12

LT1794

EXAMPLE A

13MIL VIAS USED: 30

EXAMPLE B

13MIL VIAS USED: 35

EXAMPLE C

13MIL VIAS USED: 32

EXAMPLE D

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

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,

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.