OPA2670IRGVR Datasheet, PDF (13/25 Pages) Texas Instruments – Single Port, High Output Current VDSL2 Line Driver with Power Control

OPA2670
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DUAL-SUPPLY VDSL DOWNSTREAM
Figure 39 shows an example of a dual-supply VDSL
downstream driver. Both channels of the OPA2670
are configured as a differential gain stage to provide
signal drive to the primary winding of the transformer
(in Figure 39, a step-up transformer with a turns ratio
of 1:n). The main advantage of this configuration is
the cancellation of all even harmonic-distortion
products. Another important advantage for VDSL is
that each amplifier must only swing half of the total
output required driving the load.
+12V
1/2
OPA2670
R_F
I_P
R_S
1:n
AFE
2V_PP
Max
Assumed
R_P
R_G
R_P
R_F
R_S
Z_LINE
R_L
100W
1/2
I_P
OPA2670
Figure 39. Dual-Supply VDSL Downstream Driver
The analog front-end (AFE) signal is ac-coupled to
the driver, and the noninverting input of each
amplifier is biased to the mid-supply voltage (ground
in this case). In addition to providing the proper
biasing to the amplifier, this approach also provides a
high-pass filtering with a corner frequency. Because
the signal bandwidth starts at 26kHz, this high-pass
filter does not generate any problem and has the
advantage of filtering out unwanted lower
frequencies.
The input signal is amplified with a gain set by the
following equation:
G_D = 1 +
2 ´ R_F
R_G
(2)
SBOS434 – AUGUST 2010
The two back-termination resistors (R_S) added at
each terminal of the transformer make the impedance
of the modem match the impedance of the phone
line, and also provide a means of detecting the
received signal for the receiver. The value of these
resistors (R_S) is a function of the line impedance and
the transformer turns ratio (n), given by the following
equation:
R_S =
Z_LINE
2n²
(3)
LINE DRIVER HEADROOM MODEL
The first step in a transformer-coupled, twisted-pair
driver design is to compute the peak-to-peak output
voltage from the target specifications. This calculation
is done using the following equations:
P_L = 10 ´ log
V_RMS2
(1mW)
´
R
L
(4)
with:
• P_L = power at the load
• V_RMS = voltage at the load
• R_L = load impedance
These values produce the following:
P
V_RMS =
(1mW) ´ R_L ´ 10
L
10
(5)
V_P = CrestFactor ´ V_RMS = CF ´ V_RMS
(6)
with:
• V_P = peak voltage at the load
• C_F = Crest Factor
V
LPP
=
2
´
CF
´
V
RMS
(7)
with V_LPP = peak-to-peak voltage at the load.
Consolidating Equation 4 through Equation 7 allows
us to express the required peak-to-peak voltage at
the load as a function of the crest factor, the load
impedance, and the power at the load. Thus:
V_LPP = 2 ´ CF ´
(1mW)
´
R
L
´
10
P_L
10
(8)
V_LPP is usually computed for a nominal line
impedance and may be taken as a fixed design
target.
The next step in the design is to compute the
individual amplifier output voltage and currents as a
function of peak-to-peak voltage on the line and
transformer turns ratio.
Copyright © 2010, Texas Instruments Incorporated
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