LM4844 datasheet(16/21 Pages) NSC

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Application Information (Continued)

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 LM4844 has 2 sets of bridged-tied amplifier pairs

driving LLS and RLS. The maximum internal power dissipa-

tion 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 dissipa-

tion for LLS and RLS is 634mW per channel.

DMAX-LLS = 4(VDD)

2 / (2

π2 R

L): Bridged

(3)

DMAX-RLS = 4(VDD)

2 / (2

π2 R

L): Bridged

(4)

The LM4844 also has a pair of single-ended amplifiers driv-

ing LHP and RHP. The maximum internal power dissipation

for ROUT and LOUT is given by equation (5) and (6). From

Equations (5) and (6), assuming a 5V power supply and a

Ω load, the maximum power dissipation for LOUT and

ROUT is 40mW per channel.

DMAX-LHP =(VDD)

2 /(2

π2 R

L): Single-ended

(5)

DMAX-RHP =(VDD)

2 /(2

π2 R

L): Single-ended

(6)

The maximum internal power dissipation of the LM4844

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

speaker and headphone amplifiers are simultaneously on;

and is given by Equation (7).

DMAX-TOTAL =

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

(7)

The maximum power dissipation point given by Equation (7)

must not exceed the power dissipation given by Equation

(8):

DMAX’= (TJMAX -TA)/

(8)

The LM4844’s T

JMAX

= 150˚C. In the TL package, the

LM4844’s

JA is 62˚C/W. At any given ambient temperature

A, use Equation (8) to find the maximum internal power

dissipation supported by the IC packaging. Rearranging

Equation (8) and substituting P

DMAX-TOTAL for PDMAX’ results

in Equation (9). This equation gives the maximum ambient

temperature that still allows maximum stereo power dissipa-

tion without violating the LM4844’s maximum junction tem-

perature.

A =TJMAX -PDMAX-TOTAL

(9)

For a typical application with a 5V power supply, stereo 8

Ω

loudspeaker load, and the stereo 32

Ω headphone load, the

maximum ambient temperature that allows maximum stereo

power dissipation without exceeding the maximum junction

temperature is approximately 100˚C for the TL package.

JMAX =PDMAX-TOTAL

JA +TA

(10)

Equation (10) gives the maximum junction temperature T

J-

MAX

. If the result violates the LM4844’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

power, higher ambient temperatures are allowed as output

power or duty cycle decreases. If the result of Equation (7) is

greater than that of Equation (8), then decrease the supply

voltage, increase the load impedance, or reduce the ambient

temperature. If these measures are insufficient, a heat sink

can be added to reduce

JA. The heat sink can be created

using additional copper area around the package, with con-

nections to the ground pin(s), supply pin and amplifier output

pins. External, solder attached SMT heatsinks such as the

Thermalloy 7106D can also improve power dissipation.

When adding a heat sink, the

JA is the sum of

JC,

CS, and

SA.(

JC is the junction-to-case thermal impedance,

CS is

the case-to-sink thermal impedance, and

SA is the sink-to-

ambient thermal impedance.) Refer to the Typical Perfor-

mance Characteristics curves for power dissipation informa-

tion at lower output power levels.

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