OPA2670IRGVR Datasheet, PDF (15/25 Pages) Texas Instruments – Single Port, High Output Current VDSL2 Line Driver with Power Control

OPA2670
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TOTAL DRIVER POWER FOR xDSL
APPLICATIONS
The total internal power dissipation for the OPA2670
in an xDSL line driver application is the sum of the
quiescent power and the output stage power. The
OPA2670 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 total output stage power
can be computed with reference to Figure 43.
+V_CC
I_AVG
=
I_P
CF
R_T
Figure 43. Output Stage Power Model
The two output stages used to drive the load of
Figure 40 can be seen as an H-Bridge in Figure 43.
The average current drawn from the supply into this
H-Bridge and load is the peak current in the load
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, leaving the power
dissipated internal to the drivers in the four output
stage transistors. That power is simply the target line
power used in Equation 4 plus the power lost in the
matching elements (R_S). In the following examples, a
perfect match is targeted giving the same power in
the matching elements as in the load.
For a target line power of 100mW into 100Ω, the
voltage at the load is 3.16V_RMS and the current is
31.6mA_RMS. Bringing this load current to the amplifier
side and using a 1:2 turns ratio to be 63.2mA_RMS, the
average current is then:
2
I
AVG
=
63.2mA
RMS
?
2?
p
= 56.89mA (average)
(13)
With a +12V power supply, the average driving power
is 683mW.
If the total quiescent current is used to drive the load,
the total average power consumption for Low Bias
mode is then (per port):
2
12V ? 14mA ? ₅ + 683mW = 750mW
(14)
Copyright © 2010, Texas Instruments Incorporated
SBOS434 – AUGUST 2010
OUTPUT CURRENT AND VOLTAGE
The OPA2670 provides output voltage and current
capabilities that are unsurpassed in a low-cost, dual
monolithic op amp. The output voltage for a 50Ω
differential load tested at +25°C typically swings
closer than 1.3V to either supply rail. Into a 20Ω load
(the minimum tested load), the amplifier delivers more
than ±450mA continuous and greater than ±1A peak
output current.
The specifications described above, though familiar in
the industry, consider voltage and current limits
separately. In many applications, it is the voltage
times current (or V-I product) that is more relevant to
circuit operation. Refer to the Output Voltage and
Current Limitations plot (Figure 4) in the Typical
Characteristics. The X- and Y-axes of this graph
show the zero-voltage output current limit and the
zero-current output voltage limit, respectively. The
four quadrants give a more detailed view of the
OPA2670 output drive capabilities, noting that the
graph is bounded by a safe operating area of 2W
maximum internal power dissipation (in this case, for
one channel only). Superimposing resistor load lines
onto the plot shows that the OPA2670 can drive
±5.1V into 100Ω or ±5V into 50Ω without exceeding
the output capabilities or the 1W dissipation limit. A
100Ω load line (the standard test circuit load) shows
the full +12V output swing capability, as shown in the
Electrical Characteristics table. The minimum
specified output voltage and current over temperature
are set by worst-case simulations at the cold
temperature extreme. Only at cold startup do the
output current and voltage decrease to the numbers
shown in the Electrical Characteristics table. As the
output transistors deliver power, the junction
temperature increases, decreasing the V_BEs
(increasing the available output voltage swing), and
increasing the current gains (increasing the available
output current). In steady-state operation, the
available output voltage and current are always
greater than that shown in the over-temperature
specifications, because the output stage junction
temperatures are higher than the minimum specified
operating ambient temperature. To maintain
maximum output stage linearity, no output
short-circuit protection is provided. This absence of
short-circuit protection is normally not a problem
because most applications include a series-matching
resistor at the output that limits the internal power
dissipation if the output side of this resistor is shorted
to ground. However, shorting the output pin directly to
the adjacent positive power-supply pin, in most
cases, destroys the amplifier. If additional short-circuit
protection is required, a small series resistor may be
included in the supply lines. Under heavy output
loads, this additional resistor reduces the available
output voltage swing. A 5Ω series resistor in each
power-supply lead limits the internal power
dissipation to less than 2W for an output short-circuit,
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