LTC1757A-1EMS8 Linear Technology, LTC1757A-1EMS8 Datasheet - Page 7

• Manufacturer Part Number: LTC1757A-1EMS8
• Description: IC CTRLR RF POWER SINGLE 8MSOP
• Manufacturer: Linear Technology

Specifications of LTC1757A-1EMS8

• Rf Type: Cellular, GSM, PCS, Wireless Modem, TDMA
• Frequency: 850MHz ~ 2GHz
• Features: Single Output
• Package / Case: 8-TSSOP, 8-MSOP (0.118", 3.00mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIO S I FOR ATIO
to reduce V
will be reduced to 0V as V
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 approxi-
mately 150 C. The part will be enabled when the tempera-
ture falls below 140 C.
Modes of Operation
The LTC1757A-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 <1 A. V
will be connected to ground via 100 switches.
In autozero mode (SHDN = High, TXEN = Low) V
V
in the autozero mode. The part must remain in autozero for
at least 50 s 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.
PCB
will remain connected to ground and the part will be
PCA
or V
PCB
U
when V
IN
U
SHDN
continues to increase by about
BSEL
TXEN
PCTL
V
V
PCA
PCB
IN
t
t
t
NOTE 1: THE EXTERNAL DAC DRIVING THE PCTL PIN CAN BE ENABLED DURING AUTOZERO.
S
1
2
exceeds 5V. V
SHUTDOWN
: BSEL CHANGE PRIOR TO TXEN, 200ns TYPICAL
: BSEL CHANGE AFTER TXEN, 200ns TYPICAL
: AUTOZERO SETTLING TIME, 50 s MINIMUM
W
THE AUTOZERO SYSTEM WILL CANCEL THE DAC TRANSIENT. THE DAC MUST BE
SETTLED TO AN OFFSET 400mV BEFORE TXEN IS ASSERTED HIGH.
VOLTAGE
AUTOZERO
PCA
START
t
PCA
S
LTC1757A-2 Timing Diagram
U
and V
PCA
or V
ENABLE
and
PCB
PCB
t
1
The feedback voltage 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
LTC1757A-1 Description
The LTC1757A-1 is identical in performance to the
LTC1757A-2 except that only one control output (V
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 V
t
2
VOLTAGE
LTC1757A-1/LTC1757A-2
NOTE 1
START
CC
SHDN
High
High
Low
.
TXEN
High
Low
Low
1757A TD
OPERATION
Disabled
Autozero
Power Control
PCA
7
)