Datasheet LTC4401-1, LTC4401-2 (Analog Devices) - 7

LTC4401-1/LTC4401-2
U U W U APPLICATIONS INFORMATION Operation
coupler to a low frequency voltage. This voltage is com- pared to the DAC voltage at the PCTL pin by the control The LTC4401-X RF power control amplifier integrates amplifier to close the RF power control loop. The RF pin several functions to provide RF power control over fre- input resistance is typically 250Ω and the frequency quencies ranging from 300MHz to 2.7GHz. This product is range of this pin is 300MHz to 2700MHz for the LTC4401-1 well suited to control RF power amplifiers that exhibit slow and 300MHz to 2000MHz for the LTC4401-2. The detector turn-on times. The device also prevents damage to the RF demonstrates excellent efficiency over a wide range of power amplifier due to overvoltage conditions. These input power. The Schottky detector is biased at about functions include an internally compensated power con- 60µA and drives an on-chip peak detector capacitor of trol amplifier to control the RF output power, an autozero 28pF. section to cancel internal and external voltage offsets, an RF Schottky diode peak detector and amplifier to convert
Autozero
the RF feedback signal to DC, a VPCA/B overvoltage clamp, compression and a bandgap reference. An autozero system is included to improve power pro- gramming accuracy over temperature. This section can-
Band Selection
cels internal offsets associated with the Schottky diode detector and control amplifier. External offsets associated The LTC4401-2 is designed to drive two separate power with the DAC driving the PCTL pin are also cancelled. control lines. The BSEL pin will select VPCA when low and Offset drift due to temperature is cancelled between each VPCB when high. BSEL must be established prior to SHDN burst. The maximum offset voltage allowed at the DAC being asserted high. output is limited to 400mV. Autozeroing is performed
Control Amplifier
during a 10µs period after SHDN is asserted high. An internal timer enables the VPCA/B output after 10µs. The The control amplifier supplies the power control voltage to autozero capacitors are held and the VPCA/B pin is con- the RF power amplifier. A portion (typically – 19dB for low nected to the control amplifier output. The hold droop frequencies and –14dB for high frequencies) of the RF voltage of typically < 1µV/ms provides for accurate offset output voltage is sampled, via a directional coupler, to cancellation. The part should be shut down between close the gain control loop. When a DAC voltage is applied bursts or after multiple consecutive bursts. to PCTL, the amplifier quickly servos VPCA/B positive until the detected feedback voltage applied to the RF pin matches
Filter
the voltage at PCTL. This feedback loop provides accurate There is a 270kHz two pole filter included in the PCTL path RF power control. VPCA/B is capable of driving a 6mA load to remove DAC noise. current and 100pF load capacitor.
Protection Features Control Amplifier Compression
The RF power amplifier control voltage pin is overvoltage The gain compression breakpoints are at PCTL = 80mV protected. The V and PCTL = 160mV. Above 160mV the gain does not PCA/B overvoltage clamp regulates V change. The compression changes the feedback attenua- PCA/B to 2.9V when the gain and PCTL input combination attempts to exceed this voltage. tion thereby reducing the loop gain.
Modes of Operation RF Detector Shutdown:
The part is in shutdown mode when SHDN is The internal RF Schottky diode peak detector and ampli- low. V fier converts the RF feedback voltage from the directional PCA/B is held at ground and the power supply current is typically 10µA. 4401fa 7