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LT8603

8603f

For more information www.linear.com/LT8603

APPLICATIONS INFORMATION

The recommended layer use for a 4-layer board is:

Layer 1 (Components): use 2oz (70µm) copper. Unbroken

high frequency/high current routing. (CIN loop,

SW node, BST node, inductor, COUT), high current

DC routing, ground plane fill.

Layer 2 (Internal): Unbroken ground plane.

Layer 3 (Internal): Signal routing, ground plane

on remainder.

Layer 4 (Bottom): Use 2oz (70µm) copper; high cur-

rent DC routing (PVIN, VOUT), ground plane on

remainder.

THERMAL CONSIDERATIONS

The exposed pad is the main path for conducting heat

from the silicon die to the PC board and the surrounding

air. Thermal vias should be placed under the device to

conduct heat down to internal ground planes and the back

side of the board. Multiple small vias work better than a

few large ones as copper is a much better conductor than

any solder which may or may not fill them. The planes

will distribute heat over a larger area and thus reduce the

thermal resistance from the package to the air. A good

design can achieve an effective θJA of 22°C/W. Power

dissipation within the LT8603 can be estimated by sum-

ming the power dissipated in each channel. Calculate each

channel’s power loss from an efficiency measurement

and subtract external component losses such as induc-

tors, power transistors, and diodes. The die temperature

is calculated by multiplying the total LT8603 power dis-

sipation by the thermal resistance from junction to die

θJA and adding the ambient temperature. The maximum

load current should be derated as the ambient tempera-

ture approaches the maximum junction rating. Figure 13

shows the current derating factor to avoid exceeding the

maximum junction temperature.

The thermal derating curves of Figure 13 are based on the

front page application (Ch 1: 5V, Ch 2: 3.3V, Ch 3: 1.2V,

Ch4: 8V). The currents decrease uniformly as a percent-

age of maximum. Although application dependent, this

Figure 13. Thermal Derating, E- and I-Grade

3.5in x 3.5in 4–LAYER BOARD

2oz Cu TOP AND BOTTOM

LIMITED BY MAXIMUM

JUNCTION TEMPERATURE

θJA = 22°C/W

12VIN, 1MHz

12VIN, 2.2MHz

24VIN, 1MHz

24VIN, 2.2MHz

AMBIENT TEMPERATURE (°C)

100

125

100

8603 F13

set of curves is representative of typical applications.

Final current derating should be based on temperature

measurements in the final application and environment.

The LT8603 will stop switching if the internal tempera-

ture rises too high. This thermal protection is above the

maximum reliable operating temperature and is intended

as a failsafe only.

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