LM4867 datasheet(12/26 Pages) NSC

12 / 26 page

Application Information (Continued)

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 2.5in2 (min) area is

necessary for 5V operation with a 4

Ω load. Heatsink areas

not placed on the same PCB layer as the LM4867 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

LM4867MTE 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 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.

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 LM4867 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 LM4867

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-

tical in magnitude, but 180˚ out of phase. Taking advantage

of this phase difference, a load is placed between -OUTA

and +OUTA and driven differentially (’commonly referred to

as bridge mode’). This results in a differential gain of

VD =2 * (Rf/R i)

(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 as-

sumes 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.

A bridge amplifier design has a few distinct advantages over

the single-ended configuration, as it provides differential

drive to the load, thus doubling the output swing for a speci-

fied supply voltage. Four times the output power is possible

as compared to a single-ended amplifier under the same

conditions. This increase in attainable output power as-

sumes that the amplifier is not current limited or clipped. In

order to choose an amplifier’s closed-loop gain without caus-

ing excessive clipping, please 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 cou-

pling capacitor 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

permanently 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

single-ended amplifier operating at a given supply voltage

and driving a specified output load.

DMAX =(VDD)

2/(2

π2R

L):

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 LM4867 has two operational amplifiers per channel. The

maximum internal power dissipation per channel operating in

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