THS6214 Datasheet, PDF (27/41 Pages) Texas Instruments – Dual-Port, Differential, VDSL2 Line Driver Amplifiers

THS6214
www.ti.com ....................................................................................................................................................................................................... SBOS431 – MAY 2009
TOTAL DRIVER POWER FOR xDSL
APPLICATIONS
The total internal power dissipation for the THS6214
in an xDSL line driver application is the sum of the
quiescent power and the output stage power. The
THS6214 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 is greater
than the solution given in Equation 11). The total
output stage power can 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 is 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, 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_M). In the following
examples, 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 13.
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 a 100Ω load (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 is:
P_TOT
=
21mA
(24V)
+
159mA
5.6
(24V)
-
2(115mW)
=
955mW
(15)
OUTPUT CURRENT AND VOLTAGE
The THS6214 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, the swing
limit is within 1.4V of either rail into a 100Ω differential
load. Into a 25Ω load (the minimum tested load), the
amplifier delivers more than ±408mA 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 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
THS6214 output drive capabilities, noting that the
graph is bounded by a safe operating area of 1W
maximum internal power dissipation (in this case, for
one channel only). Superimposing resistor load lines
onto the plot shows that the THS6214 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 do the output current and voltage
decrease to the numbers shown in the Electrical
Characteristics tables. 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
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