LMV228SD National Semiconductor, LMV228SD Datasheet - Page 22

• Manufacturer Part Number: LMV228SD
• Manufacturer: National Semiconductor

Specifications of LMV228SD

• Pin Count: 6
• Screening Level: Industrial
• Package Type: LLP EP
• Lead Free Status / Rohs Status: Not Compliant

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Application Notes
CONFIGURING A TYPICAL APPLICATION
The LMV225/LMV226/LMV228 are power detectors in-
tended for CDMA and WCDMA applications. Power applied
at its input translates to a DC voltage on the output through
a linear-in-dB response. The LMV225 detector is especially
suited for power measurements via a high-resistive tap,
while the LMV226/LMV228 are designed to be used in com-
bination with a directional coupler. The LMV226 has an
additional output voltage buffer and therefore a low output
impedance. The key features of the devices are shown in
table 1.
In order to match the output power range of the power
amplifier (PA) with the range of the LMV225’s input, the high
resistive tap needs to be configured correctly. In case of the
LMV226/LMV228 the coupling factor of the directional cou-
pler needs to be chosen correctly.
HIGH RESISTIVE TAP APPLICATION
The constant input impedance of the device enables the
realization of a frequency independent input attenuation to
adjust the LMV225’s range to the range of the PA. Resistor
R
this attenuation (Figure 1). To minimize insertion loss, resis-
tor R
demonstrates how to determine the proper value for R
Suppose the useful output power of the PA ranges up to +31
dBm. As the LMV225 can handle input power levels up to 0
dBm. R
dB. The attenuation realized by R
resistance R
1
LMV225
LMV226
LMV228
and the 50Ω input resistance (R
FIGURE 1. Typical LMV225 Application with High
1
needs to be sufficiently large. The following example
1
should realize a minimum attenuation of 31 - 0 = 31
TABLE 1. DEVICE CHARACTERISTICS
Input Range
IN
−15 / 15
−15 / 15
−30 / 0
(dBm)
of the detector equals:
Resistive Tap
Output
Buffer
Yes
No
No
1
IN
and the effective input
High Resistive Tap
Directional Coupler
Directional Coupler
) of the device realize
Application
20076033
1
.
(1)
22
Solving this expression for R
In Figure 1, R
31.4 dB
DIRECTIONAL COUPLER APPLICATION
The LMV226/LMV228 also has a 50Ω input resistance. How-
ever, its input range differs compared to the LMV225, i.e.
−15 dBm to +15 dBm. If a typical attenuation of a directional
coupler is 20 dB, the LMV226/LMV228 can be directly con-
nected via the directional coupler to the PA without the need
of additional external attenuator (Figure 2). Different PA
ranges can be configured using couplers with other coupling
factors.
SHUTDOWN FUNCTIONALITY
The LMV225/LMV226/LMV228 RF
tions combined:
• Enable/Shutdown
• Power input
The capacitor C and the resistor R
separate the DC shutdown functionality from the AC power
measurement. The device is active when Enable = HI, oth-
erwise it is in a low power consumption shutdown mode.
During shutdown the output will be LOW.
Capacitor C should be chosen sufficiently large to ensure a
corner frequency far below the lowest input frequency to be
measured. In case of the LMV225 the corner frequency can
be calculated using:
Where R
With R
frequency of 2.8 MHz. This corner frequency is an indicative
FIGURE 2. Typical LMV226/LMV228 Application with
1
IN
= 1800Ω and C = 100 pF, this results in a corner
= 50Ω, C
1
is set to 1800Ω resulting in an attenuation of
Directional Coupler
IN
= 45 pF typical.
1
, using that R
2
IN
(Figure 1 and Figure 2)
/E
N
pins have 2 func-
IN
= 50Ω, yields:
20076046
(2)
(3)