OPA2673 datasheet(24/40 Pages) TI

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LINE DRIVER HEADROOM MODEL

P =10

log

(1mW)

RMS

(1)

VLPP

±IP

2VLPP

1:n

RMS

(1mW)

(2)

V =CrestFactor

=CF

RMS

(3)

RMS

(4)

(V +V )

(R +R )

OPP

(8)

P ´

+(V +V )+I

(R +R )

OPP

(9)

LPP

(1mW)

(5)

±I =

4RM

LPP

(6)

R =

2LINE

(7)

OPA2673

SBOS382A – JUNE 2008 – REVISED OCTOBER 2008..................................................................................................................................................... www.ti.com

The first step in a driver design is to compute the

peak-to-peak

output

voltage

from

the

target

specifications. This calculation is done using the

following equations:

With PL power and VRMS voltage at the load, and RL

load impedance, this calculation gives:

Figure 79. Driver Peak Output Model

With the required output voltage and current versus

turns ratio set, an output stage headroom model

allows the required supply voltage versus turns ratio

to be developed.

With VP peak voltage at the load and the crest factor,

CF:

The headroom model (see Figure 80) can be

described with the following set of equations:

First, as available output voltage for each amplifier:

with VLPP: peak-to-peak voltage at the load.

Consolidating Equation 1 through Equation 4 allows

the required peak-to-peak voltage at the load function

Or, second, as required single-supply voltage:

of the crest factor, the load impedance, and the

power in the load to be expressed. Thus:

The minimum supply voltage for a set of power and

load requirements is given by Equation 9.

Table 1 gives V1, V2, R1, and R2 for +12V operation

of the OPA2673.

This VLPP is usually computed for a nominal line

impedance and may be taken as a fixed design

target.

The next step for the driver is to compute the

individual amplifier output voltage and currents as a

function of VPP on the line and transformer turns ratio.

As the turns ratio changes, the minimum allowed

supply voltage also changes. The peak current in the

amplifier is given by:

With VLPP defined in Equation 5 and RM defined in

Equation 7.

The peak current is computed in Figure 79 by noting

that the total load is 4RM and that the peak current is

Figure 80. Line Driver Headroom Model

half of the peak-to-peak calculated using VLPP.

Table 1. Line Driver Headroom Model Values

+12V

0.9V

Ω

0.9V

Ω

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