LT6300
APPLICATIO S I FOR ATIO
modes. These modes can be useful for overall system
power management when full power transmissions are
not necessary.
Shutdown and Recovery
The ultimate power saving action on a completely idle port
is to fully shut down the line driver by pulling the SHDN pin
to within 0.4V of the SHDNREF potential. As shown in
Figure 5 complete shutdown occurs in less than 10µs and,
more importantly, complete recovery from the shut down
state to full operation occurs in less than 2µs. The biasing
circuitry in the LT6300 reacts very quickly to bring the
amplifiers back to normal operation.
V
SHDN
SHDNREF = 0V
AMPLIFIER
OUTPUT
6300 F05
Figure 5. Shutdown and Recovery Timing
12V
20mA DC
24.9k – SETS I
Q
PER AMPLIFIER = 10mA
+IN
+
A
2V
RMS
SHDN
17.4Ω
–
1k
110Ω
1000pF
I
LOAD
= 57mA
RMS
110Ω
1k
1:1.7
•
•
–
B
–IN
+
–12V
SHDNREF
–2V
RMS
Figure 6. Estimating Line Driver Power Dissipation
U
Power Dissipation and Heat Management
xDSL applications require the line driver to dissipate a
significant amount of power and heat compared to other
components in the system. The large peak to RMS varia-
tions of DMT and CAP ADSL signals require high supply
voltages to prevent clipping, and the use of a step-up
transformer to couple the signal to the telephone line can
require high peak current levels. These requirements
result in the driver package having to dissipate significant
amounts of power. Several multiport cards inserted into
a rack in an enclosed central office box can add up to
many, many watts of power dissipation in an elevated
ambient temperature environment. The LT6300 has built-
in thermal shutdown circuitry that will protect the ampli-
fiers if operated at excessive temperatures, however data
transmissions will be seriously impaired. It is important in
the design of the PCB and card enclosure to take measures
to spread the heat developed in the driver away to the
ambient environment to prevent thermal shutdown (which
occurs when the junction temperature of the LT6300
exceeds 165°C).
Estimating Line Driver Power Dissipation
Figure 6 is a typical ADSL application shown for the
purpose of estimating the power dissipation in the line
driver. Due to the complex nature of the DMT signal,
100Ω
3.16V
RMS
17.4Ω
6300 F06
W
U U
9
LINER [ LINEAR TECHNOLOGY ]
