OPA2674 Datasheet, PDF (19/30 Pages) Burr-Brown (TI) – Dual Wideband, High Output Current Operational Amplifier with Current Limit

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+V_CC
R₁
V₁
V₂
R₂
V_O
I_P
Figure 8. Line Driver Headroom Model
Table 1 gives V₁, V₂, R₁, and R₂ for both +12V and +5V
operation of the OPA2674.
Table 1. Line Driver Headroom Model Values
V1
R1
V2
R2
+5V
0.9V
5Ω
0.8V
5Ω
+12V
0.9V
2Ω
0.9V
2Ω
TOTAL DRIVER POWER FOR xDSL
APPLICATIONS
The total internal power dissipation for the OPA2674 in an
xDSL line driver application will be the sum of the
quiescent power and the output stage power. The
OPA2674 holds a relatively constant quiescent current
versus supply voltage—giving a power contribution that is
simply the quiescent current times the supply voltage used
(the supply voltage will be greater than the solution given
in Equation 10). The total output stage power may be
computed with reference to Figure 9.
+V_CC
I_AVG
=
I_P
C_F
R_T
Figure 9. Output Stage Power Model
OPA2674
SBOS270 − AUGUST 2003
The two output stages used to drive the load of Figure 7
can be seen as an H-Bridge in Figure 9. The average
current drawn from the supply into this H-Bridge and load
will be the peak current in the load given by Equation 8
divided by the crest factor (CF) for the xDSL modulation.
This total power from the supply is then reduced by the
power in R_T to leave the power dissipated internal to the
drivers in the four output stage transistors. That power is
simply the target line power used in Equation 2 plus the
power lost in the matching elements (R_M). In the examples
here, a perfect match is targeted giving the same power in
the matching elements as in the load. The output stage
power is then set by Equation 11.
P _OUT
+
I_P
C_F
V_CC * 2P_L
(11)
The total amplifier power is then:
P_TOT + I_q
V_CC
)
I_P
C_F
V_CC * 2P_L
(12)
For the ADSL CPE upstream driver design of Figure 5, the
peak current is 128mA for a signal that requires a crest
factor of 5.33 with a target line power of 13dBm into 100Ω
(20mW). With a typical quiescent current of 18mA and a
nominal supply voltage of +12V, the total internal power
dissipation for the solution of Figure 5 will be:
(13)
P_TOT
+
18mA(12V)
)
128mA
5.33
(12V)
*
2(20mW)
+
464mW
DESIGN-IN TOOLS
DEMONSTRATION BOARDS
Several PC boards are available to assist in the initial
evaluation of circuit performance using the OPA2674 in
the two package styles. These are available, free, as
unpopulated PC boards delivered with descriptive
documentation. Table 2 shows the summary information
for these boards.
Table 2. Demo Board Availability
PRODUCT
PACKAGE
DEMO BOARD
NUMBER
ORDERING
NUMBER
OPA2674ID
SO-8
DEM-OPA268XU
SBOU003
OPA2674I-14D SO-14
DEM-OPA268XN
SBOU002
Go to the TI web site (www.ti.com) to request either of
these boards.
MACROMODELS AND APPLICATIONS
SUPPORT
Computer simulation of circuit performance using SPICE
is often useful when analyzing the performance of analog
circuits and systems. This is particularly true for video and
RF amplifier circuits where parasitic capacitance and
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