LT3513

18

3513fa

Printed Circuit Board Layout

For proper operation and minimum EMI, care must be

taken during printed circuit board (PCB) layout. Figure 3

shows the high current paths in the step-down regula-

tor circuit. Note that in the step-down regulators, large,

switched currents flow in the power switch, the catch

diode and the input capacitor. In the step-up regulators,

large, switched currents flow through the power switch,

the switching diode and the output capacitor. In SEPIC and

inverting regulators, the switched currents flow through

the power switch, the switching diode and the tank capaci-

tor. The loop formed by the components in the switched

current path should be as small as possible. Place these

components, along with the inductor and output capacitor,

on the same side of the circuit board, and connect them

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, keep the SW and

BOOST nodes as small as possible.

Thermal Considerations

The PCB must provide heat sinking to keep the LT3513

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 lay-

ers will spread the heat dissipated by the LT3513. 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 100LFPM airflow, this resistance can fall

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

lower thermal resistance.

SW

GND

(3a)

C1

D1

C2

3519 F03

L1

SW

GND

(3c)

SW

GND

(3b)

Figure 3. Subtracting the Current When the Switch is On (3a)

from the Current When the Switch is Off (3b) Reveals the Path of

the High Frequency Switching Current (3c) 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

Figure 4. Topside PCB Layout

OPERATION