LM4883SQ datasheet(14/23 Pages) NSC

14 / 23 page

Application Information

STEREO-INPUT MULTIPLEXER (STEREO MUX)

Typical LM4883 applications use the MUX to switch between

two stereo input signals. Each stereo channel’s gain can be

tailored to produce the required output signal level. Choos-

ing the input and feedback resistor ratio sets a MUX chan-

nel’s gain. Another configuration uses the MUX to select two

different gains or frequency compensated gains to amplify a

single pair of stereo input signals. Figure 2 shows two differ-

ent feedback networks, Network 1 and Network 2. Network 1

produces increasing gain as the input signal’s frequency

decreases. This can be used to compensate a small, full-

range speaker’s low frequency response roll-off. Network 2

sets the gain for an alternate load such as headphones.

Connecting the MUX CTRL and HP-IN pins together applies

the same control voltage to the MUX pins when connecting

and disconnecting headphones using the headphone jack

shown in Figure 3 or Figure 4. Simultaneously applying the

control voltage automatically selects the amplifier (head-

phone or bridge loads) and switches the gain (MUX channel

selection). Alternatively, leave the control pins independently

accessible. This allows a user to select bass boost as

needed. This alternative user-selectable bass-boost scheme

requires connecting equal ratio resistor feedback networks

to each MUX input channel. The value of the resistor in the

RC network is chosen to give a gain that is necessary to

achieve the desired bass-boost.

Switching between the MUX channels may change the input

signal source or the feedback resistor network. During the

channel switching transition, the average voltage level

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.

EXPOSED-DAP PACKAGE PCB MOUNTING

CONSIDERATIONS

The LM4883’s SQ exposed-DAP (die attach paddle) pack-

age 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 traces, ground plane and, finally, surrounding

air. The result is a low voltage audio power amplifier that

produces 2.1W at

≤ 1% THD with a 4Ω load. This high power

is achieved through careful consideration of necessary ther-

mal design. Failing to optimize thermal design may compro-

mise the LM4883SQ’s high power performance and activate

unwanted, though necessary, thermal shutdown protection.

The SQ package must have its DAP soldered to a copper

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

a large plane of continuous unbroken copper. This plane

forms a thermal mass and heat sink and radiation area.

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

of a two-sided PCB, or on an inner layer of a board with more

than two layers. Connect the DAP copper pad to the inner

layer or backside copper heat sink area with 6 (3x2) SQ vias.

The via diameter should be 0.012in–0.013in with a 1.27mm

pitch. Ensure efficient thermal conductivity by plating-

through and solder-filling the vias.

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 LM4883SQ should

be 5in

2 (min) for the same supply voltage and load resis-

tance. 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

conditions, the junction temperature must be held below

150˚C to prevent activating the LM4883SQ’s thermal shut-

down protection. The LM4883SQ’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 SQ package is shown in the

Demonstration Board Layout section. Further detailed and

specific information concerning PCB layout, fabrication, and

mounting an SQ package is available from National Semi-

conductor’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.

This 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.

20088770

FIGURE 2. Input MUX Example

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