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MAX2240EBL Datasheet(PDF) 7 Page - Maxim Integrated Products |
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MAX2240EBL Datasheet(HTML) 7 Page - Maxim Integrated Products |
7 / 10 page RF Input The internal input stage impedance matching network is integrated on the MAX2240, so it is possible to directly connect a 50 Ω transmission line to RFIN. No external matching is required. Output Stage (RF Output) The output stage of the MAX2240 power amplifier is the collector of a transistor. The DC bias and impedance matching network are off-chip as shown in the Typical Application Circuit. An off-chip external network, as with most PA ICs, is used to achieve higher efficiency and output power than is typically achieved using low-Q on-chip match- ing elements. Optimum output power and efficiency are achieved with a particular impedance on the output at the operating frequencies of interest and a short at the RF harmonic frequencies. This impedance is specified relative to a reference plane at the amplifier output into the matching network and load. This is the impedance that achieves the output power and current consump- tion listed in the electrical specifications. It is shown below in the chart and table of Figure 1. The primary power-matching structure is a lowpass net- work formed by the series transmission line section T1 and the open-stub transmission line section T2. The transmission line network acts like a series inductance and shunt capacitance. T1 and T2 are expressed as electrical lengths of a particular characteristic imped- ance line, but could be designed with different imped- ance lines. Choose the length of T2 to provide a short at the 2nd harmonic frequency of the fundamental, and significantly attenuate its amplitude at the output—1/4 wave at the 2nd harmonic frequency of 4.9GHz. The 3rd harmonic is attenuated through the clever use of the parasitic capac- itance in the choke. This capacitance rolls off the choke impedance at higher frequencies and appears as a low impedance at the 3rd harmonic frequency. The output series capacitor is used as a DC-blocking capacitor and a final matching element. A value of 10pF is recommended. As explained in the Power-Supply Considerations sec- tion, for proper DC biasing, the PA requires a connection to VCC through an inductor, serving as a choke. Locate the inductor on the load side of transmission line T1. The recommended inductor value is 22nH. However, its value is not critical but must provide an impedance that is several hundred ohms. Choose an inductor with a self- resonant frequency at or slightly below 2.4GHz. The inductor Q is not critical; a moderate Q (>25) is suffi- cient. Remember to provide sufficient current-handling capability for the inductor, in this case at least 200mA. Also, a 220pF bypass capacitor is recommended at the supply voltage end of the inductor. Layout Design the layout for the PA IC to be as compact as possible to minimize the magnitude of parasitics. The chipscale IC package uses a bump pitch of 0.5mm (19.7 mil) and bump diameter of 0.3mm (~12 mil). Therefore, lay out the solder pad spacing on 0.5mm (19.7 mil) cen- ters, use a pad size of 0.25mm (~10 mil) and a solder mask opening of 0.33mm (13 mil). Round or square pads are permissible. (Refer to the Maxim document, Wafer Level Ultra-Chipscale Packaging for additional detailed information on UCSP layout and handling.) Connect multiple vias from the ground plane as close to the ground pins as possible. As already described, locate the capacitors as close as possible to the IC supply voltage pin or supply end of the series inductor. Place the ground end of these capacitors near the IC GND pins to provide a low- impedance return path for the signal current. MATCHING IMPEDANCE FOR RFOUT PIN 1: (15.5 Ω + J16.7Ω), 2.4GHz 2: (15.0 Ω + 19.0Ω), 2.5GHz 1 2 2.5GHz, +20dBm Power Amplifier IC in UCSP Package _______________________________________________________________________________________ 7 0.59 0.57 2.5 2.45 2.4 FREQUENCY (GHz) 136 15.0 138 15.2 140 15.5 < Γ | Γ | R X 19.0 17.9 16.7 Figure 1. Output Impedance 0.58 MATCHING IMPEDANCE SMITH CHART |
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