1739 Просмотр технического описания (PDF) - Linear Technology

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1739

Dual 500mA, 200MHz xDSL Line Driver Amplifier

Linear Technology 

LT1739

APPLICATIO S I FOR ATIO

25

20

15

10

5

0

–240 –200 –160 –120 –80 –40 0

40 80

ILOAD (mA)

Figure 7. IQ vs ILOAD

120 160 200 240

1739 F07

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

LT1739 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 85°C and keeping the

junction temperature of the LT1739 to 140°C maximum,

the maximum thermal resistance from junction to ambient

required is:

θJA(MAX)

=

140°C – 85°C

1.332W

=

41.3°C /

W

Heat Sinking Using PCB Metal

Designing a thermal management system is often a trial

and error process as it is never certain how effective it is

until it is manufactured and evaluated. As a general rule,

the more copper area of a PCB used for spreading heat

away from the driver package, the more the operating

junction temperature of the driver will be reduced. The

limit to this approach however is the need for very

10

compact circuit layout to allow more ports to be imple-

mented on any given size PCB.

Fortunately xDSL circuit boards use multiple layers of

metal for interconnection of components. Areas of metal

beneath the LT1739 connected together through several

small 13 mil vias can be effective in conducting heat away

from the driver package. The use of inner layer metal can

free up top and bottom layer PCB area for external compo-

nent placement.

Figure 8 shows examples of PCB metal being used for heat

spreading. These are provided as a reference for what

might be expected when using different combinations of

metal area on different layers of a PCB. These examples are

with a 4-layer board using 1oz copper on each. The most

effective layers for spreading heat are those closest to the

LT1739 junction. The small TSSOP and DFN packages are

very effective for compact line driver designs. Both pack-

ages also have an exposed metal heat sinking pad on the

bottom side which, when soldered to the PCB top layer

metal, directly conducts heat away from the IC junction.

Soldering the thermal pad to the board produces a thermal

resistance from junction to case, θJC, of approximately

3°C/W.

As a minimum, the area directly beneath the package on all

PCB layers can be used for heat spreading. Limiting the

area of metal to just that of the exposed metal heat sinking

pad however is not very effective, particularly if the ampli-

fiers are required to dissipate significant power levels.

This is shown in Figure 8 for both the TSSOP and DFN

packages. Expanding the area of metal on various layers

significantly reduces the overall thermal resistance. If

possible, an entire unbroken plane of metal close to the

heat sinking pad is best for multiple drivers on one PCB

card. The addition of vias (small 13mil or smaller holes

which fill during PCB plating) connecting all layers of heat

spreading metal also helps to reduce operating tempera-

tures of the driver. These too are shown in Figure␣ 8.

Important Note: The metal planes used for heat sinking

the LT1739 are electrically connected to the negative

supply potential of the driver, typically – 12V. These

planes must be isolated from any other power planes

used in the board design.

*Note: Design techniques exist to significantly reduce this value. (See Line Driving Back Termination)

1739fas, sn1739

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