LT1940

14

1940i

PCB Layout

For proper operation and minimum EMI, care must be

taken during printed circuit board (PCB) layout. Figure 8

shows the high-di/dt paths in the buck regulator circuit.

Note that large, switched currents flow in the power

switch, the catch diode and the input capacitor. The loop

formed by these components should be as small as

possible. These components, along with the inductor and

output capacitor, should be placed on the same side of the

circuit board, and their connections should be made on

that layer. Place a local, unbroken ground plane below

these components, and tie this ground plane to system

ground at one location, ideally at the ground terminal of the

output capacitor C2. Additionally, the SW and BOOST

nodes should be kept as small as possible. Figure 9 shows

recommended component placement with trace and via

locations.

Figure 8. 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

1940 F08

L1

SW

GND

(c)

SW

GND

(b)

APPLICATIO S I FOR ATIO

Thermal Considerations

The PCB must also provide heat sinking to keep the

LT1940 cool. The exposed metal 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

LT1940. 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 re-

duced to

The power dissipation in the other power components—

catch diodes, boost diodes and inductors, cause addi-

tional copper heating and can further increase what the IC

sees as ambient temperature. See the LT1767 data sheet’s

Thermal Considerations section.