LM4873 datasheet(13/25 Pages) NSC

13 / 25 page

Application Information

LM4863 PIN CONFIGURATION COMPATIBILITY

The LM4873’s pin configuration simplifies the process of

upgrading systems that use the LM4863. Except for its four

MUX function pins, the LM4873’s pin configuration matches

the LM4863’s pin configuration. If the LM4873’s MUX func-

tionality is not needed when replacing an LM4863, connect

the MUX CTRL pin to either V

DD or ground. As shown in

Table 1, grounding the MUX CTRL pin selects stereo input 1

(–IN A1 and –IN B1), whereas applying V

DD to the MUX

CTRL pin selects stereo input 2 (–IN A2 and –IN B2).

STEREO-INPUT MULTIPLEXER (STEREO MUX)

Typical LM4873 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 1 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 2 or Figure 3. 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.

micro SMD PACKAGE PCB MOUNTING

CONSIDERATIONS

PCB layout specifications unique to the LM4873’s micro

SMD package are found in National Semiconductor’s

AN1112.

EXPOSED-DAP PACKAGE PCB MOUNTING

CONSIDERATIONS

The LM4873’s exposed-DAP (die attach paddle) packages

(MTE, MTE-1, 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

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 thermal design. Failing to optimize thermal design

may compromise the LM4873’s high power performance and

activate unwanted, though necessary, thermal shutdown

protection.

The MTE, MTE-1, and LQ packages must have their DAPs

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 32(4x8) ( (MTE), 40(4x10) (MTE-1), or 6(3x2) (LQ)

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 LM4873 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

LM4873MTE 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 LM4873MTE can continuously drive a 3

Ω load to full

10099370

FIGURE 1. Input MUX Example

LM4873 Datasheet(PDF) 13 Page - National Semiconductor (TI)

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