LD (LLP) package is available from National Semiconduc-

tor’s Package Engineering Group under application note

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-

pendant 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 2.0W to

1.95W. This problem of decreased load dissipation is exac-

erbated as load impedance decreases. Therefore, to main-

tain 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 LM4871 has two operational

amplifiers internally, allowing for a few different amplifier

configurations. The first amplifier’s gain is externally config-

urable; the second amplifier is internally fixed in a unity-gain,

inverting configuration. The closed-loop gain of the first am-

plifier is set by selecting the ratio of R

amplifier’s gain is fixed by the two internal 40k

Ω resistors.

Figure 1 shows that the output of amplifier one serves as the

input to amplifier two, which results in both amplifiers pro-

ducing signals identical in magnitude, but 180˚ out of phase.

Consequently, the differential gain for the IC is

A

By driving the load differentially through outputs Vo1 and

Vo2, an amplifier configuration commonly referred to as

“bridged mode” is established. Bridged mode operation is

different from the classical single-ended amplifier configura-

tion where one side of its load is connected to ground.

A bridge amplifier design has a few distinct advantages over

the single-ended configuration, as it provides differential

drive to the load, thus doubling output swing for a specified

supply voltage. Four times the output power is possible as

compared to a single-ended amplifier under the same con-

ditions. This increase in attainable output power assumes

that the amplifier is not current limited or clipped. In order to

choose an amplifier’s closed-loop gain without causing ex-

cessive clipping, please refer to the Audio Power Amplifier

Design section.

Another advantage of the differential bridge output is no net

DC voltage across load. This results from biasing V

V

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. The

current flow created by the half-supply bias voltage in-

creases internal IC power dissipation and my permanently

damage loads such as speakers.

POWER DISSIPATION

Power dissipation is a major concern when designing a

successful amplifier, whether the amplifier is bridged or

single-ended. A direct consequence of the increased power

delivered to the load by a bridge amplifier is an increase in

internal power dissipation. Equation 1 states the maximum

power dissipation point for a bridge amplifier operating at a

given supply voltage and driving a specified output load.

P

(1)

Since the LM4871 has two operational amplifiers in one

package, the maximum internal power dissipation is 4 times

that of a single-ended ampifier. Even with this substantial

increase in power dissipation, the LM4871 does not require

heatsinking under most operating conditions and output

loading. From Equation 1, assuming a 5V power supply and

an 8

Ω load, the maximum power dissipation point is

625 mW. The maximum power dissipation point obtained

from Equation 1 must not be greater than the power dissi-

pation that results from Equation 2:

P

θ

JA

(2)

For the SO package,

θ

θ

θ

assuming free air operation. For the LD package soldered to

a DAP pad that expands to a copper area of 1.0in

PCB, the LM4871’s

θ

LM4871. The

θ

heat sinking. The resultant

θ

θ

θ

θ

θ

(or to the exposed DAP, as is the case with the LD package),

θ

θ

heat sink to ambient thermal resistance. By adding addi-

tional copper area around the LM4871, the

θ

reduced from its free air value for the SO and MSOP pack-

ages. Increasing the copper area around the LD package

from 1.0in

θ

46˚C/W. Depending on the ambient temperature, T

θ

power dissipation supported by the IC packaging. If the

result of Equation 1 is greater than that of Equation 2, then

either the supply voltage must be decreased, the load im-

pedance increased, the

θ

perature reduced. For the typical application of a 5V power

supply, with an 8

Ω load, and no additional heatsinking, the

maximum ambient temperature possible without violating the

maximum junction temperature is approximately 61˚C pro-

vided that device operation is around the maximum power

dissipation point and assuming surface mount packaging.

For the LD package in a typical application of a 5V power

supply, with a 4

the exposed DAP pad, the maximum ambient temperature is

approximately 77˚C providing device operation is around the

maximum power dissipation point. Internal power dissipation

is a function of output power. If typical operation is not

around the maximum power dissipation point, the ambient

temperature can be increased. Refer to the Typical Perfor-

mance Characteristics curves for power dissipation infor-

mation for different output powers and output loading.

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