LM4733 datasheet(14/23 Pages) NSC

14 / 23 page

Application Information (Continued)

voltage. Until the voltage at point A is low enough to have

0.5mA of current per MUTE pin flow through R

M2, the IC will

be in mute mode. The series combination of R

M1 and RM2

needs to satisfy the mute equation above for all operating

voltages or mute mode may be activated during normal

operation. For a longer turn-on mute time, a larger time

constant,

τ =RC=R

M1CM (sec), is needed. For the values

show above and with the MUTE pins tied together, the

LM4733 will enter play mode when the voltage at point A is

-25.1V. The voltage at point A is found with Equation (1)

below.

A(t)=(Vf -VO)e

-t/

τ (Volts)

(1)

where:

t = time (sec)

τ = RC (sec)

o = Voltage on C att=0(Volts)

f = Final voltage, -VEE in this circuit (Volts)

UNDER-VOLTAGE PROTECTION

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

cuitry allows the power supplies and their corresponding

capacitors to come up close to their full values before turning

on the LM4733. Since the supplies have essentially settled

to their final value, no DC output spikes occur. At power

down, the outputs of the LM4733 are forced to ground before

the power supply voltages fully decay preventing transients

on the output.

OVER-VOLTAGE PROTECTION

The LM4733 contains over-voltage protection circuitry that

limits the output current while also providing voltage clamp-

ing. The clamp does not, however, use internal clamping

diodes. The clamping effect is quite the same because the

output transistors are designed to work alternately by sinking

large current spikes.

THERMAL PROTECTION

The LM4733 has a sophisticated thermal protection scheme

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

temperature on the die exceeds 150˚C, the LM4733 shuts

down. It starts operating again when the die temperature

drops to about 145˚C, but if the temperature again begins to

rise, shutdown will occur again above 150˚C. Therefore, the

device 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 150˚C and

145˚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 so that

thermal shutdown is not activated during normal operation.

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

limiting the output power.

Equation (2) shows the theoretical maximum power dissipa-

tion point for each amplifier in a single-ended configuration

where V

CC is the total supply voltage.

DMAX =(VCC)

2 /2

π2R

(2)

Thus by knowing the total supply voltage and rated output

load, the maximum power dissipation point can be calcu-

lated. The package dissipation is three times the number

which results from Equation (2) since there are three ampli-

fiers in each LM4733. Refer to the graphs of Power Dissipa-

tion versus Output Power in the Typical Performance Char-

acteristics section which show the actual full range of power

dissipation not just the maximum theoretical point that re-

sults from Equation (2).

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 is not activated under

normal circumstances.

The thermal resistance from the die to the outside air,

(junction to ambient), is a combination of three thermal re-

sistances,

JC (junction to case),

CS (case to sink), and

(sink to ambient). The thermal resistance,

JC (junction to

case), of the LM4733T is 0.9˚C/W. Using Thermalloy Ther-

macote thermal compound, the thermal resistance,

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

(power dissipation) is analogous to current flow, thermal

resistance is analogous to electrical resistance, and tem-

perature drops are analogous to voltage drops, the power

dissipation out of the LM4733 is equal to the following:

DMAX =(TJMAX−TAMB)/

(3)

where T

JMAX = 150˚C, TAMB is the system ambient tempera-

ture and

JA =

JC +

CS +

SA.

200794B8

Once the maximum package power dissipation has been

calculated using Equation 2, the maximum thermal resis-

tance,

SA, (heat sink to ambient) in ˚C/W for a heat sink can

be calculated. This calculation is made using Equation 4

which is derived by solving for

SA in Equation 3.

SA = [(TJMAX−TAMB)−PDMAX(

JC +

CS)]/PDMAX

(4)

Again it must be noted that the value of

SA is dependent

upon the system designer’s amplifier requirements. If the

ambient temperature that the audio amplifier is to be working

under is higher than 25˚C, then the thermal resistance for the

heat sink, given all other things are equal, will need to be

smaller.

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