7
LT6300
APPLICATIO S I FOR ATIO
WUUU
The LT6300 is a high speed, 200MHz gain bandwidth
product, dual voltage feedback amplifier with high output
current drive capability, 500mA source and sink. The
LT6300 is ideal for use as a line driver in xDSL data
communication applications. The output voltage swing
has been optimized to provide sufficient headroom when
operating from ±12V power supplies in full-rate ADSL
applications. The LT6300 also allows for an adjustment of
the operating current to minimize power consumption. In
addition, the LT6300 is available in a small footprint
surface mount package to minimize PCB area.
To minimize signal distortion, the LT6300 amplifiers are
decompensated to provide very high open-loop gain at
high frequency. As a result each amplifier is frequency
stable with a closed-loop gain of 10 or more. If a closed-
loop gain of less than 10 is desired, external frequency
compensating components can be used.
Setting the Quiescent Operating Current
Power consumption and dissipation are critical concerns
in multiport xDSL applications. Two pins, Shutdown
(SHDN) and Shutdown Reference (SHDNREF), are pro-
vided to control quiescent power consumption and allow
for the complete shutdown of the driver. The quiescent
current should be set high enough to prevent distortion
induced errors in a particular application, but not so high
that power is wasted in the driver unnecessarily. A good
starting point to evaluate the LT6300 is to set the quiescent
current to 10mA per amplifier.
The internal biasing circuitry is shown in Figure 1. Ground-
ing the SHDNREF pin and directly driving the SHDN pin with
a voltage can control the operating current as seen in the
Typical Performance Characteristics. When the SHDN pin
is less than SHDNREF + 0.4V, the driver is shut down and
consumes typically only 100µA of supply current and the
outputs are in a high impedance state. Part to part varia-
tions, however, will cause inconsistent control of the qui-
escent current if direct voltage drive of the SHDN pin is used.
Using a single external resistor, R
BIAS
, connected in one of
two ways provides a much more predictable control of the
quiescent supply current. Figure 2 illustrates the effect on
supply current per amplifier with R
BIAS
connected be-
tween the SHDN pin and the 12V V
+
supply of the LT6300
and the approximate design equations. Figure 3 illustrates
the same control with R
BIAS
connected between the
SHDNREF pin and ground while the SHDN pin is tied to V
+
.
Either approach is equally effective.
Figure 1. Internal Current Biasing Circuitry
2k
SHDN
SHDNREF
TO
START-UP
CIRCUITRY
1k
6300 F01
I
BIAS
TO AMPLIFIERS
BIAS CIRCUITRY
2I
I
2I
5I
2
5
I
BIAS
=
I
SUPPLY
PER AMPLIFIER (mA) = 64 • I
BIAS
I
SHDN
= I
SHDNREF
R
BIAS
(kΩ)
0
I
SUPPLY
PER AMPLIFIER (mA)
10
20
30
5
15
25
10
6300 F02
7 40 70 100 130 160 190
V
S
= ±12V
V
+
= 12V
R
BIAS
SHDN
SHDNREF
R
BIAS
=
• 25.6 – 2k
V
+
– 1.2V
I
S
PER AMPLIFIER (mA)
I
S
PER AMPLIFIER
(mA)
• 25.6
V
+
– 1.2V
R
BIAS
+ 2k
≈
Figure 2. R
BIAS
to V
+
Current Control
