LT1939

21

1939f

APPLICATIONS INFORMATION

one location, ideally at the ground terminal of the output

capacitor C2. Additionally, the SW and BST traces should

be kept as short as possible. The topside metal from the

DC1069A demonstration board in Figure 12 illustrates

proper component placement and trace routing.

Thermal Considerations

The PCB must also provide heat sinking to keep the

LT1939 cool. The exposed metal on the bottom of the

Figure 11. Subtracting the Current when the Switch is On (11a) from the Current when the Switch is Off (11b) Reveals the Path of the

High Frequency Switching Current (11c). Keep this Loop Small. The Voltage on the SW and BST Traces will Also be Switched; Keep

These Traces as Short as Possible. Finally, Make Sure the Circuit is Shielded with a Local Ground Plane

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

LT1939. 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 further reduce

thermal resistance. With these steps, the thermal resis-

tance from die (or junction) to ambient can be reduced to

Power dissipation within the LT1939 can be estimated

by calculating the total power loss from an efficiency

measurement and subtracting the catch diode loss. The

die temperature is calculated by multiplying the LT1939

power dissipation by the thermal resistance from junction

to ambient.

The power dissipation in the other power components

such as catch diodes, boost diodes and inductors, cause

additional copper heating and can further increase what

the IC sees as ambient temperature. See the LT1767 data

sheet’s Thermal Considerations section.

Other Linear Technology Publications

Application notes AN19, AN35 and AN44 contain more

detailed descriptions and design information for buck

regulators and other switching regulators. The LT1376

data sheet has a more extensive discussion of output

ripple, loop compensation and stability testing. Design

note DN100 shows how to generate a dual (+ and –) output

supply using a buck regulator.

Figure 12. LT1939 Demonstration Circuit Board DC1293A

GND

(11a)

LT1939 SW

GND

1939 F11

(11c)

LT1939 SW

GND

(11b)

LT1939 SW