present on the internal amplifier’s input may change. This

change can slew at a rate that may produce audible voltage

transients or clicks in the amplifier’s output signal. Using the

MUX to select between two vastly dissimilar gains is a typical

transient-producing situation. As the MUX is switched, an

audible click may occur as the gain suddenly changes.

PIN OUT COMPATIBILITY WITH THE LM4863

The LM4867 pin out was designed to simplify replacing the

LM4863: except for the four Pins(-IN A

and NC) that implement the LM4867’s extra functionality, the

LM4867MT/MTE and LM4863MT/MTE pin outs match.

(Note 19)

Note 19: If the LM4867 replaces an LM4863 and the input MUX circuitry is

the unused MUX inputs must be connected to GND.

EXPOSED-DAP MOUNTING CONSIDERATIONS

The LM4867’s exposed-DAP (die attach paddle) packages

(MTE and LQ) provide 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 surround-

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

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

power amplifier that produces 2.4W dissipation in a 4

Ω load

at

≤ 1% THD+N and over 3W 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 LM4867’s high power performance and

activate unwanted, though necessary, thermal shutdown

protection.

The MTE and LQ packages must have their DAPs soldered

to a copper pad on the PCB. The DAP’s PCB copper pad is

then, ideally, connected to a large plane of continuous un-

broken 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 32 (4 X 8) (MTE) or 6 (3

X 2) (LQ) 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

necessary for 5V operation with a 4

Ω load. Heatsink areas

not placed on the same PCB layer as the LM4867 should be

5in

The last two area recommendations apply for 25˚C ambient

temperature. Increase the area to compensate for ambient

temperatures above 25˚C. In systems using cooling fans, the

LM4867MTE can take advantage of forced air cooling. With

an air flow rate of 450 linear-feet per minute and a 2.5in

2

exposed copper or 5.0in

the LM4867MTE can continuously drive a 3

Ω load to full

power. The LM4867LQ achieves the same output power

level without forced-air cooling. In all circumstances and

under all conditions, the junction temperature must be held

below 150˚C to prevent activating the LM4867’s thermal

shutdown protection. The LM4867’s power de-rating curve in

the Typical Performance Characteristics shows the maxi-

mum power dissipation versus temperature. Example PCB

layouts for the exposed-DAP TSSOP and LQ packages are

shown in the Demonstration Board Layout section. Further

detailed and specific information concerning PCB layout and

fabrication and mounting an LQ (LLP) is found in National

Semiconductor’s AN1187.

20001339

FIGURE 6. As configured, connecting headphones to this jack automatically selects the stereo headphone amplifier

and, with the additional NC switch, changes MUX channels (Network 2 in Figure 5 )

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