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LTC1757A-1EMS8 Datasheet(PDF) 6 Page - Linear Technology |
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LTC1757A-1EMS8 Datasheet(HTML) 6 Page - Linear Technology |
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6 / 16 page  6 LTC1757A-1/LTC1757A-2 Operation The LTC1757A-2 dual band RF power control amplifier integrates several functions to provide RF power control over two frequencies ranging from 850MHz to 2GHz. The device also prevents damage to the RF power amplifier due to overvoltage or overcurrent conditions. These func- tions include an internally compensated power control amplifier to control the RF output power, an autozero section to cancel internal and external voltage offsets, a sense amplifier with an internal sense resistor to limit the maximum RF power amplifier current, an RF Schottky diode peak detector and amplifier to convert the RF feed- back signal to DC, a VPCA/B overvoltage clamp, a VIN overvoltage detector, a bandgap reference, a thermal shutdown circuit and a multiplexer to switch the control amplifier output to either VPCA or VPCB. Band Selection The LTC1757A-2 is designed for dual band operation. The BSEL pin will select output VPCA when low and output VPCB when high. For example, VPCA could be used to drive a 900MHz channel and VPCB a 1.8GHz/1.9GHz channel. BSEL must be established before the part is enabled. The LTC1757A-1 can be used to drive a single RF channel or dual channel module with integral multiplexer. 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 signal is sampled, via a directional coupler, to close the gain control loop. When a DAC signal is applied to PCTL, the amplifier quickly servos VPCA or VPCB 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 or VPCB are capable of driving a 5.5mA load current and 100pF load capacitor. 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 185 Ω and the frequency range of this pin is 850MHz to 2000MHz. The detector demonstrates excellent efficiency and linearity over a wide range of input power. The Schottky detector is biased at about 60 µA and drives an on-chip peak detector capaci- tor of 22pF. 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 by the autozero system. The maximum offset al- lowed at the DAC output is limited to 400mV. Autozeroing is performed when the part is in autozero mode (SHDN = high, TXEN = low). When the part is enabled (TXEN = high, SHDN = high) the autozero capacitors are held and the VPCA or VPCB pin is connected to the control amplifier output. The hold droop voltage of typically 10 µV/ms provides for accurate offset cancellation over the 1/8 duty cycle associated with the GSM protocol as well as multislot protocals. The part must be in the autozero mode for at least 50 µs for autozero to settle to the correct value. Protection Features The RF power amplifier is overcurrent protected by an internal sense amplifier. The sense amplifier measures the voltage across an internal 0.050 Ω resistor to determine the RF power amplifier current. VPCA or VPCB is lowered as this supply current exceeds 2.2A, thereby regulating the current to about 2.25A. The regulated current limit is temperature compensated. The 0.050 Ω resistor and the current limit feature can be removed by connecting the PA directly to VIN. The RF power amplifier control voltage pins are overvolt- age protected. The VPC overvoltage clamp regulates VPCA or VPCB to 2.85V when the gain and PCTL input combina- tion attempts to exceed this voltage. The RF power amplifier is protected against excessive input supply voltages. The VIN overvoltage detector starts APPLICATIO S I FOR ATIO |
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Similar Description - LTC1757A-1EMS8 |
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