LM4733 datasheet(15/23 Pages) NSC

15 / 23 page

Application Information (Continued)

SUPPLY BYPASSING

The LM4733 has excellent power supply rejection and does

not require a regulated supply. However, to improve system

performance as well as eliminate possible oscillations, the

LM4733 should have its supply leads bypassed with low-

inductance capacitors having short leads that are located

close to the package terminals. Inadequate power supply

bypassing will manifest itself by a low frequency oscillation

known as “motorboating” or by high frequency instabilities.

These instabilities can be eliminated through multiple by-

passing utilizing a large tantalum or electrolytic capacitor

(10µF or larger) which is used to absorb low frequency

variations and a small ceramic capacitor (0.1µF) to prevent

any high frequency feedback through the power supply lines.

If adequate bypassing is not provided, the current in the

supply 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 sup-

plies be bypassed at the package terminals with an electro-

lytic capacitor of 470µF or more.

BRIDGED AMPLIFIER APPLICATION

The LM4733 has three operational amplifiers internally, al-

lowing for a few different amplifier configurations. One of

these configurations is referred to as “bridged mode” and

involves driving the load differentially through two of the

LM4733’s outputs. This configuration is shown in Figure 2.

Bridged mode operation 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. Theoretically, four times the output power is pos-

sible 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. Using Equa-

tion (2) the load impedance should be divided by a factor of

two to find the maximum power dissipation point for each

amplifier in a bridge configuration. In the case of an 8

Ω load

in a bridge configuration, the value used for R

L in Equation

(2) would be 4

Ω for each amplifier in the bridge. When using

two of the amplifiers of the LM4733 in bridge mode, the third

amplifier should have a load impedance equal to or higher

than the equivalent impedance seen by each of the bridged

amplifiers. In the example above where the bridge load is 8

Ω

and each amplifier in the bridge sees a load value of 4

Ω then

the third amplifier should also have a 4

Ω load impedance or

higher. Using a lower load impedance on the third amplifier

will result in higher power dissipation in the third amplifier

than the other two amplifiers and may result in unwanted

activation of thermal shut down on the third amplifier. Once

the impedance seen by each amplifier is known then Equa-

tion (2) can be used to calculated the value of P

DMAX for

each amplifier. The P

DMAX of the IC package is found by

adding up the power dissipation for each amplifier within the

IC package.

This value of P

DMAX can be used to calculate the correct size

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

SA will have

to decrease accordingly as shown by Equation 4. Refer to

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

detailed discussion of proper heat sinking for a given appli-

cation.

PARALLEL AMPLIFIER APPLICATION

Parallel configuration is normally used when higher output

current is needed for driving lower impedance loads (i.e. 4

Ω

or lower) to obtain higher output power levels. As shown in

Figure 3 , the parallel amplifier configuration consist of de-

signing the amplifiers in the IC to have identical gain, con-

necting the inputs in parallel and then connecting the outputs

in parallel through a small external output resistor. Any num-

ber of amplifiers can be connected in parallel to obtain the

needed output current or to divide the power dissipation

across multiple IC packages. Ideally, each amplifier shares

the output current equally. Due to slight differences in gain

the current sharing will not be equal among all channels. If

current is not shared equally among all channels then the

power dissipation will also not be equal among all channels.

It is recommended that 0.1% tolerance resistors be used to

set the gain (R

i and Rf) for a minimal amount of difference in

current sharing.

When operating two or more amplifiers in parallel mode the

impedance seen by each amplifier is equal to the total load

impedance multiplied by the number of amplifiers driving the

load in parallel as shown by Equation (5) below:

L(parallel) =RL(total) x Number of amplifiers

(5)

Once the impedance seen by each amplifier in the parallel

configuration is known then Equation (2) can be used with

this calculated impedance to find the amount of power dis-

sipation for each amplifier. Total power dissipation (P

DMAX)

within an IC package is found by adding up the power

dissipation for each amplifier in the IC package. Using the

calculated P

DMAX the correct heat sink size can be deter-

mined. Refer to the section, Determining the Correct Heat

Sink, for more information and detailed discussion of proper

heat sinking.

If only two amplifiers of the LM4733 are used in parallel

mode then the third amplifier should have a load impedance

equal to or higher than the equivalent impedance seen by

each of the amplifiers in parallel mode. Having the same

load impedance on all amplifiers means that the power

dissipation in each amplifier will be equal. Using a lower load

impedance on the third amplifier will result in higher power

dissipation in the third amplifier than the other two amplifiers

and may result in unwanted activation of thermal shut down

on the third amplifier. Having a higher impedance on the third

amplifier than the equivalent impedance on the two amplifi-

ers in parallel will reduce total IC package power dissipation

reducing the heat sink size requirement.

BI-AMP AND TRI-AMP APPLICATIONS

Bi-amping is the practice of using two different amplifiers to

power the individual drivers in a speaker enclosure. For

example, a two-way speaker enclosure might have a tweeter

and a subwoofer. One amplifier would drive the tweeter and

another would drive the subwoofer. One advantage is that

the gain of each amplifier can be adjusted for the different

driver sensitivities. Another advantage is the crossover can

be designed before the amplifier stages with low cost op

amps instead of large passive components. With the cross-

over before the amplifier stages no power is wasted in the

passive crossover as each individual amplifier provides the

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