LTC4400-2EMS8 Linear Technology, LTC4400-2EMS8 Datasheet - Page 7

• Manufacturer Part Number: LTC4400-2EMS8
• Description: IC RF PWR CNTRL 800MHZ-2GHZ8MSOP
• Manufacturer: Linear Technology

Specifications of LTC4400-2EMS8

• Rf Type: Cellular, GSM, GPRS, PCS, Wireless Modem, TDMA
• Frequency: 300MHz ~ 2GHz
• Features: Dual Output
• Package / Case: 8-TSSOP, 8-MSOP (0.118", 3.00mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIONS
Operation
The LTC4400-X RF power control amplifier integrates
several functions to provide RF power control over fre-
quencies ranging from 300MHz to 2.7GHz. This product is
well suited to control RF power amplifiers that exhibit fast
turn-on times. The device also prevents damage to the RF
power amplifier due to overvoltage conditions. These
functions include an internally compensated power con-
trol amplifier to control the RF output power, an autozero
section to cancel internal and external voltage offsets, an
RF Schottky diode peak detector and amplifier to convert
the RF feedback signal to DC, a V
compression and a bandgap reference.
Band Selection
The LTC4400-2 is designed to drive two separate power
control lines. The BSEL pin will select V
V
being asserted high.
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 or
capacitive tap, to close the gain control loop. When a DAC
voltage is applied to PCTL, the amplifier quickly servos
V
to the RF pin matches the voltage at PCTL. This feedback
loop provides accurate RF power control. V
of driving a 6mA load current and 100pF load capacitor.
Control Amplifier Compression
The gain compression breakpoints are at PCTL = 80mV
and PCTL = 160mV. Above 160mV the gain does not
change. The compression changes the feedback attenua-
tion these by reducing the loop gain.
RF Detector
The internal RF Schottky diode peak detector and ampli-
fier converts the RF feedback voltage 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
PCB
PCA/B
when high. BSEL must be established prior to SHDN
positive until the detected feedback voltage applied
U
INFORMATION
U
PCA/B
W
overvoltage clamp,
PCA
PCA/B
when low and
U
is capable
power control loop. The RF pin input resistance is typi-
cally 250 and the frequency range of this pin is 300MHz
to 2700MHz for the LTC4400-1 and 300MHz to 2000MHz
for the LTC4400-2. The detector demonstrates excellent
efficiency over a wide range of input power. The Schottky
detector is biased at about 60 A and drives an on-chip
peak detector capacitor 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 voltage allowed at the DAC
output is limited to 400mV. Autozeroing is performed
during a 10 s period after SHDN is asserted high. An
internal timer enables the V
autozero capacitors are held and the V
nected to the control amplifier output. The hold droop
voltage of typically < 1 V/ms provides for accurate offset
cancellation. The part should be shut down between
bursts or after multiple consecutive bursts.
Filter
There is a 270kHz two pole filter included in the PCTL path
to remove DAC noise.
Protection Features
The RF power amplifier control voltage pin is overvoltage
protected. The V
V
attempts to exceed this voltage.
Modes of Operation
Shutdown: The part is in shutdown mode when SHDN is
low. V
is typically 10 A.
Enable: When SHDN is asserted high the part will auto-
matically calibrate out all offsets. This takes <10 s and is
controlled by an internal delay circuit. After 10 s V
will step up to the starting voltage of 450mV. The user can
PCA/B
PCA/B
to 2.9V when the gain and PCTL input combination
is held at ground and the power supply current
LTC4400-1/LTC4400-2
PCA/B
overvoltage clamp regulates
PCA/B
output after 10 s. The
PCA/B
pin is con-
sn4400 4400fas
PCA/B
7