LT1794CFE#PBF Linear Technology, LT1794CFE#PBF Datasheet - Page 12

• Manufacturer Part Number: LT1794CFE#PBF
• Description: IC AMP XDSL LINE DRIVER 20TSSOP
• Manufacturer: Linear Technology
• Type: Line Driver, Transmitterr

Specifications of LT1794CFE#PBF

• Number Of Drivers/receivers: 2/0
• Protocol: xDSL
• Voltage - Supply: 18V
• Mounting Type: Surface Mount
• Package / Case: 20-TSSOP Exposed Pad, 20-eTSSOP, 20-HTSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

APPLICATIO S I FOR ATIO
LT1794
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
12
13MIL VIAS USED: 30
13MIL VIAS USED: 35
13MIL VIAS USED: 32
13MIL VIAS USED: 22
EXAMPLE A
EXAMPLE B
EXAMPLE C
EXAMPLE D
TOPOLOGY
JA
JA
JA
JA
= 40 C/W
= 47 C/W
= 51 C/W
= 60 C/W
SCALE:
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
U
TOP LAYER
1 INCH
U
W
2nd LAYER
U
board produces a thermal resistance from junction to
case,
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.
3rd LAYER
JC
, of approximately 3 C/W.
BOTTOM LAYER
VIA PATTERN
1794 F08