the fault condition is temporary, but a sustained fault will

cause the device to cycle in a Schmitt Trigger fashion be-

tween the thermal shutdown temperature limits of 165˚C and

155˚C. This greatly reduces the stress imposed on the IC by

thermal cycling, which in turn improves its reliability under

sustained fault conditions.

Since the die temperature is directly dependent upon the

heat sink used, the heat sink should be chosen such that

thermal shutdown will not be reached during normal opera-

tion. Using the best heat sink possible within the cost and

space constraints of the system will improve the long-term

reliability of any power semiconductor device, as discussed

in the Determining the Correct Heat Sink Section.

DETERMINING MAXIMUM POWER DISSIPATION

Power dissipation within the integrated circuit package is a

very important parameter requiring a thorough understand-

ing if optimum power output is to be obtained. An incorrect

maximum power dissipation calculation may result in inad-

equate heat sinking causing thermal shutdown and thus lim-

iting the output power.

Equation (1) exemplifies the theoretical maximum power dis-

sipation point of each amplifier where V

voltage.

P

L

(1)

Thus by knowing the total supply voltage and rated output

load, the maximum power dissipation point can be calcu-

lated. Refer to the graphs of Power Dissipation vs Output

Power in the Typical Performance Characteristics section

which show the actual full range of power dissipation not just

the maximum theoretical point that results from equation (1).

DETERMINING THE CORRECT HEAT SINK

The choice of a heat sink for a high-power audio amplifier is

made entirely to keep the die temperature at a level such

that the thermal protection circuitry does not operate under

normal circumstances.

The thermal resistance from the die (junction) to the outside

air (ambient) is a combination of three thermal resistances,

θ

case), of the LM4701 is 2˚C/W. Using Thermalloy Therma-

cote thermal compound, the thermal resistance,

θ

sink), is about 0.2˚C/W. Since convection heat flow (power

dissipation) is analogous to current flow, thermal resistance

is analogous to electrical resistance, and temperature drops

are analogous to voltage drops, the power dissipation out of

the LM4701 is equal to the following:

P

(2)

where T

ture and

θ

Once the maximum package power dissipation has been

calculated using equation (1), the maximum thermal resis-

tance,

θ

calculation is made using equation (3) which is derived by

solving for

θ

θ

Again it must be noted that the value of

θ

upon the system designer’s amplifier requirements. If the

ambient temperature that the audio amplifier is to be working

under is higher than 25˚C, then the thermal resistance for the

heat sink, given all other things are equal, will need to be

smaller.

SUPPLY BYPASSING

The LM4701 has excellent power supply rejection and does

not require a regulated supply. However, to improve system

performance as well as eliminate possible oscillations, the

LM4701 should have its supply leads bypassed with

low-inductance capacitors having short leads that are lo-

cated close to the package terminals. Inadequate power

supply bypassing will manifest itself by a low frequency oscil-

lation known as “motorboating” or by high frequency insta-

bilities. These instabilities can be eliminated through multiple

bypassing utilizing a large tantalum or electrolytic capacitor

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

variations 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 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

One common power amplifier configuration is shown in

Fig-

ure 2 and is referred to as “bridged mode” operation. Bridged

mode operation is different from the classical single-ended

amplifier configuration where one side of the output 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. Since there

are two amplifiers used in a bridge configuration, the maxi-

mum system power dissipation point will increase by a factor

of four over the figure obtained by equation (1).

This value of P

heat sink for a bridged amplifier application, assuming that

both IC’s are mounted on the same heatsink. 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 LM4701 is a split supply ampli-

fier. But as shown in

Figure 3, the LM4701 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 LM4701

functions.

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