The LMV248 as a RF Power Amplifier (PA) Controller

The LMV248, a member of National Semiconductor’s family

of RF power amplifier (PA) power controllers, is used to

regulate the RF transmit power in TDMA (GSM, EGSM,

PCN, PCS AND DCS) mobile phones. Capable of controlling

up to two RF output amplifiers and of triple band operation

(GSM, DCS, and PCS) the device supports both bipolar and

FET RF power amplifiers including Silicon BJT, CMOS, SiGe

HBT, and GaAs HBT.

Figure 1 shows the basics of a typical LMV248 dual band

application. The key components are:

•

Two power amplifiers, usually for the GSM or DCS/PCS

bands.

•

RF directional couplers where two single or one dual

channel RF coupler could be used.

•

Up to three Schottky RF detector diodes, one for each

directional coupler output and one for temperature com-

pensation.

•

A RF diplexer.

•

A dual or tri-band antenna.

The block diagram in

Figure 2 shows the LMV248’s internal

architecture. The LMV248 contains input filters and condi-

tioning amplifiers, an input summing network, detector bias-

ing current source, error amplifier, output band select func-

tion, and output drivers.

Power Supplies

The LMV248 supports a single supply with the battery volt-

age at V

Digital Input Signals:

The LMV248 has two digital control signals:

•

Transmit enable signal, Tx_En.

•

Band select signal, BS.

The band select pin, BS, selects which band (i.e which

output and input channel) is active. A high enables Out_A, a

low enables Out_B. The transmit enable pin, Tx_En, is used

to enable the BS selected output during transmit (Tx) slots

and disable the outputs during receive (Rx) slots. Disabling

the output during the receive (Rx) slot shuts down the

LMV248’s output stages and the RF power amplifiers, con-

serving battery power. A high input on the transmit enable

signal, Tx_En, brings the amplifier out of shutdown within

about 4µs. The output is glitch-free when enabled by this pin.

When an output is either not selected by BS or Tx_En is low,

its level is near GND. Internally, the band select pin, BS,

selects the correct input line and output amplifier, and places

the external compensation network across the active ampli-

fier using analog switches.

Error Amplifier/Loop Compensation

The error amplifier (A1) controls the overall loop regulation

and response. Frequency compensation and stabilization of

the RF output power regulating loop is accomplished by a

capacitor (or resistor/capacitor network) across Comp1 and

Comp2 of the LMV248. This external network sets the

closed loop frequency response. In most cases this network

will simply be a capacitor. Conventional loop stability tech-

niques can be used in selecting this network, such as Bode

plots.

Analog Inputs

At VfA and VfB are voltages proportional to the RF power

output of channel A and channel B respectively. Each of

these signals is derived from the RF output power via a RF

directional coupler a capacitor and a Schottky RF detector

diode. A single two-channel RF coupler could be used in-

stead of the two single-channel RF couplers shown in

Figure

1.

If only one input and output is needed, just connect BS high

or low and use the selected channel. At the TC input, a

reference diode identical to the detector diodes, and ther-

mally coupled to them, is used for temperature compensa-

tion of the VfA and VfB signals.

VfA, VfB and TC are referenced to V

internal to the LMV248 connects to the VfA, VfB and TC pins

to bias the diodes. The quiescent voltage on all three pins is

one diode drop below V

the RF output power. The recommended Ramp voltage

range for RF power control is 0.2V to 1.8V. The Ramp input

will tolerate voltages from 0V to V

damage. This signal usually comes from the baseband con-

troller’s DAC (digital to analog converter), its shape being

defined by the relevant GSM, PCN, or DCS standard.

The Ramp input does not change the output level from the

idle level set by V

so offset voltages in the DAC or amplifier supplying the

Ramp signal will not cause excess RF signal output and

increased power consumption. An internal RC filter with a

corner frequency of approximately 1.6MHz smooths the

Ramp signal, to eliminate step discontinuities at the base-

band DAC’s output. Ramp is ground referenced, so supply

variations are rejected.

V

threshold voltage of the RF power amplifier. The variable

V

as well as boost output voltage to minimize phase delay at

low power levels. The V

reference, resistive voltage divider, or DAC output. The rec-

ommended V

to 2V. The V

without malfunction or damage. The minimum output voltage

at Out_A or Out_B set by V

V

the minimum voltage of the output not selected by BS.

For maximum performance a fine adjust is needed for

V

voltage is slightly different from the nominal datasheet value.

Analog Outputs

Outputs Out_A and Out_B are driven by rail-to-rail amplifiers

capable of both sourcing and sinking. Either output can

source 6mA and sink 2mA with less than 200mV voltage

drop over recommended operating conditions. The output is

free from glitches when enabled by Tx_En. When an output

is not selected by BS, its level is near GND.

Understanding the LMV248

Input Structure

The LMV248 simplified block diagram of

Figure 2 shows the

IC’s internal structure. Input VfA goes through a resistor

(approximately 48.5k

Ω) and analog switch to the inverting

input of the error amplifier A1. Input VfB goes through a

different resistor (also approximately 48.5k

Ω) and analog

switch to the inverting input of the error amplifier A1. These

two analog switches are controlled by BS. VfA is selected

when BS is high and VfB is selected when BS is low. The

temperature compensation signal, TC, goes through a series

resistance of about 48.5k

Ω, to the non-inverting input of the

error amplifier A1. This way the relatively large static DC

offset of the input signal is mostly removed from the differ-

ential signal to be amplified by A1, and the temperature

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