LMV221SD/NOPB National Semiconductor, LMV221SD/NOPB Datasheet - Page 28

• Manufacturer Part Number: LMV221SD/NOPB
• Description: IC RF DETECTOR CDMA/WCDMA 6-LLP
• Manufacturer: National Semiconductor

Specifications of LMV221SD/NOPB

• Frequency: 50MHz ~ 3.5GHz
• Rf Type: Cellular, CDMA, CDMA2000, EDGE, GSM, GPRS, TDMA, W-CDMA
• Input Range: -45dBm ~ 5dBm
• Accuracy: 0.5dB
• Voltage - Supply: 2.7 V ~ 3.3 V
• Current - Supply: 10mA
• Package / Case: 6-LLP
• Operating Temperature (min): -40C
• Operating Temperature (max): 85C
• Operating Temperature Classification: Industrial
• Mounting: Surface Mount
• Pin Count: 6
• Package Type: LLP EP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LMV221SDTR

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Low frequency supply voltage variations due to PA switching
might result in a ripple at the output voltage. The LMV221 has
a Power Supply Rejection Ration of 60 dB for low frequencies.
4.1.2 Ground (GND)
The LMV221 needs a ground plane free of noise and other
disturbing signals. It is important to separate the RF ground
return path from the other grounds. This is due to the fact that
the RF input handles large voltage swings. A power level of
0 dBm will cause a voltage swing larger than 0.6 V
internal 50Ω input resistor. This will result in a significant RF
return current toward the source. It is therefore recommended
that the RF ground return path not be used for other circuits
in the design. The RF path should be routed directly back to
the source without loops.
4.2 RF Input Interface
The LMV221 is designed to be used in RF applications, hav-
ing a characteristic impedance of 50Ω. To achieve this
impedance, the input of the LMV221 needs to be connected
via a 50Ω transmission line. Transmission lines can be easily
created on PCBs using microstrip or (grounded) coplanar
waveguide (GCPW) configurations. This section will discuss
both configurations in a general way. For more details about
designing microstrip or GCPW transmission lines, a mi-
crowave designer handbook is recommended.
4.2.1 Microstrip Configuration
One way to create a transmission line is to use a microstrip
configuration. A cross section of the configuration is shown in
Figure 15, assuming a two layer PCB.
FIGURE 14. Recommended Board Layout
PP
, over the
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A conductor (trace) is placed on the topside of a PCB. The
bottom side of the PCB has a fully copper ground plane. The
characteristic impedance of the microstrip transmission line
is a function of the width W, height H, and the dielectric con-
stant ε
Characteristics such as height and the dielectric constant of
the board have significant impact on transmission line dimen-
sions. A 50Ω transmission line may result in impractically wide
traces. A typical 1.6 mm thick FR4 board results in a trace
width of 2.9 mm, for instance. This is impractical for the
LMV221, since the pad width of the LLP-6 package is 0.25
mm. The transmission line has to be tapered from 2.9 mm to
0.25 mm. Significant reflections and resonances in the fre-
quency transfer function of the board may occur due to this
tapering.
4.2.2 GCPW Configuration
A transmission line in a (grounded) coplanar waveguide
(GCPW) configuration will give more flexibility in terms of
trace width. The GCPW configuration is constructed with a
conductor surrounded by ground at a certain distance, S, on
the top side. Figure 16 shows a cross section of this configu-
ration. The bottom side of the PCB is a ground plane. The
ground planes on both sides of the PCB should be firmly con-
nected to each other by multiple vias. The characteristic
impedance of the transmission line is mainly determined by
r
.
FIGURE 15. Microstrip Configuration
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