Application Information

MUTE MODE

By placing a logic-high voltage on the mute pins, the signal

going into the amplifiers will be muted. If the mute pins are

left floating or connected to a logic-low voltage, the amplifi-

ers will be in a non-muted state. There are two mute pins,

one for each amplifier, so that one channel can be muted

without muting the other if the application requires such a

configuration. Refer to the Typical Performance Character-

istics section for curves concerning Mute Attenuation vs

Mute Pin Voltage.

STANDBY MODE

The standby mode of the LM4765 allows the user to drasti-

cally reduce power consumption when the amplifiers are

idle. By placing a logic-high voltage on the standby pins, the

amplifiers will go into Standby Mode. In this mode, the cur-

rent drawn from the V

total for both amplifiers. The current drawn from the V

ply is typically 4.2 mA. Clearly, there is a significant reduction

in idle power consumption when using the standby mode.

There are two Standby pins, so that one channel can be put

in standby mode without putting the other amplifier in

standby if the application requires such flexibility. Refer to

the Typical Performance Characteristics section for

curves showing Supply Current vs. Standby Pin Voltage for

both supplies.

UNDER-VOLTAGE PROTECTION

Upon system power-up, the under-voltage protection cir-

cuitry allows the power supplies and their corresponding ca-

pacitors to come up close to their full values before turning

on the LM4765 such that no DC output spikes occur. Upon

turn-off, the output of the LM4765 is brought to ground be-

fore the power supplies such that no transients occur at

power-down.

OVER-VOLTAGE PROTECTION

The LM4765 contains over-voltage protection circuitry that

limits the output current to approximately 3.5 Apk while also

providing voltage clamping, though not through internal

clamping diodes. The clamping effect is quite the same,

however, the output transistors are designed to work alter-

nately by sinking large current spikes.

SPiKe PROTECTION

The

LM4765

is

protected

from

instantaneous

peak-temperature stressing of the power transistor array.

The Safe Operating graph in the Typical Performance

Characteristics section shows the area of device operation

where SPiKe Protection Circuitry is not enabled. The wave-

form to the right of the SOA graph exemplifies how the dy-

namic protection will cause waveform distortion when en-

abled. Please refer to AN-898 for more detailed information.

THERMAL PROTECTION

The LM4765 has a sophisticated thermal protection scheme

to prevent long-term thermal stress of the device. When the

temperature on the die reaches 165˚C, the LM4765 shuts

down. It starts operating again when the die temperature

drops to about 155˚C, but if the temperature again begins to

rise, shutdown will occur again at 165˚C. Therefore, the de-

vice is allowed to heat up to a relatively high temperature if

the fault condition is temporary, but a sustained fault will

cause the device to cycle in a Schmitt Trigger fashion be-

tween the thermal shutdown temperature limits of 165˚C and

155˚C. This greatly reduces the stress imposed on the IC by

thermal cycling, which in turn improves its reliability under

sustained fault conditions.

Since the die temperature is directly dependent upon the

heat sink used, the heat sink should be chosen such that

thermal shutdown will not be reached during normal opera-

tion. Using the best heat sink possible within the cost and

space constraints of the system will improve the long-term

reliability of any power semiconductor device, as discussed

in the Determining the Correct Heat Sink Section.

DETERMlNlNG MAXIMUM POWER DISSIPATION

Power dissipation within the integrated circuit package is a

very important parameter requiring a thorough understand-

ing if optimum power output is to be obtained. An incorrect

maximum power dissipation calculation may result in inad-

equate heat sinking causing thermal shutdown and thus lim-

iting the output power.

Equation (1) exemplifies the theoretical maximum power dis-

sipation point of each amplifier where V

voltage.

P

L

(1)

Thus by knowing the total supply voltage and rated output

load, the maximum power dissipation point can be calcu-

lated. The package dissipation is twice the number which re-

sults from

Equation (1) since there are two amplifiers in each

LM4765. Refer to the graphs of Power Dissipation versus

Output Power in the Typical Performance Characteristics

section which show the actual full range of power dissipation

not just the maximum theoretical point that results from

Equation (1).

DETERMINING THE CORRECT HEAT SINK

The choice of a heat sink for a high-power audio amplifier is

made entirely to keep the die temperature at a level such

that the thermal protection circuitry does not operate under

normal circumstances.

The thermal resistance from the die (junction) to the outside

air (ambient) is a combination of three thermal resistances,

θ

(junction to case), of the LM4765 is 1˚C/W. Using Thermalloy

Thermacote thermal compound, the thermal resistance,

θ

CS

(case to sink), is about 0.2˚C/W. Since convection heat flow

(power dissipation) is analogous to current flow, thermal re-

sistance is analogous to electrical resistance, and tempera-

ture drops are analogous to voltage drops, the power dissi-

pation out of the LM4765 is equal to the following:

P

(2)

where T

ture and

θ

Once the maximum package power dissipation has been

calculated using

Equation (1), the maximum thermal resis-

tance,

θ

be calculated. This calculation is made using

Equation (3)

which is derived by solving for

θ

DS100927-52

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