Application Information

BRIDGE CONFIGURATION EXPLANATION

As shown in Figure 1, the LM4876 consists of two opera-

tional amplifiers. External resistors R

loop gain of Amp1, whereas two internal 40k

Ω resistors set

Amp2’s gain at -1. The LM4876 drives a load, such as a

speaker, connected between the two amplifier outputs, V

and V

Figure 1 shows that the Amp1 output serves as the Amp2

input, which 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 V

V

and driven differentially (commonly referred to as

"bridge mode"). This results in a differential gain of

A

(1)

Bridge mode is different from single-ended amplifiers that

drive loads connected between a single amplifier’s output

and ground. For a given supply voltage, bridge mode has a

distinct advantage over the single-ended configuration: its

differential output doubles the voltage swing across the load.

This results in 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 ampli-

fier is not current limited or that the output signal is not

clipped. To ensure minimum output signal clipping when

choosing an amplifier’s closed-loop gain, refer to the Audio

Power Amplifier Design section.

Another advantage of the differential bridge output is no net

DC voltage across the load. This results from biasing V

and V

tor that single supply, single-ended amplifiers require. Elimi-

nating an output coupling capacitor in a single-ended con-

figuration forces a single-supply amplifier’s half-supply bias

voltage across the load. The current flow created by the

half-supply bias voltage increases internal IC power dissipa-

tion and may permanently damage loads such as speakers.

POWER DISSIPATION

Power dissipation is a major concern when designing a

successful bridged or single-ended 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.

P

(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 LM4876 has two operational amplifiers in one package

and the maximum internal power dissipation is four times

that of a single-ended amplifier. Equation (3) states the

maximum power dissipation for a bridge amplifier. However,

even with this substantial increase in power dissipation, the

LM4876 does not require heatsinking. From Equation (3),

assuming a 5V power supply and an 8

Ω load, the maximum

power dissipation point is 633mW.

P

(3)

The maximum power dissipation point given by Equation (3)

must not exceed the power dissipation given by Equation

(4):

P

θ

JA

(4)

The LM4876’s T

LM4876’s

θ

T

dissipation supported by the IC packaging. Rearranging

Equation (4) results in Equation (5). This equation gives the

maximum ambient temperature that still allows maximum

power dissipation without violating the LM4876’s maximum

junction temperature.

T

θ

JA

(5)

For a typical application with a 5V power supply and an 8W

load, the maximum ambient temperature that allows maxi-

mum power dissipation without exceeding the maximum

junction temperature is approximately 61˚C.

T

θ

(6)

For the MSOP10A package,

θ

shows that T

ambient temperature of 25˚C and using the same 5V power

supply and an 8

Ω load. This violates the LM4876’s 150˚C

maximum junction temperature when using the MSOP10A

package. Reduce the junction temperature by reducing the

power supply voltage or increasing the load resistance. Fur-

ther, allowance should be made for increased ambient tem-

peratures. To achieve the same 61˚C maximum ambient

temperature found for the MO8 package, the MSOP10 pack-

aged part should operate on a 4.1V supply voltage when

driving an 8

Ω load. Alternatively, a 5V supply can be used

when driving a load with a minimum resistance of 12

Ω for the

same 61˚C maximum ambient temperature.

Fully charged Li-ion batteries typically supply 4.3V to por-

table applications such as cell phones. This supply voltage

allows the LM4876 to drive loads with a minimum resistance

of 9

Ω without violating the maximum junction temperature

when the maximum ambient temperature is 61˚C.

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 (3) is greater than that of Equation

(4), 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

θ

copper area around the package, with connections to the

ground pin(s), supply pin and amplifier output pins. When

adding a heat sink, the

θ

θ

θ

θ

(

θ

θ

case-to-sink thermal impedance, and

θ

ambient thermal impedance.) Refer to the Typical Perfor-

mance Characteristics curves for power dissipation infor-

mation 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 10µF in parallel with a 0.1µF filter capacitors to stabi-

lize 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 local bypass ca-

pacitance at the LM4876’s supply pins. Keep the length of

leads and traces that connect capacitors between the

LM4876’s power supply pin and ground as short as possible.

Connecting a 1µF capacitor between the BYPASS pin and

ground improves the internal bias voltage’s stability and

improves the amplifier’s PSRR. The PSRR improvements

increase as the bypass pin capacitor value increases. Too

large, however, and the amplifier’s click and pop perfor-

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