FB

LM25085A

PGATE

GND

Q1

L1

GND

D1

R4

LM25085A

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SNVS601B – JANUARY 2009 – REVISED MARCH 2013

Figure 34. Lowest Cost Ripple Generating Configuration

PC Board Layout

In most applications, the heat sink pad or tab of Q1 is connected to the switch node, i.e. the junction of Q1, L1

and D1. While it is common to extend the PC board pad from under these devices to aid in heat dissipation, the

pad size should be limited to minimize EMI radiation from this switching node. If the PC board layout allows, a

similarly sized copper pad can be placed on the underside of the PC board, and connected with as many vias as

possible to aid in heat dissipation.

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

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

as neat and compact as possible with all the components as close as possible to their associated pins. Two

major current loops conduct currents which switch very fast, requiring the loops to be as small as possible to

be as close as possible to the VIN and VCC pins.

If the anticipated internal power dissipation of the LM25085A will produce excessive junction temperatures during

normal operation, a package option with an exposed pad must be used (HVSSOP-PowerPAD-8 or WSON-8).

Effective use of the PC board ground plane can help dissipate heat. Additionally, the use of wide PC board

traces, where possible, helps conduct heat away from the IC. Judicious positioning of the PC board within the

end product, along with the use of any available air flow (forced or natural convection) also helps reduce the

junction temperature.

Copyright © 2009–2013, Texas Instruments Incorporated

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