19

LT1941

1941f

APPLICATIO S I FOR ATIO

THERMAL CONSIDERATIONS

The PCB must provide heat sinking to keep the LT1941

cool. The Exposed Pad on the bottom of the package must

be soldered to a ground plane. This ground should be tied

to other copper layers below with thermal vias; these

layers will spread the heat dissipated by the LT1941. Place

additional vias near the catch diodes. Adding more copper

to the top and bottom layers and tying this copper to the

internal planes with vias can reduce thermal resistance

further. With these steps, the thermal resistance from die

(or junction) to ambient can be reduced to

or less. With 100 LFPM airflow, this resistance can fall by

another 25%. Further increases in airflow will lead to lower

thermal resistance.

Because of the large output current capability of the

LT1941, it is possible to dissipate enough heat to raise

the junction temperature beyond the absolute maximum

of 125

°C. If two of the channels are running at full output

current, the third channel may have reduced output

current capability, limited by the maximum junction

Figure 7. Subtracting the Current when the Switch is ON (a) From the Current when the Switch is OFF (b) Reveals the Path

of the High Frequency Switching Current (c) Keep This Loop Small. The Voltage on the SW and BOOST Nodes will also be

Switched; Keep these Nodes as Small as Possible. Finally, Make Sure the Circuit is Shielded with a Local Ground Plane

SW

GND

(a)

C1

D1

C2

1941 F07

L1

SW

GND

(c)

SW

GND

(b)

GND

GND

GND

L3

L4

L1

1941 F08

C7

C12

C10

C11

D1 C8

D3

PLACE VIAS UNDER GROUND PAD

TO GROUND PLANE FOR GOOD

THERMAL CONDUCTIVITY

D4

D5

D2

U1

GND

C9

Figure 8. Power Path Components and Topside Layout