LM4867 datasheet(15/28 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 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 coupling ca-

pacitor 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 perma-

nently 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 =(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

the bridge mode is four times that of a single-ended ampli-

fier. From Equation (3), assuming a 5V power supply and an

Ω load, the maximum single channel power dissipation is

1.27W or 2.54W for stereo operation.

DMAX =4*(VDD)

2/(2

π2R

L):

Bridge Mode

(3)

The LM4867’s power dissipation is twice that given by Equa-

tion (2) or Equation (3) when operating in the single-ended

mode or bridge mode, respectively. Twice the maximum

power dissipation point given by Equation (3) must not ex-

ceed the power dissipation given by Equation (4):

DMAX’= (TJMAX −TA)/

(4)

The LM4867’s TJMAX = 150˚C. In the LQ package soldered

to a DAP pad that expands to a copper area of 5in

2 on a

PCB, the LM4867’s

JA is 20˚C/W. In the MTE package

soldered to a DAP pad that expands to a copper area of 2in

on a PCB, the LM4867’s

JA is 41˚C/W. At any given ambient

temperature T

A, use Equation (4) to find the maximum inter-

nal power dissipation supported by the IC packaging. Rear-

ranging Equation (4) and substituting P

DMAX for PDMAX’ re-

sults in Equation (5). This equation gives the maximum

ambient temperature that still allows maximum stereo power

dissipation without violating the LM4867’s maximum junction

temperature.

A =TJMAX −2XPDMAX

(5)

For a typical application with a 5V power supply and an 4

Ω

load, the maximum ambient temperature that allows maxi-

mum stereo power dissipation without exceeding the maxi-

mum junction temperature is approximately 99˚C for the LQ

package and 45˚C for the MTE package.

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