10
LT1739
1739fas, sn1739
APPLICATIO S I FOR ATIO
WUUU
At full power to the line the driver power dissipation is:
P
D(FULL)
= 24V • 8mA + (12V – 2V
RMS
) • 57mA
RMS
+ [|–12V – (–2V
RMS
)|] • 57mA
RMS
P
D(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:
T
J
= T
AMBIENT
(°C) + P
D(FULL)
(W) • θ
JA
(°C/W)
θ
JA
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:
θ
JA MAX
CC
W
CW
()
–
.
./=
°°
=°
140 85
1 332
41 3
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
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, θ
JC
, 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.
Figure 7. I
Q
vs I
LOAD
I
LOAD
(mA)
–240 –200 –160 –120 –80 –40 0 40 80 120 160 200 240
TOTAL I
Q
(mA)
10
15
20
1739 F07
5
0
25
*Note: Design techniques exist to significantly reduce this value. (See Line Driving Back Termination)
