LM4854 datasheet(19/29 Pages) NSC

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Application Information (Continued)

the amplifier that drives the headphone jack’s sleeve. If this

current exceeds 500mA

PK, the amplifier is shutdown, pro-

tecting the LM4854 and the external equipment. For more

information, see the section titled ’Single-Ended Output

Power Performance and Measurement Considerations’.

EXPOSED-DAP MOUNTING CONSIDERATIONS

The LM4854’s exposed-DAP (die attach paddle) package

(LD) provides a low thermal resistance between the die and

the PCB to which the part is mounted and soldered. This

allows rapid heat transfer from the die to the surrounding

PCB copper area heatsink, copper traces, ground plane, and

finally, surrounding air. The result is a low voltage audio

power amplifier that produces 1.7W dissipation in a 4

Ω load

≤ 1% THD+N and over 1.9W in a 3Ω load at 10% THD+N.

This high power is achieved through careful consideration of

necessary thermal design. Failing to optimize thermal design

may compromise the LM4854’s high power performance and

activate unwanted, though necessary, thermal shutdown

protection.

The LD package 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 6 (3 X 2) (LD) vias. The via diameter

should be 0.012in - 0.013in with a 1.27mm pitch. Ensure

efficient thermal conductivity by plugging and tenting the vias

with plating and solder mask, respectively.

Best thermal performance is achieved with the largest prac-

tical copper heat sink area. If the heatsink and amplifier

share the same PCB layer, a nominal 2.5in

2 (min) area is

necessary for 5V operation with a 4

Ω load. Heatsink areas

not placed on the same PCB layer as the LM4854 should be

5in

2 (min) for the same supply voltage and load resistance.

The last two area recommendations apply for 25˚C ambient

temperature. Increase the area to compensate for ambient

temperatures above 25˚C. In all circumstances and under all

conditions, the junction temperature must be held below

150˚C to prevent activating the LM4854’s thermal shutdown

protection. The LM4854’s power de-rating curve in the Typi-

cal Performance Characteristics shows the maximum

power dissipation versus temperature. Example PCB layouts

for the exposed-DAP TSSOP and LD packages are shown in

the Demonstration Board Layout section. Further detailed

and specific information concerning PCB layout and fabrica-

tion and mounting an LD (LLP) is found in National Semi-

conductor’s AN1187.

20038207

FIGURE 2. Typical Audio Amplifier Application Circuit

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