1739 Просмотр технического описания (PDF) - Linear Technology

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производитель

1739

Dual 500mA, 200MHz xDSL Line Driver Amplifier

Linear Technology 

LT1739

APPLICATIO S I FOR ATIO

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 LT1739

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,

which looks very much like noise, it is easiest to use the

RMS values of voltages and currents for estimating the

driver power dissipation. The voltage and current levels

shown for this example are for a full-rate ADSL signal

driving 20dBm or 100mWRMS of power on to the 100Ω

telephone line and assuming a 0.5dBm insertion loss in

the transformer. The quiescent current for the LT1739 is

set to 10mA per amplifier.

The power dissipated in the LT1739 is a combination of the

quiescent power and the output stage power when driving

a signal. The two amplifiers are configured to place a

differential signal on to the line. The Class AB output stage

in each amplifier will simultaneously dissipate power in

the upper power transistor of one amplifier, while sourc-

ing current, and the lower power transistor of the other

amplifier, while sinking current. The total device power

dissipation is then:

PD = PQUIESCENT + PQ(UPPER) + PQ(LOWER)

PD = (V+ – V–) • IQ + (V+ – VOUTARMS) •

ILOAD + (V – – VOUTBRMS) • ILOAD

With no signal being placed on the line and the amplifier

biased for 10mA per amplifier supply current, the quies-

cent driver power dissipation is:

PDQ = 24V • 20mA = 480mW

This can be reduced in many applications by operating

with a lower quiescent current value.

When driving a load, a large percentage of the amplifier

quiescent current is diverted to the output stage and

becomes part of the load current. Figure 7 illustrates the

total amount of biasing current flowing between the + and

– power supplies through the amplifiers as a function of

load current. As much as 60% of the quiescent no load

operating current is diverted to the load.

12V

20mA DC

24.9k – SETS IQ PER AMPLIFIER = 10mA

+IN

1000pF

110Ω

110Ω

+

A

–

1k

2VRMS

SHDN 17.4Ω

1:1.7

ILOAD = 57mARMS

1k

100Ω

–

B

17.4Ω

1739 F06

–IN

+

SHDNREF

–12V

–2VRMS

Figure 6. Estimating Line Driver Power Dissipation

3.16VRMS

1739fas, sn1739

9

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