Datasheet
P =10 log´
L
(1mW) R´
L
V
RMS
2
R
M
R
M
V
LPP
n
V
LPP
R
L
±I
P
±I
P
2V
LPP
n
1:n
V =
RMS
10
P
(1mW) R 10´ ´
L
L
V =CrestFactor V =CF V
P
RMS RMS
´ ´
V =2 CF V
LP RMS
P
´ ´
V =V (V +V ) I (R +R )-
OPP
CC 1 2
- ´
P
1 2
V =
CC 1 2
P
´V +(V +V )+I (R +R )
OPP
1 2
V =2 CF
LPP
´ ´
10
P
(1mW) R 10´ ´
L
L
±I =
P
4R
M
1
´
2
1
n
2 V´
LPP
´
R =
M
Z
LINE
2n
2
V
O
R
1
V
1
+V
CC
R
2
V
2
I
P
OPA2673
SBOS382F –JUNE 2008–REVISED MAY 2010
www.ti.com
LINE DRIVER HEADROOM MODEL
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:
(1)
With P
L
power and V
RMS
voltage at the load, and R
L
load impedance, this calculation gives:
Figure 79. Driver Peak Output Model
With the required output voltage and current versus
(2)
turns ratio set, an output stage headroom model
(3)
allows the required supply voltage versus turns ratio
to be developed.
With V
P
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:
(4)
First, as available output voltage for each amplifier:
with V
LPP
: peak-to-peak voltage at the load.
(8)
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
(9)
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.
(5)
Table 1 gives V
1
, V
2
, R
1
, and R
2
for +12V operation
of the OPA2673.
This V
LPP
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 V
PP
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:
(6)
With V
LPP
defined in Equation 5 and R
M
defined in
Equation 7.
(7)
The peak current is computed in Figure 79 by noting
that the total load is 4R
M
and that the peak current is
Figure 80. Line Driver Headroom Model
half of the peak-to-peak calculated using V
LPP
.
Table 1. Line Driver Headroom Model Values
V
1
R
1
V
2
R
2
+12V 0.9V 2Ω 0.9V 2Ω
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