LT6300IGN datasheet(9/16 Pages) LINER

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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 10

µs and,

more importantly, complete recovery from the shut down

state to full operation occurs in less than 2

µs. The biasing

circuitry in the LT6300 reacts very quickly to bring the

amplifiers back to normal operation.

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,

VSHDN

SHDNREF = 0V

AMPLIFIER

OUTPUT

6300 F05

Figure 5. Shutdown and Recovery Timing

Figure 6. Estimating Line Driver Power Dissipation

6300 F06

–

–IN

–

+IN

12V

20mA DC

SHDN

–12V

–2VRMS

17.4

Ω

24.9k – SETS IQ PER AMPLIFIER = 10mA

1:1.7

110

Ω

1000pF

110

Ω

17.4

Ω

SHDNREF

100

Ω

3.16VRMS

ILOAD = 57mARMS

2VRMS

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