L4782TA datasheet(18/26 Pages) NSC

18 / 26 page

Application Information (Continued)

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 (4) below:

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

(4)

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 LM4782 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

correct frequencies for the driver. Tri-Amping is using three

different amplifier stages in the same way bi-amping is done.

Bi-amping can also be done on a three-way speaker design

by using one amplifier for the subwoofer and another for the

midrange and tweeter.

The LM4782 is perfectly suited for bi-amp or tri-amp appli-

cations with it’s three amplifiers. Two of the amplifiers can be

configured for bridge or parallel mode to drive a subwoofer

with the third amplifier driving the tweeter or tweeter and

midrange. An example would be to use a 4

Ω subwoofer and

Ω tweeter/midrange with the LM4782 in parallel and single-

ended modes. Each amplifier would see an 8

Ω load but the

subwoofer would have twice the output power as the

tweeter/midrange. The gain of each amplifier may also be

adjusted for the desired response. Using the LM4782 in a

tri-amp configuration would allow the gain of each amplifier

to be adjusted to achieve the desired speaker response.

SINGLE-SUPPLY AMPLIFIER APPLICATION

The typical application of the LM4782 is a split supply am-

plifier. But as shown in Figure 4, the LM4782 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 LM4782

functions, like the mute function.

The LM4782 possesses a mute and standby function with

internal logic gates that are half-supply referenced. Thus, to

enable either the Mute or Standby function, the voltage at

these pins must be a minimum of 2.5V above half-supply. In

single-supply systems, devices such as microprocessors

and simple logic circuits used to control the mute and

standby functions, are usually referenced to ground, not

half-supply. Thus, to use these devices to control the logic

circuitry of the LM4782, a “level shifter,” like the one shown in

Figure 6, must be employed. A level shifter is not needed in

a split-supply configuration since ground is also half-supply.

When the voltage at the Logic Input node is 0V, the 2N3904

is “off” and thus resistor R

c pulls up mute or standby input to

the supply. This enables the mute or standby function. When

the Logic Input is 5V, the 2N3904 is “on” and consequently,

the voltage at the collector is essentially 0V. This will disable

the mute or standby function, and thus the amplifier will be in

its normal mode of operation. R

shift, along with Cshift, creates

an RC time constant that reduces transients when the mute

or standby functions are enabled or disabled. Additionally,

shift limits the current supplied by the internal logic gates of

the LM4782 which insures device reliability. Refer to the

200811D9

FIGURE 6. Level Shift Circuit

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