LM4868 datasheet(14/28 Pages) NSC

14 / 28 page

Application Information (Continued)

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 LM4868 pin out was designed to simplify replacing the

LM4863: except for the four Pins(-IN A

2, MUX CTRL, -IN B2,

and NC) that implement the LM4868’s extra functionality, the

LM4868MT/MTE and LM4863MT/MTE pin outs match.

(Note 19)

Note 19: If the LM4868 replaces an LM4863 and the input MUX circuitry is

not being used, the LM4868 MUX CTRL pin must be tied to VDD or GND and

the unused MUX inputs must be connected to GND.

EXPOSED-DAP MOUNTING CONSIDERATIONS

The LM4868’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

≤ 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 LM4868’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

2 (min) area is

necessary for 5V operation with a 4

Ω load. Heatsink areas

not placed on the same PCB layer as the LM4868 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 systems using cooling fans, the

LM4868MTE can take advantage of forced air cooling. With

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

exposed copper or 5.0in

2 inner layer copper plane heatsink,

the LM4868MTE can continuously drive a 3

Ω load to full

power. The LM4868LQ 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 LM4868’s thermal

shutdown protection. The LM4868’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.

PCB LAYOUT AND SUPPLY REGULATION

CONSIDERATIONS FOR DRIVING 3

Ω AND 4Ω LOADS

Power dissipated by a load is a function of the voltage swing

across the load and the load’s impedance. As load imped-

ance decreases, load dissipation becomes increasingly de-

pendent on the interconnect (PCB trace and wire) resistance

between the amplifier output pins and the load’s connec-

tions. Residual trace resistance causes a voltage drop,

which results in power dissipated in the trace and not in the

load as desired. For example, 0.1

Ω trace resistance reduces

the output power dissipated by a 4

Ω load from 2.1W to 2.0W.

The problem of decreased load dissipation is exacerbated

as load impedance decreases. Therefore, to maintain the

highest load dissipation and widest output voltage swing,

PCB traces that connect the output pins to a load must be as

wide as possible.

Poor power supply regulation adversely affects maximum

output power. A poorly regulated supply’s output voltage

decreases with increasing load current. Reduced supply

voltage causes decreased headroom, output signal clipping,

and reduced output power. Even with tightly regulated sup-

plies, trace resistance creates the same effects as poor

supply regulation. Therefore, making the power supply

traces as wide as possible helps maintain full output voltage

swing.

BRIDGE CONFIGURATION EXPLANATION

As shown in Figure 4, the LM4868 consists of two pairs of

operational amplifiers, forming a two-channel (channel A and

channel B) stereo amplifier. (Though the following discusses

channel A, it applies equally to channel B.) External resistors

f and Ri set the closed-loop gain of Amp1A, whereas two

internal 20k

Ω resistors set Amp2A’s gain at -1. The LM4868

drives a load, such as a speaker, connected between the two

amplifier outputs, -OUTA and +OUTA.

Figure 4 shows that Amp1A’s output serves as Amp2A’s

input. This results in both amplifiers producing signals iden-

20026739

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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