LT6300 Linear Technology Corporation, LT6300 Datasheet - Page 7

• Manufacturer Part Number: LT6300
• Description: 500ma, 200mhz Xdsl Line Driver In 16-lead Ssop Package
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
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.
Figure 1. Internal Current Biasing Circuitry
CIRCUITRY
START-UP
I
I
I
U
BIAS
SUPPLY
TO
=
2
5
PER AMPLIFIER (mA) = 64 • I
5I
I
SHDNREF
SHDN
U
30
25
20
15
10
SHDN
5
0
7
= I
V
2k
S
SHDNREF
= 12V
2I
W
10
6300 F01
1k
2I
I
TO AMPLIFIERS
BIAS CIRCUITRY
BIAS
BIAS
Figure 2. R
40
U
70
BIAS
V
R
+
BIAS
= 12V
to V
R
100
SHDN
SHDNREF
(k )
BIAS
+
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 100 A 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
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
tween the SHDN pin and the 12V V
and the approximate design equations. Figure 3 illustrates
the same control with R
SHDNREF pin and ground while the SHDN pin is tied to V
Either approach is equally effective.
Current Control
I
R
S
BIAS
PER AMPLIFIER (mA)
=
130
I
S
PER AMPLIFIER (mA)
V
+
– 1.2V
160
R
V
BIAS
+
– 1.2V
• 25.6 – 2k
+ 2k
190
• 25.6
6300 F02
BIAS
connected between the
BIAS
+
supply of the LT6300
, connected in one of
BIAS
connected be-
LT6300
7
+
.