SA7347

HANGZHOU MICROELECTRONICS CO.,LTD

REV:1.1

2007.07.27

Http: www.silan.com.cn

Page 5 of 9

protection circuit will start to switch off the bias of channel when local temperature reaches approximately 185

°C

or global temperature reaches approximately 150

°C. As a result, there is low dissipating in the chip. It starts

operating again when the chip temperature drop to same value below the die temperature. But if the temperature

again begins to rise, the SA7347 shuts down again. The process will not stop until the local temperature is under

185

°C and the global temperature is under 150°C.

This protects the SA7347 against shorts to ground, to the supply voltage and across the load, and against too

high chip temperatures. However, not that there are abnormal events as shorts to ground, to the supply voltage

and across the load in the chip, the protection circuit should start-up. In fact, just as much as the temperature

does not exceed the critical level, the protection circuit will not operate.

Power Dissipation and Heat Sinking

Proper heat sinking is necessary to ensure that SA7347 will function correctly under all operating conditions. A

heat sink that is too small will cause the die to heat excessively and will result in a degraded output signal as the

thermal protection circuitry begins to operate.

In order to determine the appropriate heat sink for a given application, the power dissipation of the SA7347 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:

Where VS is the total power supply voltage across the SA7347, RL is the load resistance; PQ is the quiescent

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

output stage and constant power dissipation in all other parts of the circuit. As an example, if the SA7347 is

operated on a 15V power supply with a resistive load of 4

Ω, it can develop up to 13.5W of internal power

dissipation. If the die temperature is to remain below 150

°C for ambient temperatures up to 60°C, the total

junction-to-ambient thermal resistance must be less than:

(150

°C 60°C)/13.5W 6.7°C /W

Using Rth(j-c) = 1.3

°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 5.4

°C/W. The case-to-heat-sink thermal resistance of the HSIP-15

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

°C /W if lubricated, and

about 1.5

°C /W if dry.

6.7

°C /W-1.3°C /W-1°C /W 4.4°C /W

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.