LM4851 datasheet(16/22 Pages) NSC

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Application Information (Continued)

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 LM4851 consists of three pairs of

output amplifier blocks (A4-A6). A4, A5, and A6 consist of

bridged-tied amplifier pairs that drive LOUT, ROUT, and

SPKROUT respectively. The LM4851 drives a load, such as

a speaker, connected between 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’). Assuming R

F =RI, this results in a differ-

ential 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 LM4851 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 LM4851 also has 2 pairs of bridged-tied 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 158mW, or 316mW total.

DMAX-LOUT = 4(VDD)

2/(2

π2 R

L): Bridge Mode

(3)

DMAX-ROUT = 4(VDD)

2/(2

π2 R

L): Bridge Mode

(4)

The maximum internal power dissipation of the LM4851

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 LM4851’s TJMAX = 150˚C. In the IBL and ITL packages,

the LM4851’s

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

a DAP pad that expands to a copper area of 2.5in

2 on a

PCB, the LM4851’s

JA is 42˚C/W. At any given ambient

temperature T

A, use Equation (6) to find the maximum inter-

nal power dissipation supported by the IC packaging. Rear-

ranging Equation (6) and substituting P

DMAX-TOTAL for PD-

MAX

’ results in Equation (7). This equation gives the

maximum ambient temperature that still allows maximum

stereo power dissipation without violating the LM4851’s

maximum junction temperature.

A =TJMAX -PDMAX-TOTAL

(7)

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 104˚C for the

IBL package.

JMAX =PDMAX-TOTAL

JA +TA

(8)

Equation (8) gives the maximum junction temperature T

J-

MAX

. If the result violates the LM4851’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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