LT1794ISW#TR Linear Technology, LT1794ISW#TR Datasheet - Page 11

• Manufacturer Part Number: LT1794ISW#TR
• Description: IC OPAMP 200MHZ DUAL 20-SOIC
• Manufacturer: Linear Technology
• Type: Line Driver, Transmitterr

Specifications of LT1794ISW#TR

• Number Of Drivers/receivers: 2/0
• Protocol: xDSL
• Voltage - Supply: 18V
• Mounting Type: Surface Mount
• Package / Case: 20-SOIC (7.5mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: LT1794ISWTR

APPLICATIO S I FOR ATIO
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:
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:
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:
LT1794 to the ambient air, which can be minimized by
JA
P
P
P
T
J
DQ
D(FULL)
D(FULL)
is the thermal resistance from the junction of the
= T
= 24V • 20mA = 480mW
AMBIENT
= 24V • 8mA + (12V – 2V
= 192mW + 570mW + 570mW = 1.332W
+ [|–12V – (– 2V
( C) + P
U
D(FULL)
U
25
20
15
10
5
0
–240
RMS
–200
(W) •
)|] • 57mA
W
RMS
–160
JA
) • 57mA
–120
( C/W)
RMS
U
–80
Figure 7. I
RMS
–40
I
LOAD
0
Q
(mA)
vs I
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:
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.
40
LOAD
JA MAX
(
80
)
120
140
160
1 332
.
C
200
–
1794 F07
85
W
240
C
41 3
.
C W
/
LT1794
11