LM4781 datasheet(16/25 Pages) NSC

16 / 25 page

Application Information (Continued)

tics section shows the area of device operation where

SPiKe Protection Circuitry is not enabled. The SPiKe Pro-

tection Response waveform graph shows the waveform dis-

tortion when SPiKe is enabled. Please refer to AN-898 for

more detailed information.

THERMAL PROTECTION

The LM4781 has a sophisticated thermal protection scheme

to prevent long-term thermal stress of the device. When the

temperature on the die exceeds 150˚C, the LM4781 shuts

down. It starts operating again when the die temperature

drops to about 145˚C, but if the temperature again begins to

rise, shutdown will occur again above 150˚C. Therefore, the

device is allowed to heat up to a relatively high temperature

if 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 150˚C and

145˚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 so that

thermal shutdown is not activated during normal operation.

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.

DETERMlNlNG 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

limiting the output power.

Equation (2) shows the theoretical maximum power dissipa-

tion point for each amplifier in a single-ended configuration

where V

CC is the total supply voltage.

DMAX =(VCC)

2 /2

π2R

(2)

Thus by knowing the total supply voltage and rated output

load, the maximum power dissipation point can be calcu-

lated. The package dissipation is three times the number

which results from Equation (2) since there are three ampli-

fiers in each LM4781. Refer to the graphs of Power Dissipa-

tion versus Output Power in the Typical Performance Char-

acteristics section which show the actual full range of power

dissipation not just the maximum theoretical point that re-

sults from Equation (2).

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 is not activated under

normal circumstances.

The thermal resistance from the die to the outside air,

(junction to ambient), is a combination of three thermal re-

sistances,

JC (junction to case),

CS (case to sink), and

(sink to ambient). The thermal resistance,

JC (junction to

case), of the LM4781T is 0.9˚C/W. Using Thermalloy Ther-

macote thermal compound, the thermal resistance,

(case to 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 tem-

perature drops are analogous to voltage drops, the power

dissipation out of the LM4781 is equal to the following:

DMAX =(TJMAX−TAMB)/

(3)

where T

JMAX = 150˚C, TAMB is the system ambient tempera-

ture and

JA =

JC +

CS +

SA.

200673B8

Once the maximum package power dissipation has been

calculated using Equation 2, the maximum thermal resis-

tance,

SA, (heat sink to ambient) in ˚C/W for a heat sink can

be calculated. This calculation is made using Equation 4

which is derived by solving for

SA in Equation 3.

SA = [(TJMAX−TAMB)−PDMAX(

JC +

CS)]/PDMAX

(4)

Again it must be noted that the value of

SA is dependent

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 LM4781 has excellent power supply rejection and does

not require a regulated supply. However, to improve system

performance as well as eliminate possible oscillations, the

LM4781 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 LM4781 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

LM4781’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-

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