LM2694

SNVS444A – MAY 2006 – REVISED APRIL 2013

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PC BOARD LAYOUT and THERMAL CONSIDERATIONS

The LM2694 regulation, over-voltage, and current limit comparators are very fast, and will respond to short

duration noise pulses. Layout considerations are therefore critical for optimum performance. The layout must be

as neat and compact as possible, and all the components must be as close as possible to their associated pins.

The two major current loops have currents which switch very fast, and so the loops should be as small as

possible to minimize conducted and radiated EMI. The first loop is that formed by C1, through the VIN to SW

pins, L1, C2, and back to C1. The second loop is that formed by D1, L1, C2, and the SGND and ISEN pins. The

ground connection from C2 to C1 should be as short and direct as possible, preferably without going through

vias. Directly connect the SGND and RTN pin to each other, and they should be connected as directly as

possible to the C1/C2 ground line without going through vias. The power dissipation within the IC can be

free-wheeling diode and the inductor. The power loss in the diode is approximately:

where

•

Io is the load current

•

•

D is the duty cycle

(23)

The power loss in the inductor is approximately:

where

•

•

1.1 factor is an approximation for the AC losses

(24)

If it is expected that the internal dissipation of the LM2694 will produce high junction temperatures during normal

operation, good use of the PC board’s ground plane can help considerably to dissipate heat. The exposed pad

on the WSON package bottom should be soldered to a ground plane, and that plane should both extend from

beneath the IC, and be connected to exposed ground plane on the board’s other side using as many vias as

possible. The exposed pad is internally connected to the IC substrate. The use of wide PC board traces at the

pins, where possible, can help conduct heat away from the IC. The four No Connect pins on the TSSOP package

are not electrically connected to any part of the IC, and may be connected to ground plane to help dissipate heat

from the package. Judicious positioning of the PC board within the end product, along with the use of any

available air flow (forced or natural convection) can help reduce the junction temperature.

16

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