LMV248 datasheet(9/11 Pages) NSC

9 / 11 page

Application Section (Continued)

The LMV248 has separate inputs for the coupler/detector

feedback signal(s). These may be combined as a single

input when using a single coupler/detector that accommo-

dates both RF bands. When the two are combined as shown

Figure 5 internal current sources are placed in parallel. To

maintain correct temperature compensation characteristics

for the applications circuit, an external biasing resistor, R'

should be added externally. For this arrangement, VfA, VfB,

and TC all are returned to a common supply, V

CC.

Selecting Coupler/Detector

Coupler Calculations:

The applied Ramp voltage controls the RF power output.

Because of this the maximum Ramp voltage sets the mini-

mum attenuation that is allowed in the direction coupler used

to sense output power.

The basic equation is;

O =Vrms

2/R

For peak detection it is;

The coupling factor, at maximum power, is defined as C

or,

Cf=(V

rampmax)

2/2x(P

outmax xRO) for couplers that sense

peak power

The limit conditions for the maximum ramp and max power

out is,

rampmax = 2.0V

outmax = 2Watts

O =50Ω

fdB = 10 x log10 [(1/2)(Vrampmax)2/Poutmax xRO)]

fdB = −16.98dB

Attenuation in the directional coupler should be at least

−17dB, to assure maximum power out.

General Device Equations

Calculation:

Let Vf be VfA or VfB as selected by BS. Let RVf be the

external series resistance from Vf to the Detector diode.

Voltage at the non-inverting terminal of A1 V

NI = TC, since

current into A2’s input is small.

Voltage at the inverting terminal of A1 V

INV

=Vf -

[Ramp/(20k

Ω)] x (RVf + 48.5kΩ)

Assuming the external series resistance for VfA and VfB is

<< 48.5k

Ω:

Voltage at the inverting terminal of A1 V

INV = Vf – 2.42 x

Ramp

Differential voltage at A1 inputs = V

NI -VINV = TC - (Vf - 2.42

x Ramp) = 2.42 x Ramp - (Vf - TC).

Closed loop regulation of RF output power will force the

average differential voltage at A1 inputs to approach zero:

Average

Including the V

ramp deadband and the device offset voltage,

the equation for the VfA, B vs. V

ramp relationship becomes:

VfA, B = 2.42 x (V

ramp-200mV) + TC - 80mV, all variables

being ground referenced DC voltages.

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