In

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

and attenuating effect of the LM4753 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.

P

(2)

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

into account the effects of the unregulated power supply volt-

age variation and the highly reactive load impedance varia-

tion 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 resis-

tive, 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 in-

creases.

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 re-

quired 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 cir-

cuitry 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 mini-

mum 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,

θ

thermal resistances are provided by National,

θ

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 LM4753 is equal to

the following:

P

(3)

The thermal resistance,

θ

where

θ

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

and

θ

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.

θ

Example:

V

R

θ

θ

(1) P

(2)

θ

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 re-

sistances. Good washers can be obtained from Thermalloy

or Berquist. Refer to the References list for contact informa-

tion for these manufacturers.

Supply Bypassing

The LM4753 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

and power op amps should have their supply leads by-

passed with low-inductance capacitors having short leads.

All high frequency bypass capacitors should be located as

close to the package terminals as possible and have a clear

unobstructed current return path to ground. It is typical to use

capacitor values that are a factor of 100 different from each

other to minimize interaction with each other. The LM4753

should be bypassed with 0.1 µF ceramic and 100 µF tanta-

lum capacitors for optimum performance. The 100 µF tanta-

lum can be replaced with an electrolytic, but the bypassing

DS100043-31

FIGURE 7. Thermal Shutdown Response

DS100043-32

FIGURE 8. Thermal Model

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