Application Information

EXPOSED-DAP MOUNTING CONSIDERATIONS

The LM4849’s exposed-DAP (die attach paddle) package

(MH) 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 LM4849’s high power performance and activate

unwanted, though necessary, thermal shutdown protection.

The MH package must have its exposed DAP soldered to a

grounded 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)

(MH)

vias.

The

via

diameter

should

be

0.012in–0.013in with a 1.27mm pitch. Ensure efficient ther-

mal 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

necessary for 5V operation with a 4

Ω load. Heatsink areas

not placed on the same PCB layer as the LM4849 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

LM4849MH 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 LM4849MH can continuously drive a 3

Ω load to full

power. The LM4849’s power de-rating curve in the Typical

Performance Characteristics shows the maximum power

dissipation versus temperature. Example PCB layouts for

the exposed-DAP TSSOP package is shown in the Demon-

stration Board Layout section.

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.

BRIDGE CONFIGURATION EXPLANATION

As shown in Figure 1, the LM4849 output stage 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.)

Figure 1 shows that the first amplifier’s negative (-) output

serves as the second amplifier’s input. This results in both

amplifiers producing signals identical in magnitude, but 180˚

out of phase. Taking advantage of this phase difference, a

load is placed between −OUTA and +OUTA and driven dif-

ferentially (commonly referred to as “bridge mode”). This

results in a differential gain of

A

(1)

Bridge mode amplifiers are different from single-ended am-

plifiers that drive loads connected between a single amplifi-

er’s output and ground. For a given supply voltage, bridge

mode has a distinct advantage over the single-ended con-

figuration: its differential output doubles the voltage

swing across the load. This produces four times the output

power when compared to a single-ended amplifier under the

same conditions. This increase in attainable output power

assumes that the amplifier is not current limited or that the

output signal is not clipped. To ensure minimum output sig-

nal clipping when choosing an amplifier’s closed-loop gain,

refer to the Audio Power Amplifier Design section.

Another advantage of the differential bridge output is no net

DC voltage across the load. This is accomplished by biasing

channel A’s and channel B’s outputs at half-supply. This

eliminates the coupling capacitor that single supply, single-

ended amplifiers require. Eliminating an output coupling ca-

pacitor in a single-ended configuration forces a single-supply

amplifier’s half-supply bias voltage across the load. This

increases internal IC power dissipation and may perma-

nently damage loads such as speakers.

POWER DISSIPATION

Power dissipation is a major concern when designing a

successful single-ended or bridged amplifier. Equation (2)

states the maximum power dissipation point for a single-

ended amplifier operating at a given supply voltage and

driving a specified output load.

P

Single-Ended

(2)

However, a direct consequence of the increased power de-

livered to the load by a bridge amplifier is higher internal

power dissipation for the same conditions.

The LM4849 has two operational amplifiers per channel. The

maximum internal power dissipation per channel operating in

the bridge mode is four times that of a single-ended ampli-

fier. From Equation (3), assuming a 5V power supply and a

4

Ω load, the maximum single channel power dissipation is

1.27W or 2.54W for stereo operation.

P

Bridge Mode

(3)

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