LM4991

SNAS217A – MAY 2004 – REVISED APRIL 2013

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A bridge amplifier design has a few distinct advantages over the single-ended configuration, as it provides

differential drive to the load, thus doubling output swing for a specified 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 assumes that the amplifier is not current limited or clipped. In order to choose an amplifier's closed-

loop gain without causing excessive clipping, please refer to the AUDIO POWER AMPLIFIER DESIGN section.

Another advantage of the differential bridge output is no net DC voltage across load. This results from biasing

VO1 and VO2 at the same DC voltage, in this case VDD/2 . This eliminates the coupling capacitor that single

supply, single-ended amplifiers require. Eliminating an output coupling capacitor in a single-ended configuration

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 dissipation and my permanently damage loads such as

speakers.

POWER DISSIPATION

Power dissipation is a major concern when designing a successful amplifier, whether the amplifier is bridged or

single-ended. A direct consequence of the increased power delivered to the load by a bridge amplifier is an

increase in internal power dissipation. Equation (2) states the maximum power dissipation point for a bridge

amplifier operating at a given supply voltage and driving a specified output load.

PDMAX = 4*(VDD)

2/(2π2R

L)

(2)

Since the LM4991 has two operational amplifiers in one package, the maximum internal power dissipation is 4

times that of a single-ended ampifier. Even with this substantial increase in power dissipation, the LM4991 does

not require heatsinking under most operating conditions and output loading. From Equation (2), assuming a 5V

power supply and an 8

Ω load, the maximum power dissipation point is 625 mW. The maximum power dissipation

point obtained from Equation (2) must not be greater than the power dissipation that results from Equation (3):

PDMAX = (TJMAX–TA)/θJA

(3)

For the SO package,

θJA = 140°C/W. For the LD package soldered to a DAP pad that expands to a copper area

of 1.0in2 on a PCB, the LM4991's

θJA is 56°C/W. TJMAX = 150°C for the LM4991. The θJA can be decreased by

using some form of heat sinking. The resultant

θJA will be the summation of the θJC, θCS, and θSA. θJC is the

junction to case of the package (or to the exposed DAP, as is the case with the LD package),

θCS is the case to

heat sink thermal resistance and

θSA is the heat sink to ambient thermal resistance. By adding additional copper

area around the LM4991, the

θJA can be reduced from its free air value for the SO package. Increasing the

copper area around the LD package from 1.0in2 to 2.0in2 area results in a

θJA decrease to 46°C/W. Depending

on the ambient temperature, TA, and the θJA, Equation (3) can be used to find the maximum internal power

dissipation supported by the IC packaging. If the result of Equation (2) is greater than that of Equation (3), then

either the supply voltage must be decreased, the load impedance increased, the

θJA decreased, or the ambient

temperature reduced. For the typical application of a 5V power supply, with an 8

Ω load, and no additional

heatsinking, the maximum ambient temperature possible without violating the maximum junction temperature is

approximately 61°C provided that device operation is around the maximum power dissipation point and assuming

surface mount packaging. For the LD package in a typical application of a 5V power supply, with a 4

Ω load, and

1.0in2 copper area soldered to the exposed DAP pad, the maximum ambient temperature is approximately 77°C

providing device operation is around the maximum power dissipation point. Internal power dissipation is a

function of output power. If typical operation is not around the maximum power dissipation point, the ambient

temperature can be increased. Refer to the Typical Performance Characteristics curves for power dissipation

information for different output powers and output loading.

POWER SUPPLY BYPASSING

As with any amplifier, proper supply bypassing is critical for low noise performance and high power supply

rejection. The capacitor location on both the bypass and power supply pins should be as close to the LM4991 as

possible. The capacitor connected between the bypass pin and ground improves the internal bias voltage's

stability, producing improved PSRR. The improvements to PSRR increase as the bypass pin capacitor increases.

Typical applications employ a 5V regulator with 10µF and a 0.1µF bypass capacitors which aid in supply stability.

This does not eliminate the need for bypassing the supply nodes of the LM4991 with a 1µF tantalum capacitor.

The selection of bypass capacitors, especially CB, is dependent upon PSRR requirements, click and pop

performance as explained in the section, PROPER SELECTION OF EXTERNAL COMPONENTS, system cost,

and size constraints.

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