2
W U U
APPLICATIO S I FOR ATIO
U
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.
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 oscil-
lations 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.
Another factor that affects power ramp sidebands is the
DAC signal to PCTL. The bandwidth of the LTC1957 may
notbelowenoughtoadequatelyfilteroutstepsassociated
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.
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. How-
ever, larger losses reduce the RF signal to the LTC1957
and detector performance may be degraded at low power
levels. (See RF Detector Characteristics.)
The power control ramp should be started in the range of
1µs to 10µs after TXEN is asserted high.
LTC1957
2k
PTCL
DAC
330pF
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
effectiveloopbandwidthfrequency. Generallyloopsusing
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.
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
storingeightrampprofiles.Theboardissuppliedpreloaded
withfourGSMpowerprofilesandfourDCSpowerprofiles
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.
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 fre-
quency is generally in the 100kHz to 10MHz range. Power
supplyrelatedoscillationsuppressionrequireslargevalue
ceramic decoupling capacitors placed close to the RF
powerampsupplypins.Therangeofdecouplingcapacitor
values is typically 1nF to 3.3µF.
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.
5) Poor layout techniques associated with the directional
coupler area may result in high frequency signals bypass-
ing the coupler. This could result in stability problems due
to the reduction in the coupler loss.
9
LTC1957-1/LTC1957-2
W U U
U
APPLICATIO S I FOR ATIO
Determining External Loop Gain and Bandwidth
For PCTL voltages <650mV, the RF detected voltage is
0.6PCTL. ForPCTLvoltages>650mV, RFdetectedvoltage
is 1.18PCTL – 0.38. This change in gain is due to an
internalcompressioncircuitdesignedtoextendthedetec-
tor range.
The external loop voltage gain contributed by the RF chan-
nelanddirectionalcouplernetworkshouldbemeasuredin
a closed loop configuration. A voltage step is applied to
PCTL and the change in VPCA (or VPCB) is measured. The
detectedvoltageis0.6•PCTLandtheexternalvoltagegain
contributed by the RF power amplifier and directional
coupler network is 0.6 • ∆VPCTL/∆VVPCA. Measuring volt-
age gain in the closed loop configuration accounts for the
nonlinear detector gain that is dependent on RF input
voltage and frequency.
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 ampli-
fier and directional coupler network can be calculated by
dividing the 60mV change at the RF pin by the 30mV
changeattheVPCA pin.Thenetexternalvoltagegainwould
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 deter-
minedfromFigures3and4. Repeattheabovevoltagegain
measurement over the full power and frequency range.
The LTC1957 unity gain bandwidth specified in the data
sheetassumesthatthenetvoltage gaincontributedbythe
RF power amplifier and directional coupler is unity. The
bandwidth is calculated by measuring the rise time be-
tween 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
TheLTC1957controlamplifierunitygainbandwidth(BW1)
is typically 370kHz. The phase margin of the control
amplifier is typically 86°.
80
70
60
50
40
180
160
140
120
100
80
80
70
60
50
40
180
160
140
120
100
80
R
C
= 2k
= 33pF
R
C
= 2k
= 33pF
LOAD
LOAD
LOAD
LOAD
PHASE
PHASE
30
30
20
10
60
40
20
10
60
40
GAIN
GAIN
0
20
0
20
–10
–20
–30
–40
–50
–60
0
–10
–20
–30
–40
–50
–60
0
–20
–40
–60
–80
–100
–20
–40
–60
–80
–100
100
1k
10k
100k
1M
10M
100
1k
10k
100k
1M
10M
FREQUENCY (Hz)
FREQUENCY (Hz)
1957 F03
1957 F04
Figure 3. Measured Open Loop Gain and Phase, PCTL < 640mV
Figure 4. Measured Open Loop Gain and Phase, PCTL > 640mV
10
LTC1957-1/LTC1957-2
W U U
U
APPLICATIO S I FOR ATIO
80
70
60
50
40
30
20
10
180
CONTROL
AMPLIFER
R
= 2k
= 33pF
LOAD
LOAD
160
140
120
100
80
C
BW1 370kHz
RF POWER AMP
G2
CONTROLLED
RF OUTPUT
POWER
V
+
PCA/B
PHASE
PCTL
G1
–
GAIN
LTC1957
H1
60
I
FB
40
20
0
RF
H2
0
1957 F05
–10
–20
–30
–40
–50
–60
RF DETECTOR
DIRECTIONAL
COUPLER
14dB to 20dB LOSS
–20
–40
–60
–80
–100
Figure 5. Closed Loop Block Diagram
100
1k
10k
100k
1M
10M
FREQUENCY (Hz)
1957 F06
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
marginthanpredictedfromtheopenloopBodeplot. Alow
frequencyACSPICEmodeloftheLTC1957powercontrol-
ler is included to better determine pole and zero interac-
tions. The user can apply external gains and poles to
determinebandwidthandphasemargin.DC,transientand
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 band-
width, therefore, use the RF input as demonstrated in the
model.
Figure 6. SPICE Model Open Loop Gain and Phase
Characteristics from RF to VPCA, PCTL < 640mV
within a customer circuit or system. Further, Linear Tech-
nology Corporation reserves the right to change these
models without prior notice.
Inallcases, thecurrentdatasheetinformationisyourfinal
design guideline, and is the only performance guarantee.
Forfurthertechnicalinformation, refertoindividualdevice
datasheets. Yourfeedbackandsuggestionsonthismodel
is appreciated.
Linear Technology Corporation hereby grants the users of
this model a nonexclusive, nontransferable license to use
this model under the following conditions:
This model (Figure 7) is being supplied to LTC users as an
aid to circuit designs. While the model reflects reasonably
closesimilaritytocorrespondingdevicesinlowfrequency
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.
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.
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
andallcomputersystems.Modelusersareherebynotified
that these models are supplied “as is”, with no direct or
impliedresponsibilityonthepartofLTCfortheiroperation
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, collat-
eral, 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
W U 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
LTC1957-1/LTC1957-2
W U U
APPLICATIO S I FOR ATIO
U
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
0.034
(0.86)
REF
0.007
(0.18)
0° – 6° TYP
SEATING
PLANE
0.009 – 0.015
(0.22 – 0.38)
0.021 ± 0.006
(0.53 ± 0.015)
0.005 ± 0.002
(0.13 ± 0.05)
0.0256
(0.65)
BSC
MSOP (MS8) 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
14
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
SEATING
PLANE
0.007 – 0.011
(0.17 – 0.27)
0.021 ± 0.006
(0.53 ± 0.015)
0.005 ± 0.002
(0.13 ± 0.05)
MSOP (MS10) 1100
0.0197
(0.50)
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
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 represen-
tationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights.
15
LTC1957-1/LTC1957-2
U
TYPICAL APPLICATION
Single Band Cellular Telephone Transmitter
68Ω
LTC1957-1
8
33pF
V
IN
1
2
3
4
DIRECTIONAL
COUPLER
V
V
CC
IN
7
6
5
Li-Ion
RF
V
PCA
TXEN
SHDN
GND
TXEN
SHDN
RF IN
RF PA
PCTL
DAC
1957 TA02
Dual Band Cellular Telephone Transmitter Without Current Limiting
68Ω
DIRECTIONAL
COUPLER
DIPLEXER
LTC1957-1
RF POWER MODULE WITH MUX
33pF
V
IN
1
2
3
4
8
7
6
5
V
V
V
CC
RFOUT1
900MHz
IN
CC
Li-Ion
RF
V
PCA
PWRCTRL
TXEN
SHDN
GND
TXEN
PCTL
BANDSELECT RFOUT2
1800MHz
SHDN
RF1 IN
RF2 IN
50Ω
1957 TA03
900MHz 1800MHz
DAC
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
Regulated –5V from 3V, REG Pin Indicates Regulation, Up to 15mA, Micropower
LTC1261
Regulated Inductorless Voltage Inverter
Low Noise Inductorless Voltage Inverter
Li-Ion Pulse Charger
LTC1550/LTC1551
LTC1730
Regulated Output, <1mV Ripple, 900kHz
P-P
Complete Pulse Charger for 1-Cell Li-Ion Battery
LTC1732
Li-Ion Linear Charger
ThinSOTTM Li-Ion Linear Charger
Complete Linear Charger for 1- and 2-Cell Li-Ion Battery
LTC1734
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
I
= 100mA, Low Noise: 20µV
OUT RMS
LTC3200/LTC3200-5
Low Noise, Regulated Charge Pump
2MHz Constant Frequency, I
ThinSOT and MSOP Packages
= 100mA, 2.7V ≤ V ≤ 4.5V,
OUT IN
ThinSOT is a trademark of Linear Technology Corporation.
1957f LT/TP 0601 2K • PRINTED IN THE USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
16
●
●
(408)432-1900 FAX:(408)434-0507 www.linear-tech.com
LINEAR TECHNOLOGY CORPORATION 2001
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