LT6300IGN datasheet(10/16 Pages) LINER

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LT6300

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 LT6300 is

set to 10mA per amplifier.

The power dissipated in the LT6300 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.

At full power to the line the driver power dissipation is:

PD(FULL) = 24V • 8mA + (12V – 2VRMS) • 57mARMS

+ [|–12V – (– 2VRMS)|] • 57mARMS

PD(FULL) = 192mW + 570mW + 570mW = 1.332W

The junction temperature of the driver must be kept less

than the thermal shutdown temperature when processing

a signal. The junction temperature is determined from the

following expression:

TJ = TAMBIENT (°C) + PD(FULL) (W) • θJA (°C/W)

θJA is the thermal resistance from the junction of the

LT6300 to the ambient air, which can be minimized by

heat-spreading PCB metal and airflow through the enclo-

sure as required. For the example given, assuming a

maximum ambient temperature of 50

°C and keeping the

junction temperature of the LT6300 to 150

°C maximum,

the maximum thermal resistance from junction to ambient

required is:

θJA MAX

()

–

°° =°

150

1 332

75 1

APPLICATIO S I FOR ATIO

Figure 7. IQ vs ILOAD

ILOAD (mA)

–240

–200

–160

–120

–80

–40

120

160

200

240

6300 F07

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