These instabilities can be eliminated through multiple by-

passing utilizing a large tantalum or electrolytic capacitor (10

µF or larger) which is used to absorb low frequency varia-

tions and a small ceramic capacitor (0.1 µF) 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 470 µF or more.

BRIDGED AMPLIFIER APPLICATION

The LM4731 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 LM4731’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 LM4731, which has two

operational amplifiers in one package, the package dissipa-

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

LM4731’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 LM4731 is a split supply am-

plifier. But as shown in Figure 3, the LM4731 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 LM4731

functions.

The LM4731 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 LM4731, a “level shifter,” like the one shown in

Figure 5, 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

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

an RC time constant that reduces transients when the mute

or standby functions are enabled or disabled. Additionally,

R

the LM4731 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 LM4731 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

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

LM4731.

20060354

FIGURE 5. Level Shift Circuit

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