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MAX2245EBL-T Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX2245EBL-T Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 14 page 2.5GHz, 22dBm/20dBm Power Amplifiers with Analog Closed-Loop Power Control _______________________________________________________________________________________ 9 RF Input/SHDN RFIN/SHDN is a dual-function input for a 2.4GHz to 2.5GHz RF signal and a DC-coupled shutdown function. The input port is internally matched to 50 Ω, making it simple to interface the PAs to a 50 Ω source without external matching components. The PAs are designed to amplify input signal levels of 0 to 4dBm and, although the PAs function for input signals outside this range, out- put power and efficiency degrade. Note: Ensure that the RF signal is present at the input when the PA is enabled. If the RF signal is not present at startup, the PA functions like any closed-loop control system and automatically goes into a high-gain state, amplifying and transmitting noise. Avoid this mode of operation. The second function of the RFIN/SHDN is shutdown con- trol. A DC voltage at the input port digitally controls the on/off state with standard CMOS levels. The PA is in low- current shutdown when the DC voltage is a valid logic low and is active for a valid logic high. Connect the SHDN signal to the RFIN/SHDN through a 1k Ω resistor. Connect the RF signal to the RFIN/SHDN with a 10pF capacitor in series to block any DC from corrupting the SHDN signal. Output Matching The output structure of these nonlinear PAs is an open- collector transistor that requires external impedance matching and pullup inductance for biasing. The recom- mended output matching network is shown in the Typical Application Circuits (Figure 1). The impedance presented to the RFOUT pin is shown in Figure 2 and Table 2. This impedance is specified relative to a reference plane at the amplifier output into the matching network and load. The matching network is for impedance transformation that transforms 6 Ω to 50Ω with the specified maximum output power. The network also forms a lowpass filter that provides attenuation for the 2nd and 3rd harmonics. A shunt capacitor (C7) is needed to perform the transforma- tion, and the inductive 50 Ω transmission line (T2) is need- ed to match that capacitance. A larger capacitor can be used to increase the maximum output power, but the transmission line also must be increased to maintain a match with C7. A DC-blocking capacitor (C6) of 5pF to 10pF is necessary between the PA output and the trans- mission line. The pullup inductance from RFOUT to VCC serves three main purposes: it resonates out the capacitive PA output, provides biasing for the output stage, and becomes a high-frequency choke to reduce RF energy from coupling into VCC. The pullup inductance normally is a 50 Ω trans- mission line (T1); however, chip inductors can be used instead. The typical application circuit terminates the transmission line with a capacitor (C6). Analog Power Control (PC) The PAs use a closed-loop power-control system for consistent output power across input power, supply volt- age, and temperature. Output power is internally moni- tored and compared to the desired setting on PC. The control amplifier then adjusts the first-stage variable-gain amplifier until the output power matches the desired set- ting. The result is that the output power is controlled by the voltage applied to PC. The power-control voltage range at PC for the MAX2244/MAX2246 is 0 to 2V. Output power remains at its minimum for VPC between 0 and 0.4V. At approxi- mately 0.4V, output power increases exponentially until VPC = 2V, where output power is 22dBm (MAX2244) or 20dBm (MAX2246). See Figures 3a and 3c for the rela- tionship between VPC and output power for the MAX2244 and MAX2246, respectively. Likewise, the MAX2245 output power is controlled by VPC, but with a different power-control range. The power- control voltage range of the MAX2245 is 0 to 2.2V, with output power beginning to increase when VPC = 0.9V. Figure 3b shows the VPC and output power relationship for the MAX2245. MATCHING IMPEDANCE FOR RFOUT PIN MAX2244: 6.26 Ω + j13.56Ω AT 2.45GHz MAX2245: 6.35 Ω + j13.25Ω AT 2.45GHz MAX2246: 5.50 Ω + j13.50Ω AT 2.45GHz SMITH CHART Figure 2. Impedance of Matching Network at RFOUT Pin MAX2244 MAX2245 MAX2246 FREQUENCY GHz REAL ( Ω) IMAG ( Ω) REAL ( Ω) IMAG ( Ω) REAL ( Ω) IMAG ( Ω) 2.40 6.47 13.2 6.61 12.94 5.73 13.01 2.45 6.26 13.5 6.35 13.25 5.50 13.50 2.50 6.06 13.9 6.11 13.59 5.27 14.02 Table 2. Matching Network Impedance |
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