LMV225TL datasheet(16/21 Pages) NSC

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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.

TABLE 1. DEVICE CHARACTERISTICS

Input Range

(dBm)

Output

Buffer

Application

LMV225

−30 / 0

High Resistive Tap

LMV226

−15 / 15

Yes

Directional Coupler

LMV228

−15 / 15

Directional Coupler

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

1 and the 50

Ω input resistance (R

IN) of the device realize

this attenuation (Figure 1). To minimize insertion loss, resis-

tor R

1 needs to be sufficiently large. The following example

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

1 should realize a minimum attenuation of 31-0=31

dB. The attenuation realized by R

1 and the effective input

resistance R

IN of the detector equals:

(1)

Solving this expression for R

1, using that RIN =50

Ω, yields:

(2)

In Figure 1,R

1 is set to 1800

Ω resulting in an attenuation of

31.4 dB

DIRECTIONAL COUPLER APPLICATION

The LMV226/LMV228 also has a 50

Ω 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

IN/EN pins have 2 func-

tions combined:

•

Enable/Shutdown

•

Power input

The capacitor C and the resistor R

2 (Figure 1 and Figure 2)

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:

(3)

Where R

IN =50

Ω,C

IN = 45 pF typical.

With R

1 = 1800

Ω and C = 100 pF, this results in a corner

frequency of 2.8 MHz. This corner frequency is an indicative

20076033

FIGURE 1. Typical LMV225 Application with High

Resistive Tap

20076046

FIGURE 2. Typical LMV226/LMV228 Application with

Directional Coupler

LMV225TL Datasheet(PDF) 16 Page - National Semiconductor (TI)

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