LM4856 datasheet(17/23 Pages) NSC

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Application Information (Continued)

not placed on the same PCB layer as the LM4856 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 all circumstances and under all

conditions, the junction temperature must be held below

150˚C to prevent activating the LM4856’s thermal shutdown

protection. Further detailed and specific information con-

cerning PCB layout and fabrication and mounting an LD

(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 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 1, the LM4856 consists of three pairs of

output amplifier blocks (A4-A6). Amplifier block A6 consists

of a bridged-tied amplifier pair that drives SPKROUT. The

LM4856 drives a load, such as a speaker, connected be-

tween outputs, SPKROUT+ and SPKROUT-. In the amplifier

block A6, the output of the amplifier that drives SPKROUT-

serves as the input to the unity gain inverting amplifier that

drives SPKROUT+.

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 SPKROUT- and

SPKROUT+ and driven differentially (commonly referred to

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

VD = 2(Rf /Ri)=2

(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.

Another advantage of the differential bridge output is no net

DC voltage across the load. This is accomplished by biasing

SPKROUT- and SPKROUT+ outputs at half-supply. This

eliminates the coupling capacitor that single supply, single-

ended amplifiers require. Eliminating an output coupling ca-

pacitor in a typical 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.

A direct consequence of the increased power delivered to

the load by a bridge amplifier is higher internal power dissi-

pation. The LM4856 has a pair of bridged-tied amplifiers

driving a handsfree speaker, SPKROUT. The maximum in-

ternal power dissipation operating in the bridge mode is

twice that of a single-ended amplifier. From Equation (2),

assuming a 5V power supply and an 8

Ω load, the maximum

SPKROUT power dissipation is 634mW.

DMAX-SPKROUT = 4(VDD)

2 / (2

π2 R

L): Bridge Mode

(2)

The LM4856 also has a pair of single-ended amplifiers driv-

ing stereo headphones, ROUT and LOUT. The maximum

internal power dissipation for ROUT and LOUT is given by

equation (3) and (4). From Equations (3) and (4), assuming

a 5V power supply and a 32

Ω load, the maximum power

dissipation for LOUT and ROUT is 40mW, or 80mW total.

DMAX-LOUT =(VDD)

2 /(2

π2 R

L): Single-ended Mode (3)

DMAX-ROUT =(VDD)

2 /(2

π2 R

L): Single-ended Mode (4)

The maximum internal power dissipation of the LM4856

occurs when all 3 amplifiers pairs are simultaneously on; and

is given by Equation (5).

DMAX-TOTAL =

DMAX-SPKROUT +PDMAX-LOUT +PDMAX-ROUT

(5)

The maximum power dissipation point given by Equation (5)

must not exceed the power dissipation given by Equation

(6):

DMAX’= (TJMAX -TA)/

(6)

The LM4856’s T

JMAX = 150˚C. In the ITL package, the

LM4856’s

JA is 48˚C/W. In the LD package soldered to a

DAP pad that expands to a copper area of 2.5in

2 on a PCB,

the LM4856’s

JA is 42˚C/W. At any given ambient tempera-

ture T

A, use Equation (6) to find the maximum internal power

dissipation supported by the IC packaging. Rearranging

Equation (6) and substituting P

DMAX-TOTAL for PDMAX’ results

in Equation (7). This equation gives the maximum ambient

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