LTC5505-1/LTC5505-2
6
5505f
APPLICATIO S I FOR ATIO
WUU
U
Operation
The LTC5505-X RF detector integrates several functions to
provide RF power detection over frequencies ranging from
300MHz to 3.5GHz. These functions include an internally
compensated buffer amplifier, an RF Schottky diode peak
detector and level shift amplifier to convert the RF feedback
signal to DC, a delay circuit to avoid voltage transients at
V
OUT
when coming out of shutdown and a gain compres-
sion circuit to extend the detector dynamic range.
Buffer Amplifier
The buffer amplifier has a gain of two and is capable of
driving a 2mA load. The buffer amplifier typically has an
output voltage range of 0.25V to 1.75V.
RF Detector
The internal RF Schottky diode peak detector and level
shift amplifier converts the RF input signal to a low
frequency signal. The detector demonstrates excellent
efficiency and linearity over a wide range of input power.
The Schottky detector is biased at about 60µA and drives
a peak detector capacitor of 28pF.
Gain Compression
The gain compression circuit changes the feedback ratio
as the RF peak-detected input voltage increases above
100mV. Below 100mV, the voltage gain from the peak
detector to the buffer output is 1.5. Above 200mV, the
voltage gain is reduced to 0.7. The compression expands
the low power detector range due to higher gain.
Modes of Operation
MODE SHDN OPERATION
Shutdown Low Disabled
Enable High Power Detect
Applications
The LTC5505-1 and LTC5505-2 can be used as self-
standing signal strength measuring receivers for a wide
range of input signals from –32dBm to 18dBm for fre-
quencies from 300MHz to 3.5GHz.
The LTC5505-1 and LTC5505-2 can be used as demodu-
lators for AM and ASK modulated signals with data rates
up to 5MHz. Depending on specific application needs, the
RSSI output can be split into two branches, providing
AC-coupled data (or audio) output and DC-coupled, RSSI
output for signal strength measurements and AGC.
The LTC5505-1 and LTC5505-2 can be used for dual band
mobile phone transmitter power control (refer to Typical
Application schematic on first page). The circuit uses a
capacitive tap at the Tx PA outputs. For example, a 0.3pF
capacitor (C1) followed by a 100Ω resistor (R1) forms a
coupling circuit with about a 20dB loss at the cellular band
and 18dB loss at the PCS band, referenced to the LTC5505-2
IC RF input pin. For improved coupling accuracy, the C1
capacitor should be a high tolerance component (±0.05pF.)
Example of LTC5505-X GSM/DCS Power Control Timing Diagram
5505 TD
t
1
t
1
: PART COMES OUT OF SHUTDOWN 20µs MAXIMUM PRIOR TO BURST.
t
2
: CIRCUITS POWER UP AND SETTLE.
t
3
: BASEBAND CONTROLLER STARTS RF POWER RAMP UP AT 22µs AFTER SHDN IS ASSERTED HIGH.
t
2
t
3
t
4
t
5
t
6
20µs2µs28µs 543µs28µs
SHDN
V
OUT
RF INPUT
POWER
t
4
: BASEBAND CONTROLLER COMPLETES RAMP UP.
t
5
: BASEBAND CONTROLLER STARTS RF POWER RAMP DOWN AT END OF BURST.
t
6
: LTC5505-X RETURNS TO SHUTDOWN MODE BETWEEN BURSTS
