LM4918 datasheet(11/17 Pages) NSC

11 / 17 page

Application Information (Continued)

mize thermal design may compromise the LM4918’s perfor-

mance and activate unwanted, though necessary, thermal

shutdown protection.

The LM4918LD must have its DAP soldered to a copper pad

on the PCB. The DAP’s PCB copper pad is then, ideally,

connected to a large plane of continuous unbroken copper.

This plane forms a thermal mass, heat sink, and radiation

area. Place the heat sink area on either outside plane in the

case of a two-sided or multi-layer PCB. (The heat sink area

can also be placed on an inner layer of a multi-layer board.

The thermal resistance, however, will be higher.) Connect

the DAP copper pad to the inner layer or backside copper

heat sink area with 2 vias. The via diameter should be

0.012in - 0.013in with a 1.27mm pitch. Ensure efficient ther-

mal conductivity by plugging and tenting the vias with plating

and solder mask, respectively. Further detailed and specific

information concerning PCB layout, fabrication, and mount-

ing an LQ (LLP) package is available in National Semicon-

ductor’s AN1187.

POWER SUPPLY BYPASSING

As with any power amplifier, proper supply bypassing is

critical for low noise performance and high power supply

rejection. Applications that employ a 5V regulator typically

use a 10µF in parallel with a 0.1µF filter capacitors to stabi-

lize the regulator’s output, reduce noise on the supply line,

and improve the supply’s transient response. However, their

presence does not eliminate the need for a local 1.0µF

tantalum bypass capacitance connected between the

LM4918’s supply pins and ground. Keep the length of leads

and traces that connect capacitors between the LM4918’s

power supply pin and ground as short as possible. Connect-

ing a 2.2µF capacitor, C

B, between the BYPASS pin and

ground improves the internal bias voltage’s stability and

improves the amplifier’s PSRR. The PSRR improvements

increase as the bypass pin capacitor value increases. Too

large, however, increases turn-on time and can compromise

the amplifier’s click and pop performance. The selection of

bypass capacitor values, especially C

B, depends on desired

PSRR requirements, click and pop performance system

cost, and size constraints.

200913I8

FIGURE 2. REFERENCE DESIGN BOARD SCHEMATIC

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