LM4846TL datasheet(14/19 Pages) NSC

14 / 19 page

Application Information (Continued)

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 LM4846 drives a load, such as a speaker, connected

between outputs, MONO+ and MONO-.

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 MONO- and

MONO+ and driven differentially (commonly referred to as

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

VD = 2(Rf /Ri)=2

(7)

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

MONO- and MONO+ outputs at half-supply. This eliminates

the coupling capacitor that single supply, single-ended am-

plifiers require. Eliminating an output coupling capacitor in a

typical single-ended configuration forces a single-supply am-

plifier’s half-supply bias voltage across the load. This in-

creases 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 LM4846 has a pair of bridged-tied amplifiers

driving a handsfree speaker, MONO. The maximum internal

power dissipation operating in the bridge mode is twice that

of a single-ended amplifier. From Equation (8), assuming a

5V power supply and an 8

Ω load, the maximum MONO

power dissipation is 634mW.

DMAX-SPKROUT = 4(VDD)

2 / (2

π2 R

L): Bridge Mode

(8)

The LM4846 also has a pair of single-ended amplifiers driv-

ing stereo headphones, R

OUT and LOUT. The maximum in-

ternal power dissipation for R

OUT and LOUT is given by

equation (9) and (10). From Equations (9) and (10), assum-

ing a 5V power supply and a 32

Ω load, the maximum power

dissipation for L

OUT and ROUT is 40mW, or 80mW total.

DMAX-LOUT =(VDD)

2 /(2

π2 R

L): Single-ended Mode (9)

DMAX-ROUT =(VDD)

2 /(2

π2 R

L): Single-ended Mode(10)

The maximum internal power dissipation of the LM4846

occurs when all 3 amplifiers pairs are simultaneously on; and

is given by Equation (11).

DMAX-TOTAL =

DMAX-SPKROUT +PDMAX-LOUT +PDMAX-ROUT

(11)

The maximum power dissipation point given by Equation

(11) must not exceed the power dissipation given by Equa-

tion (12):

DMAX =(TJMAX -TA)/

(12)

The LM4846’s T

JMAX = 150˚C. In the ITL package, the

LM4846’s

JA is 65˚C/W. At any given ambient temperature

A, use Equation (12) to find the maximum internal power

dissipation supported by the IC packaging. Rearranging

Equation (12) and substituting P

DMAX-TOTAL for PDMAX’ re-

sults in Equation (13). This equation gives the maximum

ambient temperature that still allows maximum stereo power

dissipation without violating the LM4846’s maximum junction

temperature.

A =TJMAX -PDMAX-TOTAL

(13)

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

ITL package.

JMAX =PDMAX-TOTAL

JA +TA

(14)

Equation (14) gives the maximum junction temperature T

J-

MAX

. If the result violates the LM4846’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 (11)

is greater than that of Equation (12), 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 connections to the ground pin(s), supply pin and ampli-

fier 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 im-

pedance,

CS is the case-to-sink thermal impedance, and

SA is the sink-to-ambient thermal impedance). Refer to the

Typical Performance Characteristics curves for power dissi-

pation information at lower output power levels.

POWER SUPPLY BYPASSING

As with any power amplifier, proper supply bypassing is

critical for low noise performance and high power supply

rejection. Applications that employ a 5V regulator typically

use a 1µF in parallel with a 0.1µF filter capacitors to stabilize

the regulator’s output, reduce noise on the supply line, and

improve the supply’s transient response. However, their

presence does not eliminate the need for a local 1.1µF

tantalum bypass capacitance connected between the

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