LTC1758-1EMS8 Linear Technology, LTC1758-1EMS8 Datasheet - Page 6

• Manufacturer Part Number: LTC1758-1EMS8
• Description: IC RF PWR CONTRLLR SINGLE 8-MSOP
• Manufacturer: Linear Technology

Specifications of LTC1758-1EMS8

• Rf Type: Cellular, GSM, GPRS, 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

APPLICATIONS
LTC1758-1/LTC1758-2
Forward
The LTC1758 has a wider dynamic range than the
LTC1757A. The Schottky diode detector dynamic range
has been extended to over 40dB. The start voltage accu-
racy 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 bandwidth has been reduced to 250kHz to control
slow turn-on RF power amplifiers.
Operation
The LTC1758-2 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 V
compression, a bandgap reference, a thermal shutdown
circuit and a multiplexer to switch the control amplifier
output to either V
Band Selection
The LTC1758-2 is designed for dual band operation. The
BSEL pin will select output V
V
a 900MHz channel and V
BSEL must be established before the part is enabled. The
LTC1758-1 can be used to drive a single RF channel or
dual channel with integral multiplexer.
6
PCB
when high. For example, V
PCA
U
or V
INFORMATION
PCA/B
PCB
U
PCB
.
a 1.8GHz/1.9GHz channel.
PCA
PCA
overvoltage clamp, gain
when low and output
could be used to drive
W
U
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 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 V
until the detected feedback voltage applied to the RF pin
matches the voltage at PCTL. This feedback loop provides
accurate RF power control. V
driving a 5.5mA load current and 100pF load capacitor.
RF Detector
The internal RF Schottky diode peak detector and ampli-
fier converts the RF feedback voltage 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 200
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 60 A and drives an on-chip peak detector capaci-
tor of 28pF.
Autozero
An autozero system is included to improve power pro-
gramming accuracy over temperature. This section can-
cels 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 V
connected to the control amplifier output. The hold droop
voltage of typically < 1 V/ms provides for accurate offset
cancellation over the normal 1/8 duty cycle associated
with the GSM protocol as well as with multislot protocols.
The part must be in the autozero mode for at least 50 s for
autozero to settle to the correct value.
PCA
or V
PCA
and the frequency
PCB
PCA
or V
are capable of
or V
PCB
PCB
positive
pin is