LM4916 datasheet(13/18 Pages) NSC

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Application Information (Continued)

POWER DISSIPATION

Power dissipation is a major concern when designing a

successful amplifier, whether the amplifier is bridged or

single-ended. A direct consequence of the increased power

delivered to the load by a bridge amplifier is an increase in

internal power dissipation. Since the LM4916 has two opera-

tional amplifiers in one package, the maximum internal

power dissipation is 4 times that of a single-ended amplifier.

The maximum power dissipation for a given BTL application

can be derived from the power dissipation graphs or from

Equation 1.

DMAX = 4*(VDD)

2 /(2

π2R

(1)

When operating in Single Ended mode, Equation 2 states

the maximum power dissipation point for a single-ended

amplifier operating at a given supply voltage and driving a

specified output load.

DMAX =(VDD)

2 /(2

π2R

(2)

Since the LM4916 has two operational amplifiers in one

package, the maximum internal power dissipation point is

twice that of the number that results from Equation 2. From

Equation 2, assuming a 1.5V power supply and a 16

Ω load,

the maximum power dissipation point is 7mW per amplifier.

Thus the maximum package dissipation point is 14mW.

The maximum power dissipation point obtained from either

Equations 1, 2 must not be greater than the power dissipa-

tion that results from Equation 3:

DMAX =(TJMAX -TA)/

(3)

For package MUB10A,

JA = 175˚C/W. TJMAX = 150˚C for

the LM4916. Depending on the ambient temperature, T

A,of

the system surroundings, Equation 3 can be used to find the

maximum internal power dissipation supported by the IC

packaging. If the result of Equation 1 or 2 is greater than that

of Equation 3, then either the supply voltage must be de-

creased, the load impedance increased or T

A reduced. For

the typical application of a 1.5V power supply, with a 16

Ω

load, the maximum ambient temperature possible without

violating the maximum junction temperature is approximately

146˚C provided that device operation is around the maxi-

mum power dissipation point. Thus, for typical applications,

power dissipation is not an issue. Power dissipation is a

function of output power and thus, if typical operation is not

around the maximum power dissipation point, the ambient

temperature may be increased accordingly. Refer to the

Typical Performance Characteristics curves for power dissi-

pation information for lower output powers.

20048754

FIGURE 5. Turn-On, Shutdown, and Mute Timing for Cap-Coupled Mode

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