LM4854 datasheet(20/29 Pages) NSC

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Application Information (Continued)

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 1.7W to 1.6W.

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 2, the LM4854 consists of three opera-

tional amplifiers. In mono mode, AMP1 and AMP2 operate in

series to drive a speaker connected between their outputs.

In stereo mode, AMP1 and AMP3 are used to drive stereo

headphones or other SE load.

In mono mode, external resistors R

fL and RiL set the closed-

loop gain of AMP1, whereas two internal 20k

Ω resistors set

AMP2’s gain at -1. The LM4854 drives a load, such as a

speaker, connected between the two amplifier outputs,

L-OUT and BTL-OUT.

Figure 2 shows that AMP1’s output serves as AMP2’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 L-OUT and BTL-

OUT and driven differentially (commonly referred to as

"bridge mode"). This results in a differential,or BTL, gain of:

VD = 2(Rf/Ri)

(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. Theoretically, 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 and

that the output signal is not clipped. To ensure minimum

output signal 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

AMP1’s and AMP2’s outputs at half-supply. This eliminates

the coupling capacitor that single supply, single-ended am-

plifiers require. Eliminating an output coupling capacitor in a

typical single-ended configuration forces a single-supply am-

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

creases 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 a single-

ended amplifier operating at a given supply voltage and

driving a specified output load.

DMAX-SE =(VDD)

2/(2

π2 R

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 LM4854 has

two operational amplifiers driving a mono bridge load. The

maximum internal power dissipation operating in the bridge

mode is twice that of a single-ended amplifier. From Equa-

tion (3), assuming a 5V power supply and an 8

Ω load, the

maximum BTL-mode power dissipation is 317mW.

DMAX-MONOBTL = 4(VDD)

2/(2

π2 R

L): Bridge Mode

(3)

The maximum power dissipation point given by Equation (3)

must not exceed the power dissipation given by Equation

(4):

DMAX’= (TJMAX -TA)/

(4)

The LM4854’s TJMAX = 150˚C. In the IBL package, the

LM4854’s

JA is 121˚C/W. The LM4854’s TJMAX = 150˚C. In

the LD package soldered to a DAP pad that expands to a

copper area of 2.0in

2 on a PCB, the LM4854’s

JA is 42˚C/W.

In the MT package, the LM4854’s

JA is 109˚C/W. At any

given ambient temperature T

A, use Equation (4) to find the

maximum internal power dissipation supported by the IC

packaging. Rearranging Equation (4) and substituting P

DMAX

for P

DMAX’ results in Equation (5). This equation gives the

maximum ambient temperature that still allows maximum

stereo power dissipation without violating the LM4854’s

maximum junction temperature.

A =TJMAX -PDMAX-MONOBTL

(5)

For a typical application with a 5V power supply and an 8

Ω

load, the maximum ambient temperature that allows maxi-

mum stereo power dissipation without exceeding the maxi-

mum junction temperature is approximately 73˚C for the IBL

package.

JMAX =PDMAX-MONOBTL

JA +TA

(6)

Equation (6) gives the maximum junction temperature T

J-

MAX

. If the result violates the LM4854’s 150˚C, reduce the

maximum junction temperature by reducing the power sup-

ply voltage or increasing the load resistance. Further allow-

ance should be made for increased ambient temperatures.

The above examples assume that a device is a surface

mount part operating around the maximum power dissipation

point. Since internal power dissipation is a function of output

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