SA7264

HANGZHOU SILAN MICROELECTRONICS CO.,LTD

REV:1.1

2006.05.25

Http: www.silan.com.cn

Page 5 of 13

POWER DISSIPATION AND HEAT SINKING

The SA7264 must always be operated with a heat sink, even when it is not required to drive a load. The idling

current of the device is 80mA, so that on a

±20V power supply an unloaded SA7264 must dissipate about 3W of

power. The 54

°C/W junction-to-ambient thermal resistance of a HSIP-11 package would cause the die

temperature to rise 162

°C above ambient, so the thermal protection circuitry will shut the amplifier down if

operation without a heat sink is attempted.

In order to determine the appropriate heat sink for a given application, the power dissipation of the SA7264 in

that application must be known. When the load is resistive, the maximum average power that the IC will be

required to dissipate is approximately:

PD(MAX)=Vs

power dissipation of the amplifier. The above equation is only an approximation which assume an “ideal”class B

output stage and constant power dissipation in all other parts of the circuit. The curves of “Power Dissipation vs.

Power Output”give a better representation of the behaviour of the SA7264 with various power supply voltages

and resistive loads. As an example, if the SA7264 is operated on a

±20V power supply with a resistive load of 8Ω,

it can develop up to 23W of internal power dissipation. If the die temperature is to remain below 150

°C for

ambient temperatures up to 50

°C, the total junction-to-ambient thermal resistance must be less than:

(150

°C 50°C)/23W 4.3°C/W

Using Rth(j-c) = 2

°C /W, the sum of the case-to-heat-sink interface thermal resistance and the heat-sink-to-

ambient thermal resistance must be less than 2.3

°C/W. The case-to-heat-sink thermal resistance of the HSIP-11

package varies with the mounting method used. A metal-to-metal interface will be about 1

°C /W if lubricated, and

about 1.2

°C /W if dry.

If a mica insulator is used, the thermal resistance will be about 1.6

°C /W lubricated and 3.4°C /W dry. For this

example, we assume a lubricated mica insulator between the SA7264 and the heat sink. The heat sink thermal

resistance must then be less than:

4.3

°C/W-2°C/W-1.6°C/W 0.7°C/W

This is a rather large heat sink and may not be practical in some applications. If a smaller heat sink is required

for reasons of size or cost, there is an alternative. The heat sink can be isolated from the chassis so the mica

washer is not needed. This will change the required heat sink to a 1.3

°C /W unit if the case-to-heat-sink interface

is lubricated.

The thermal requirements can become more difficult when an amplifier is driving a reactive load. For a given

magnitude of load impedance, a higher degree of reactance will cause a higher level of power dissipation within

the amplifier. As a general rule, the power dissipation of an amplifier driving a 60º reactive load (usually

considered to be a worst-case loudspeaker load) will be roughly that of the same amplifier driving the resistive

part of that load. For example, a loudspeaker may at some frequency have an impedance with a magnitude of 8

Ω

and a phase angle of 60º. The real part of this load will then be 4

Ω, and the amplifier power dissipation will

roughly follow the curve of power dissipation with a 4

Ω load.