OPA2673 Datasheet, PDF (24/40 Pages) Texas Instruments – Dual, Wideband, High Output Current Operational Amplifier with Active Off-Line Control

OPA2673
SBOS382 – JUNE 2008...................................................................................................................................................................................................... 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:
V²
P_L = 10 ´ log
RMS
(1mW)
´
R
L
(1)
With P_L power and V_RMS voltage at the load, and R_L
load impedance, this calculation gives:
P
V_RMS =
(1mW) ´ R_L ´ 10
L
10
(2)
V_P = CrestFactor ´ V_RMS = CF ´ V_RMS
(3)
With V_P peak voltage at the load and the crest factor,
CF:
V_LPP = 2 ´ CF ´ V_RMS
(4)
with V_LPP: peak-to-peak voltage at the load.
Consolidating Equation 1 through Equation 4 allows
the required peak-to-peak voltage at the load function
of the crest factor, the load impedance, and the
power in the load to be expressed. Thus:
V_LPP = 2 ´ CF ´
(1mW) ´ R_L ´ 10
P_L
10
(5)
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:
±I_P =
1
2
´
2 ´ V_LPP
n
´
1
4R_M
(6)
With V_LPP defined in Equation 5 and R_M defined in
Equation 7.
R = ^2LINE
M _2n2
(7)
The peak current is computed in Figure 79 by noting
that the total load is 4R_M and that the peak current is
half of the peak-to-peak calculated using V_LPP.
±I_P R_M
1:n
2V_LPP
n
V_LPP
n
R_M
±I_P
R_L
V_LPP
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.
The headroom model (see Figure 80) can be
described with the following set of equations:
First, as available output voltage for each amplifier:
V_OPP
=
V
CC
-
(V
1
+
V)
2
-
I_P
´
(R
1
+
R)
2
(8)
Or, second, as required single-supply voltage:
V
CC
=
V_OPP
+
(V
1
+
V)
2
+
I_P
´
(R
1
+
R)
2
(9)
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.
+V_CC
R₁
V₁
V_O
I_P
V₂
R₂
Figure 80. Line Driver Headroom Model
Table 1. Line Driver Headroom Model Values
V₁
R₁
V₂
R₂
+12V
0.9V
2Ω
0.9V
2Ω
24
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