LM4875 datasheet(29/37 Pages) NSC

29 / 37 page

Application Information (Continued)

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 2in

2 area is necessary

for 5V operation with a 4

Ω load. Heatsink areas not placed

on the same PCB layer as the LM4857 should be 4in

2 for the

same supply voltage and load resistance. The last two area

recommendations apply for 25˚C ambient temperature. In-

crease 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 pre-

vent activating the LM4857’s thermal shutdown protection.

An example PCB layout for the exposed-DAP SP package is

shown in the Demonstration Board Layout section. Further

detailed and specific information concerning PCB layout and

fabrication and mounting an SP (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 1.6W to 1.5W.

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

The LM4857 consists of three sets of a bridged-tied amplifier

pairs that drive the left loudspeaker (LLS), the right loud-

speaker (RLS), and the mono earpiece (EP). For this discus-

sion, only the LLS bridge-tied amplifier pair will be referred

to. The LM4857 drives a load, such as a speaker, connected

between outputs, LLS+ and LLS-. In the LLS amplifier block,

the output of the amplifier that drives LLS- serves as the

input to the unity gain inverting amplifier that drives LLS+.

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 LLS- and LLS+

and driven differentially (commonly referred to as ’bridge

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

VD = 2(Rf /Ri)=2

(2)

Both the feedback resistor, R

f, and the input resistor, Ri, are

internally set.

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

LLS- and LLS+ outputs at half-supply. This eliminates the

coupling capacitor that single supply, single-ended amplifiers

require. Eliminating an output coupling capacitor 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 LM4857 has 3 sets of bridged-tied amplifier pairs

driving LLS, RLS, and EP. The maximum internal power

dissipation operating in the bridge mode is twice that of a

single-ended amplifier. From Equation (3) and (4), assuming

a 5V power supply and an 8

Ω load, the maximum power

dissipation for LLS and RLS is 634mW per channel. From

equation (5), assuming a 5V power supply and a 32

Ω load,

the maximum power dissipation for EP is 158mW.

DMAX-LLS = 4(VDD)

2 / (2

π2 R

L): Bridged

(3)

DMAX-RLS = 4(VDD)

2 / (2

π2 R

L): Bridged

(4)

DMAX-EP = 4(VDD)

2 / (2

π2 R

L): Bridged

(5)

The LM4857 also has 3 sets of single-ended amplifiers

driving LHP, RHP, and LINEOUT. The maximum internal

power dissipation for ROUT and LOUT is given by equation

(6) and (7). From Equations (6) and (7), assuming a 5V

power supply and a 32

Ω load, the maximum power dissipa-

tion for LOUT and ROUT is 40mW per channel. From equa-

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

Ω load, the

maximum power dissipation for LINEOUT is negligible.

DMAX-LHP =(VDD)

2 /(2

π2 R

L): Single-ended

(6)

DMAX-RHP =(VDD)

2 /(2

π2 R

L): Single-ended

(7)

DMAX-LINE =(VDD)

2 /(2

π2 R

L): Single-ended

(8)

The maximum internal power dissipation of the LM4857

occurs during output modes 3, 8, and 13 when both loud-

speaker and headphone amplifiers are simultaneously on;

and is given by Equation (9).

DMAX-TOTAL =

DMAX-LLS +PDMAX-RLS +PDMAX-LHP +PDMAX-RHP

(9)

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