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LTC4401-2 Datasheet(PDF) 7 Page - Linear Technology |
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LTC4401-2 Datasheet(HTML) 7 Page - Linear Technology |
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7 / 16 page  7 LTC4401-1/LTC4401-2 4401fa APPLICATIONS INFORMATION Operation The LTC4401-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 slow 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 VPCA/B overvoltage clamp, compression and a bandgap reference. Band Selection The LTC4401-2 is designed to drive two separate power control lines. The BSEL pin will select VPCA when low and VPCB 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, to close the gain control loop. When a DAC voltage is applied to PCTL, the amplifier quickly servos VPCA/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. VPCA/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 thereby reducing the loop gain. 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 250 Ω and the frequency range of this pin is 300MHz to 2700MHzfor the LTC4401-1 and 300MHz to 2000MHz for the LTC4401-2. The detector demonstrates excellent efficiency over a wide range of input power. The Schottky detector is biased at about 60 µA 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 µs period after SHDN is asserted high. An internal timer enables the VPCA/B output after 10µs. The autozero capacitors are held and the VPCA/B pin is con- nected to the control amplifier output. The hold droop voltage of typically < 1 µV/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 VPCA/B overvoltage clamp regulates VPCA/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. VPCA/B is held at ground and the power supply current is typically 10 µA. |
Similar Part No. - LTC4401-2 |
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Similar Description - LTC4401-2 |
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