Application Information

BRIDGE CONFIGURATION EXPLANATION

Audio Amplifier portion of the LM4804 has two operational

amplifiers internally, allowing for a few different amplifier

configurations. The first amplifier’s gain is externally config-

urable, while the second amplifier is internally fixed in a

unity-gain, inverting configuration. The closed-loop gain of

the first amplifier is set by selecting the ratio of Rf to Ri while

the second amplifier’s gain is fixed by the two internal 20k

Ω

resistors. Figure 1 shows that the output of amplifier one

serves as the input to amplifier two which results in both

amplifiers producing signals identical in magnitude, but out

of phase by 180˚. Consequently, the differential gain for the

IC is

A

By driving the load differentially through outputs Vo1 and

Vo2, an amplifier configuration commonly referred to as

“bridged mode” is established. Bridged mode operation is

different from the classical single-ended amplifier configura-

tion where one side of the load is connected to ground.

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 con-

ditions. 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 ex-

cessive clipping, please refer to the Audio Power Amplifier

Design section.

A bridge configuration also creates a second advantage over

single-ended amplifiers. Since the differential outputs, Vo1

and Vo2, are biased at half-supply, no net DC voltage exists

across the load. This eliminates the need for an output

coupling capacitor which is required in a single supply,

single-ended amplifier configuration. Without an output cou-

pling capacitor, the half-supply bias across the load would

result in both increased internal IC power dissipation and

also possible loudspeaker damage.

AMPLIFIER 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. Since the amplifier portion of the

LM4804 has two operational amplifiers, the maximum inter-

nal power dissipation is 4 times that of a single-ended am-

plifier. The maximum power dissipation for a given BTL

application can be derived from Equation 1.

P

(1)

BOOST CONVERTER POWER DISSIPATION

At higher duty cycles, the increased ON time of the FET

means the maximum output current will be determined by

power dissipation within the LM2731 FET switch. The switch

power dissipation from ON-state conduction is calculated by

Equation 2.

P

(2)

There will be some switching losses as well, so some derat-

ing needs to be applied when calculating IC power dissipa-

tion.

TOTAL POWER DISSIPATION

The total power dissipation for the LM4804 can be calculated

by adding Equation 1 and Equation 2 together to establish

Equation 3:

P

2

(3)

The result from Equation 3 must not be greater than the

power dissipation that results from Equation 4:

P

θJA

(4)

For package LQA28A,

θ

LM4804. Depending on the ambient temperature, T

system surroundings, Equation 4 can be used to find the

maximum internal power dissipation supported by the IC

packaging. If the result of Equation 3 is greater than that of

Equation 4, then either the supply voltage must be in-

creased, the load impedance increased or T

the typical application of a 3V power supply, with V1 set to

6.0V and 8

Ω load, the maximum ambient temperature pos-

sible without violating the maximum junction temperature is

approximately TBD˚C provided that device operation is

around the maximum power dissipation point. Thus, for typi-

cal applications, power dissipation is not an issue. Power

dissipation is a function of output power and thus, if typical

operation is not around the maximum power dissipation

point, the ambient temperature may be increased accord-

ingly. Refer to the Typical Performance Characteristics

curves for power dissipation information for lower output

levels.

EXPOSED-DAP PACKAGE PCB MOUNTING

CONSIDERATIONS

The LM4804’s exposed-DAP (die attach paddle) package

(LD) provides a low thermal resistance between the die and

the PCB to which the part is mounted and soldered. This

allows rapid heat transfer from the die to the surrounding

PCB copper traces, ground plane, and surrounding air. The

LD package should have its DAP soldered to a copper pad

on the PCB. The DAP’s PCB copper pad may be connected

to a large plane of continuous unbroken copper. This plane

forms a thermal mass, heat sink, and radiation area. Further

detailed and specific information concerning PCB layout,

fabrication, and mounting an LD (LLP) package is available

from National Semiconductor’s Package Engineering Group

under application note AN1187.

SHUTDOWN FUNCTION

In many applications, a microcontroller or microprocessor

output is used to control the shutdown circuitry to provide a

quick, smooth transition into shutdown. Another solution is to

use a single-pole, single-throw switch, in conjunction with an

external pull-up resistor to drive both shutdown pins simul-

taneously. When the switch is closed, the shutdown pin is

connected to ground which disables the amplifier. If the

switch is open, then the external pull-up resistor to V

enable the LM4804. This scheme guarantees that the shut-

down pins will not float thus preventing unwanted state

changes.

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