If adequate bypassing is not provided, the current in the sup-

ply leads which is a rectified component of the load current

may be fed back into internal circuitry. This signal causes

distortion at high frequencies requiring that the supplies be

bypassed at the package terminals with an electrolytic ca-

pacitor of 470 µF or more.

BRIDGED AMPLIFIER APPLICATION

The LM4766 has two operational amplifiers internally, allow-

ing for a few different amplifier configurations. One of these

configurations is referred to as “bridged mode” and involves

driving the load differentially through the LM4766’s outputs.

This configuration is shown in

Figure 2. Bridged mode op-

eration is different from the classical single-ended amplifier

configuration where one side of its load is connected to

ground.

A bridge amplifier design has a distinct advantage over the

single-ended configuration, as it provides differential drive to

the load, thus doubling output swing for a specified supply

voltage. Consequently, theoretically 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.

A direct consequence of the increased power delivered to

the load by a bridge amplifier is an increase in internal power

dissipation. For each operational amplifier in a bridge con-

figuration, the internal power dissipation will increase by a

factor of two over the single ended dissipation. Thus, for an

audio power amplifier such as the LM4766, which has two

operational amplifiers in one package, the package dissipa-

tion will increase by a factor of four. To calculate the

LM4766’s maximum power dissipation point for a bridged

load, multiply

Equation (1) by a factor of four.

This value of P

heat sink for a bridged amplifier application. Since the inter-

nal dissipation for a given power supply and load is in-

creased by using bridged-mode, the heatsink’s

θ

to decrease accordingly as shown by

Equation (3). Refer to

the section, Determining the Correct Heat Sink, for a more

detailed discussion of proper heat sinking for a given appli-

cation.

SINGLE-SUPPLY AMPLIFIER APPLICATION

The typical application of the LM4766 is a split supply ampli-

fier. But as shown in

Figure 3, the LM4766 can also be used

in a single power supply configuration. This involves using

some external components to create a half-supply bias

which is used as the reference for the inputs and outputs.

Thus, the signal will swing around half-supply much like it

swings around ground in a split-supply application. Along

with proper circuit biasing, a few other considerations must

be accounted for to take advantage of all of the LM4766

functions, like the mute function.

CLICKS AND POPS

In the typical application of the LM4766 as a split-supply au-

dio power amplifier, the IC exhibits excellent “click” and “pop”

performance when utilizing the mute and standby modes. In

addition, the device employs Under-Voltage Protection,

which eliminates unwanted power-up and power-down tran-

sients. The basis for these functions are a stable and con-

stant half-supply potential. In a split-supply application,

ground is the stable half-supply potential. But in a

single-supply application, the half-supply needs to charge up

just like the supply rail, V

a clickless and popless turn-on more challenging. Any un-

even charging of the amplifier inputs will result in output

clicks and pops due to the differential input topology of the

LM4766.

To achieve a transient free power-up and power-down, the

voltage seen at the input terminals should be ideally the

same. Such a signal will be common-mode in nature, and

will be rejected by the LM4766. In

Figure 3, the resistor R

INP

serves to keep the inputs at the same potential by limiting the

voltage difference possible between the two nodes. This

should significantly reduce any type of turn-on pop, due to an

uneven charging of the amplifier inputs. This charging is

based on a specific application loading and thus, the system

designer may need to adjust these values for optimal perfor-

mance.

As shown in

Figure 3, the resistors labeled R

the LM4766 off the half-supply node at the emitter of the

2N3904. But due to the input and output coupling capacitors

in the circuit, along with the negative feedback, there are two

different values of R

sistors bring up the inputs at the same rate resulting in a pop-

less turn-on. Adjusting these resistors values slightly may re-

duce pops resulting from power supplies that ramp

extremely quick or exhibit overshoot during system turn-on.

AUDIO POWER AMPLlFIER DESIGN

Design a 30W/8

Ω Audio Amplifier

Given:

Power Output

30 Wrms

Load Impedance

8

Ω

Input Level

1 Vrms(max)

Input Impedance

47 k

Ω

Bandwidth

20 Hz−20 kHz

A designer must first determine the power supply require-

ments in terms of both voltage and current needed to obtain

the specified output power. V

Equation (4) and I

(4)

(5)

To determine the maximum supply voltage the following con-

ditions must be considered. Add the dropout voltage to the

peak output swing V

of I

loaded voltage which is usually about 15% higher. The sup-

ply voltage will also rise 10% during high line conditions.

Therefore the maximum supply voltage is obtained from the

following equation.

Max supplies

For 30W of output power into an 8

Ω load, the required

V

from adding V

Equation (5). It should be noted that for a dual 30W amplifier

into an 8

Ω load the I

2.74 Apk or 5.48 Apk. At this point it is a good idea to check

the Power Output vs Supply Voltage to ensure that the re-

quired output power is obtainable from the device while

maintaining low THD+N. In addition, the designer should

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