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| OPA2670IRGVR Datasheet, PDF (14/25 Pages) Texas Instruments – Single Port, High Output Current VDSL2 Line Driver with Power Control | |||
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OPA2670
SBOS434 â AUGUST 2010
As this turns ratio changes, the minimum allowed
supply voltage changes along with it. The peak
current in the amplifier output is given by:
±IP =
1
2
´
V
LPP
2´ n
´
1
4RS
(9)
with VPP as defined in Equation 8, and RM as defined
in Equation 3 and shown in Figure 40.
RS
VPP
=
2VLPP
n
VLPP
n
RS
RL
VLPP
Figure 40. Driver Peak Output Voltage
With the previous information available, it is now
possible to select a supply voltage and the turns ratio
desired for the transformer, as well as calculate the
headroom for the OPA2670.
The model, shown in Figure 41, can be described
with the following set of equations:
1. As the available output swing:
VPP = VCC - (V1 + V2) - IP ´ (R1 + R2)
(10)
2. Or as the required supply voltage:
VCC = VPP + (V1 + V2) + IP ´ (R1 + R2)
(11)
The minimum supply voltage for power and load
requirements is given by Equation 11.
+VCC
R1
V1
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V1,V2, R1, and R2 are given in Table 1 for +12V
operation.
Table 1. Line Driver Headroom Model Values
+12V
V1
0.8V
R1
0.3Ω
V2
0.8V
R2
0.6Ω
When using a synthetic output impedance circuit (see
Figure 39), a significant drop is noticed in bandwidth
from the specification that appears in the Electrical
Characteristics table. This apparent drop in
bandwidth for the differential signal is a result of the
apparent increase in the feedback transimpedance as
seen for each amplifier. This feedback
transimpedance equation is given below.
ZFB = RF ´
1+2´
RS
RL
+
RS
RP
1+2´
RS
RL
+
RS
RP
-
RF
RP
(12)
To increase 0.1dB flatness to the frequency of
interest, adding a serial RC in parallel with the gain
resistor may be needed, as shown in Figure 42.
RS
1/2
OPA2670
RF
RP
RM
VIN
RG
RP
CM
RF
1/2
RS
OPA2670
ZLINE
100W
Figure 42. +0.1dB Flatness Compensation Circuit
VOUT
IP
V2
R2
Figure 41. Line Driver Headroom Model
14
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