Datasheet

TOTAL DRIVER POWER FOR xDSL
P =21mA(24V)+
TOT
5.6
159mA
(24V) 2(115mW)=955mW-
OUTPUT CURRENT AND VOLTAGE
R
T
+V
CC
I =
AVG
I
P
CF
P =
OUT
CF
I
P
´ -
CC
V 2P
L
(13)
P =I V +
TOT
Q
´
CC
CF
I
P
´ -
CC
V 2P
L
(14)
THS6214
www.ti.com ....................................................................................................................................................................................................... SBOS431 – MAY 2009
With a typical quiescent current of 21mA and a
APPLICATIONS nominal supply voltage of ± 12V, the total internal
power dissipation for the solution of Figure 82 is:
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
(15)
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
The THS6214 provides output voltage and current
than the solution given in Equation 11 ). The total
capabilities that are unsurpassed in a low-cost, dual
output stage power can be computed with reference
monolithic op amp. Under no-load conditions at
to Figure 86 .
+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 100 Ω differential
load. Into a 25 Ω 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
Figure 86. Output Stage Power Model
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
The two output stages used to drive the load of
maximum internal power dissipation (in this case, for
Figure 83 can be seen as an H-Bridge in Figure 86 .
one channel only). Superimposing resistor load lines
The average current drawn from the supply into this
onto the plot shows that the THS6214 can drive
H-Bridge and load is the peak current in the load
± 10.9V into 100 Ω or ± 10.5V into 50 Ω without
given by Equation 9 divided by the crest factor (CF)
exceeding the output capabilities or the 1W
for the xDSL modulation. This total power from the
dissipation limit. A 100 Ω load line (the standard test
supply is then reduced by the power in R
T
, leaving
circuit load) shows the full ± 12V output swing
the power dissipated internal to the drivers in the four
capability, as shown in the Electrical Characteristics
output stage transistors. That power is simply the
tables. The minimum specified output voltage and
target line power used in Equation 4 plus the power
current over temperature are set by worst-case
lost in the matching elements (R
M
). In the following
simulations at the cold temperature extreme. Only at
examples, a perfect match is targeted giving the
cold startup do the output current and voltage
same power in the matching elements as in the load.
decrease to the numbers shown in the Electrical
The output stage power is then set by Equation 13 .
Characteristics tables. As the output transistors
deliver power, the junction temperature increases,
decreasing the V
BE
s (increasing the available output
voltage swing), and increasing the current gains
The total amplifier power is then:
(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
For the ADSL CO driver design of Figure 82 , the
stage junction temperatures are higher than the
peak current is 159mA for a signal that requires a
minimum specified operating ambient temperature.
crest factor of 5.6 with a target line power of 20.5dBm
To maintain maximum output stage linearity, no
into a 100 Ω load (115mW).
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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