Application Information

BRIDGE CONFIGURATION EXPLANATION

As shown in

Figure 1, the LM4818 consist of two operational

amplifiers. External resistors, R

gain of the first amplifier (and the amplifier overall), whereas

two internal 20k

Ω resistors set the second amplifier’s gain at

-1. The LM4818 is typically used to drive a speaker con-

nected between the two amplifier outputs.

Figure 1 shows that the output of Amp1 servers as the input

to Amp2, which results in both amplifiers producing signals

identical in magnitude but 180˚ out of phase. Taking advan-

tage of this phase difference, a load is placed between V

01

and V

’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 assumes that the amplifier is

not current limited or 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 Am-

plifier Design Example section.

Another advantage of the differential bridge output is no net

DC voltage across the load. This results from biasing V

01

and V

that single supply, single-ended amplifiers require. Eliminat-

ing an output coupling capacitor in a single-ended configu-

ration forces a single supply amplifier’s half-supply bias volt-

age across the load. The current flow created by the half-

supply bias voltage 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 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 load.

P

(2)

However, a direct consequence of the increased power de-

livered to the load by a bridged amplifier is an increase in the

internal power dissipation point for a bridge amplifier oper-

ating at the same given conditions. Equation (3) states the

maximum power dissipation point for a bridged amplifier

operating at a given supply voltage and driving a specified

load.

P

(3)

The LM4818 has two operational amplifiers in one package

and the maximum internal power dissipation is four times

that of a single-ended amplifier. However, even with this

substantial increase in power dissipation, the LM4818 does

not require heatsinking. From Equation (3), assuming a 5V

power supply and an 8

Ω load, the maximum power dissipa-

tion point is 633mW. The maximum power dissipation point

obtained from Equation (3) must not exceed the power dis-

sipation predicted by Equation (4):

P

(4)

For the M08A package,

θ

for the LM4818. For a given ambient temperature, T

tion (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 the result of Equation (4), then

decrease the supply voltage, increase the load impedance,

or reduce the ambient temperature. For a typical application

using the M08A packaged LM4818 with a 5V power supply

and an 8

Ω load, the maximum ambient temperature that

does not violate the maximum junction temperature is ap-

proximately 42˚C. It is assumed that a device is a surface

mount part operating around the maximum power dissipation

point. The assumption that the device is operating around

the maximum power dissipation point is incorrect for an 8

Ω

load. The maximum power dissipation point occurs when the

output power is equal to the maximum power dissipation or

50% efficiency. The LM4818 is not capable of the output

power level (633mW) required to operate at the maximum

power dissipation point for an 8

Ω load. To find the maximum

power dissipation, the graph Power Dissipation vs. Output

Power must be used. From the graph, the maximum power

dissipation for an 8

Ω load and a 5V supply is approximately

575mW. Substituting this value back into equation (4) for

P

maximum ambient temperature is 52˚C. Refer to the Typical

Performance Characteristics curves for power dissipation

information for lower output powers and maximum power

dissipation for each package at a given ambient tempera-

ture.

POWER SUPPLY BYPASSING

As with any power amplifier, proper supply bypassing is

critical for low noise performance and high power supply

rejection. The capacitors connected to the bypass and power

supply pins should be placed as close to the LM4818 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 value increases. Typi-

cal applications employ a 5V regulator with 10µF and 0.1µF

filter capacitors that aid in supply stability. Their presence,

however, does not eliminate the need for bypassing the

supply nodes of the LM4818. The selection of bypass ca-

pacitor values, especially C

requirements, click and pop performance as explained in the

section, Proper Selection of External Components, as

well as system cost and size constraints.

SHUTDOWN FUNCTION

The voltage applied to the LM4818’s SHUTDOWN pin con-

trols the shutdown function. Activate micro-power shutdown

by applying V

LM4818’s micro-power shutdown feature turns off the ampli-

fier’s bias circuitry, reducing the supply current. The logic

threshold is typically 1/2V

down current is achieved by applying a voltage that is as

near as V

that is less than V

Avoid intermittent or unexpected micro-power shutdown by

ensuring that the SHUTDOWN pin is not left floating but

connected to either V

There are a few ways to activate micro-power shutdown.

These included using a single-pole, single-throw switch, a

microcontroller, or a microprocessor. When using a switch,

connect an external 10k

Ω to 100kΩ pull-up resistor between

the SHUTDOWN pin and V

the SHUTDOWN pin and ground. Select normal amplifier

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