LT8602 datasheet(24/30 Pages) LINER

24 / 30 page

LT8602

8602fb

For more information www.linear.com/LT8602

3.5in × 3.5in 4-LAYER BOARD

2oz Cu TOP AND BOTTOM

LIMITED BY MAXIMUM

JUNCTION TEMPERATURE

θJA = 22°C/W

12VIN, 1MHz

12VIN, 2.2MHz

28VIN, 1MHz

28VIN, 2.2MHz

AMBIENT TEMPERATURE (°C)

100

125

100

8602 F10

applicaTions inForMaTion

and near the LT8602 to additional ground planes within

the circuit board and on the bottom side.

Thermal Considerations

Care should be taken in the layout of the PCB to ensure

good heat sinking of the LT8602. The exposed pad on

the bottom of the package must be soldered to a ground

plane. This ground should be tied to large copper layers

below with thermal vias; these layers will spread heat

dissipated by the LT8602. Recommended layer use for

a 4 layer board is:

Layer 1 (Components): use 2oz copper; unbroken high

frequency/high current routing (CIN loop, SW node, BST

node, inductor, COUT), high current DC routing, ground

plane on remainder

Layer 2 (Internal): Unbroken ground plane

Layer 3 (Internal): Signal routing, ground plane on re-

mainder

Layer 4 (Bottom): Use 2oz copper; high current DC routing

(VIN, VOUT), ground plane on remainder

Placing additional vias can reduce thermal resistance fur-

ther. Many small thermal vias are better than a few large

ones. Following these PCB design guidelines can reduce

the QFN θJA to 22°C/W.

Power dissipation within the LT8602 can be estimated by

adding the power dissipated in each channel. Calculate

each channel’s power loss from an efficiency measure-

ment and subtract the inductor loss. The die temperature

is calculated by multiplying the total LT8602 power dis-

sipation by the thermal resistance from junction to ambi-

ent and adding the ambient temperature. The maximum

load current should be derated as the die temperature

approaches the maximum junction rating. The LT8602 will

stop switching if the internal temperature rises too high.

This thermal protection is above the maximum operating

temperature and is intended as a failsafe only.

Even with the best thermal practices, the LT8602 must be

deratedathighambienttemperature.Thethermalderating

curves in Figure 10 show the front page application (Ch1:

5VOUT, Ch 2: 3.3VOUT, Ch 3:1.8VOUT, Ch 4:1.2VOUT). The

PCBlayoutisasdescribedaboveandtheθJAis22°C/W.The

output currents are decreased uniformly as a percentage

of maximum. Although derating is application dependent,

this set of curves is representative of typical applications

with a range of frequencies and input voltages.

Figure 10. Thermal Derating, I-Grade

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