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參數(shù)資料
| 型號： | LTC4400-2EMS8 |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 通信及網(wǎng)絡(luò) |
| 英文描述： | RF Power Controllers with 450kHz Loop BW and 45dB Dynamic Range |
| 中文描述： | SPECIALTY TELECOM CIRCUIT, PDSO8 |
| 封裝： | PLASTIC, MSOP-8 |
| 文件頁數(shù)： | 7/16頁 |
| 文件大?。?/td> | 186K |
| 代理商： | LTC4400-2EMS8 |
7
LTC4400-1/LTC4400-2
sn4400 4400fas
APPLICATIO
S I
N
FOR
ATIO
U
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
PCA/B
overvoltage clamp,
compression and a bandgap reference.
W
U
Band Selection
The LTC4400-2 is designed to drive two separate power
control lines. The BSEL pin will select V
PCA
when low and
V
PCB
when high. BSEL must be established prior to SHDN
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
PCA/B
positive until the detected feedback voltage applied
to the RF pin matches the voltage at PCTL. This feedback
loop provides accurate RF power control. V
PCA/B
is capable
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
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 excel
lent
efficiency over a wide range of input power. The Schottky
detector is biased at about 60
μ
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
μ
s period after SHDN is asserted high. An
internal timer enables the
V
PCA/B
output after 10
μ
s. The
autozero capacitors are held and the
V
PCA/B
pin is con-
nected to the control amplifier output. The hold droop
voltage of typically <1
μ
V/ms provides for accurate offset
cancella
tion. 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
PCA/B
overvoltage clamp regulates
V
PCA/B
to 2.9V when the gain and PCTL input combination
attempts to exceed this voltage.
Modes of Operation
Shutdown:
The part is in shutdown mode when SHDN is
low. V
PCA/B
is held at ground and the power supply current
is typically 10
μ
A.
Enable:
When SHDN is asserted high the part will auto-
matically calibrate out all offsets. This takes <10
μ
s and is
controlled by an internal delay circuit. After 10
μ
s V
PCA/B
will step up to the starting voltage of 450mV. The user can
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