LM4782 datasheet(19/27 Pages) NSC

19 / 27 page

Application Information (Continued)

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

Mute Mode and Standby Mode sections in the Application

Information section for a more detailed description of these

functions.

CLICKS AND POPS

In the typical application of the LM4782 as a split-supply

audio power amplifier, the IC exhibits excellent “click” and

“pop” performance when utilizing the mute and standby

modes. In addition, the device employs Under-Voltage Pro-

tection, which eliminates unwanted power-up and power-

down transients. The basis for these functions are a stable

and constant half-supply potential. In a split-supply applica-

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

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

at the same rate as the supply rail, V

CC. This makes the task

of attaining a clickless and popless turn-on more challeng-

ing. Any uneven charging of the amplifier inputs will result in

output clicks and pops due to the differential input topology

of the LM4782.

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 LM4782. In Figure 4, 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 4, the resistors labeled R

BI help bias up

the LM4782 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

BI, namely 10k

Ω and 200kΩ. These

resistors bring up the inputs at the same rate resulting in a

popless turn-on. Adjusting these resistors values slightly

may reduce pops resulting from power supplies that ramp

extremely quick or exhibit overshoot during system turn-on.

200811D9

FIGURE 6. Level Shift Circuit

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