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LT1794IFE#PBF

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10
LT1794
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 on the
order of 1W. 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 LT1794 has built-in ther-
mal shutdown cir
cuitry that will protect the amplifiers if
operated at excessive temperatures, however data trans-
missions 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 LT1794
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 100mW
RMS
of power on to the 100Ω
telephone line and assuming a 0.5dBm insertion loss in
the transformer. The quiescent current for the LT1794 is
set to 10mA per amplifier.
The power dissipated in the LT1794 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:
P
D
= P
QUIESCENT
+ P
Q(UPPER)
+ P
Q(LOWER)
P
D
= (V
+
– V
–
) • I
Q
+ (V
+
– V
OUTARMS
) •
I
LOAD
+ (V
–
– V
OUTBRMS
) • I
LOAD
APPLICATIO S I FOR ATIO
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Figure 6. Estimating Line Driver Power Dissipation
1794 F06
+
–
B
–IN
–
+
A
+IN
12V
20mA DC
SHDN
–12V
–2V
RMS
17.4Ω
24.9k – SETS I
Q
PER AMPLIFIER = 10mA
1:1.7
110Ω
1000pF
110Ω
1k
1k
17.4Ω
SHDNREF
100Ω 3.16V
RMS
I
LOAD
= 57mA
RMS
•
•
2V
RMS
11
LT1794
Figure 7. I
Q
vs I
LOAD
APPLICATIO S I FOR ATIO
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I
LOAD
(mA)
–240 –200 –160 –120 –80 –40 0 40 80 120 160 200 240
TOTAL I
Q
(mA)
10
15
20
1794 F07
5
0
25
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:
P
DQ
= 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:
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
LT1794 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 LT1794 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 com-
pact circuit layout to allow more ports to be implemented
on any given size PCB.
Fortunately xDSL circuit boards use multiple layers of
metal for interconnection of components. Areas of metal
beneath the LT1794 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.
12
LT1794
EXAMPLE A
θ
JA
= 40°C/W
13MIL VIAS USED: 30
EXAMPLE B
θ
JA
= 47°C/W
13MIL VIAS USED: 35
EXAMPLE C
θ
JA
= 51°C/W
13MIL VIAS USED: 32
EXAMPLE D
θ
JA
= 60°C/W
13MIL VIAS USED: 22
TOPOLOGY
TOP LAYER 2nd LAYER 3rd LAYER BOTTOM LAYER VIA PATTERN
1794 F08
SCALE:
1 INCH
APPLICATIO S I FOR ATIO
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Figure 8. Examples of PCB Metal Used for Heat Dissipation. LT1794IFE Driver Mounted on Top Layer.
Heat Sink Pad Soldered to Top Layer Metal. External Components Mounted on Bottom Layer
Figure 8 shows four 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
LT1794 junction. The LT1794IFE is used because the
small TSSOP package is most effective for very compact
line driver designs. This package also has 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.
Example A utilizes the most total metal area and provides
the lowest thermal resistance. Example B however uses
less metal on the top and bottom layers and still achieves
reasonable thermal performance. For the most compact
board design, inner layer metal can be used for heat
dissipation. This is shown in examples C and D where
minimum metal is used on the top and none on the bottom
layers, only the 2nd and 3rd layers have a heat-conducting
plane. Example C, with the larger metal areas performs
better.
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LT1794IFE#PBF

Mfr. #:
Buy LT1794IFE#PBF
Manufacturer:
Analog Devices Inc.
Description:
High Speed Operational Amplifiers 2x 500mA, 200MHz xDSL Line Drvr Amp
Lifecycle:
New from this manufacturer.
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