LM4756 datasheet(12/17 Pages) NSC

12 / 17 page

Application Information (Continued)

voltage swing will be reduced, creating a clipping effect as

shown in Figure 7. With further increases in die temperature

the maximum voltage swing will be further reduced.

The thermal sensing mechanism monitors the global die

temperature and is not intended to operate quickly enough to

shutdown the IC for extremely high power dissipation pulses

created by driving very low impedance loads.

In Figure 7, a 50kHz input signal is used to show the clipping

and attenuating effect of the LM4756 when coming out of

thermal shutdown.

THERMAL CONSIDERATIONS

Determining Maximum Power Dissipation

It is important to determine the maximum amount of package

power dissipation in order to choose an adequate heat sink.

Improper heat sinking can lead to premature thermal shut-

down operation, causing music to cut out. Equation (2) can

be used to calculate the approximate maximum integrated

circuit power dissipation for your amplifier design, given the

supply voltage, and rated load, with both channels being

driven simultaneously.

DMAX = 2(VCCtot

2/2

π2R

(2)

To ensure that a proper heat sink is chosen, be sure to take

into account the effects of the unregulated power supply

voltage variation and the highly reactive load impedance

variation over frequency.

A poorly regulated power supply can have a supply voltage

variation of more than 10V. Be sure to take into account the

no-load power supply voltage.

A nominally rated 8

Ω load can have an impedance dip down

to 5

Ω at low frequencies. As well, the load is not purely

resistive, and this causes the amplifier output current to be

out of phase with the output voltage. When the current and

voltage are out of phase, the internal power dissipation

actually increases.

Equation (2) can be directly applied to the Power Dissipation

vs Output Power curves in the Typical Performance Charac-

teristics section. However, the curves take into account qui-

escent power dissipation which Equation (2) does not. The

curves are to be used as a guideline in determining the

required heat sink and are not intended to provide exact

power dissipation values.

Heat Sinking

Choosing a heat sink for a high-power audio amplifier is

made entirely to keep the die temperature below its maxi-

mum junction temperature, so that the thermal protection

circuitry does not operate under normal circumstances. The

heat sink should be chosen to dissipate the maximum IC

power for the maximum no-load supply voltage and the

minimum load impedance.

Referring to Figure 8, the thermal resistance from the die

(junction) to the outside air (ambient) is a combination of

three thermal resistances,

JC,

CS and

SA. Two of these

thermal resistances are provided by National,

JC and

CS.

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 LM4756 is equal to

the following:

DMAX =(TJMAX –TAMB)/

(3)

The thermal resistance,

JA is equal to

JC +

CS +

SA,

where

JC is the junction-to-case thermal resistance,

CS is

the case-to-sink thermal resistance (thermal compound),

and

SA is the sink-to-ambient thermal resistance.

Once the maximum power dissipation is calculated from

Equation (2) above, the minimum heat sink thermal resis-

tance can be calculated from Equation (4) below.

SA = [(TJMAX –TAMB)– PDMAX (

JC +

CS)]/PDMAX (4)

Example:

CC = +22V

L =8

Ω

JC = 1˚C/W

CS = 0.5˚C/W

(1) P

DMAX = 2((22V)

2/2

π2(8Ω))=6W

(2)

SA = [(150˚C–25˚C) – 6W(1˚C/W + 0.5˚C/W)]/6W =

19˚C/W

Therefore, the minimum heat sink thermal resistance re-

quired is 19˚C/W for both channels being driven simulta-

neously at maximum power dissipation into an 8

Ω load using

a +22V voltage supply. Again, remember to take into account

the unregulated supply voltage and reactive load impedance

dips.

Should it be necessary to isolate the tab of the IC from the

heat sink, an insulating washer can be used. There are many

different types of insulating washers with varying thermal

resistances. Good washers can be obtained from Thermal-

loy or Berquist. Refer to the References list for contact

information for these manufacturers.

Supply Bypassing

The LM4756 has good power supply rejection, however, for

all power amplifiers, proper power supply bypassing is re-

quired. To prevent oscillations and instability, all op amps

20064431

FIGURE 7. Thermal Shutdown Response

20064432

FIGURE 8. Thermal Model

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