THS6204 Datasheet, PDF (27/43 Pages) Texas Instruments – Dual-Port, Differential VDSL2 Line Driver Amplifiers

THS6204
www.ti.com ................................................................................................................................................... SBOS416C – OCTOBER 2007 – REVISED APRIL 2009
TOTAL DRIVER POWER FOR xDSL
APPLICATIONS
The total internal power dissipation for the THS6204
in an xDSL line driver application will be the sum of
the quiescent power and the output stage power. The
THS6204 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 11). The
total output stage power may be computed with
reference to Figure 86.
+V_CC
I_AVG
=
I_P
CF
R_T
Figure 86. Output Stage Power Model
The two output stages used to drive the load of
Figure 83 can be seen as an H-Bridge in Figure 86.
The average current drawn from the supply into this
H-Bridge and load will be the peak current in the load
given by Equation 9 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 4 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 12.
P_OUT =
I_P
CF
´ V_CC - 2P_L
(13)
The total amplifier power is then:
P_TOT
=
I
Q
´
V
CC
+
I_P
CF
´
V
CC
-
2P
L
(14)
For the ADSL CO driver design of Figure 82, the
peak current is 159mA for a signal that requires a
crest factor of 5.6 with a target line power of 20.5dBm
into 100Ω (115mW). With a typical quiescent current
of 21mA and a nominal supply voltage of ±12V, the
total internal power dissipation for the solution of
Figure 82 will be:
159mA
P_TOT = 21mA (24V) + _5.6 (24V) - 2(115mW) = 955mW
(15)
OUTPUT CURRENT AND VOLTAGE
The THS6204 provides output voltage and current
capabilities that are unsurpassed in a low-cost dual
monolithic op amp. Under no-load conditions at
+25°C, the output voltage typically swings closer than
1.1V to either supply rail; tested at +25°C swing limit
is within 1.4V of either rail into a 100Ω differential
load. Into a 25Ω load (the minimum tested load), it
delivers more than ±408mA continuous and > ±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 14) 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
THS6204 output drive capabilities, noting that the
graph is bounded by a safe operating area of 1W
maximum internal power dissipation (in this case for 1
channel only). Superimposing resistor load lines onto
the plot shows that the THS6204 can drive ±10.9V
into 100Ω or ±10.5V 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 tables. The minimum
specified output voltage and current over temperature
are set by worst-case simulations at the cold
temperature extreme. Only at cold startup will the
output current and voltage decrease to the numbers
shown in the Electrical Characteristics tables. As the
output transistors deliver power, the junction
temperatures 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 will always be
greater than that shown in the over-temperature
specifications, since the output stage junction
temperatures will be higher than the minimum
specified operating ambient. To maintain maximum
output stage linearity, no output short-circuit
protection is provided. This 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 (24-pin
package), will in most cases, destroy the amplifier. If
additional short-circuit protection is required, a small
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