LM4780 datasheet(15/24 Pages) NSC

15 / 24 page

Application Information

MUTE MODE

The muting function allows the user to mute the amplifier.

This can be accomplished as shown in the Typical Applica-

tion Circuit. The resistor R

M is chosen with reference to the

negative supply voltage and is used in conjunction with a

switch. The switch, when opened or switched to GND, cuts

off the current flow from the MUTE pins to −V

EE, thus placing

the LM4780 into mute mode. Refer to the Mute Attenuation

vs Mute Current curves in the Typical Performance Char-

acteristics section for values of attenuation per current out

of each MUTE pin. The resistance R

M is calculated by the

following equation:

≤ (|−V

EE| − 2.6V) / IMUTE

Where I

MUTE

≥ 0.5mA for each MUTE pin.

The MUTE pins can be tied together so that only one resistor

is required for the mute function. The mute resistor value

must be chosen so that a minimum of 1mA is pulled through

the resistor R

M. This ensures that each amplifier is fully

operational. Taking into account supply line fluctuations, it is

a good idea to pull out 1mA per MUTE pin or 2mA total if

both pins are tied together.

A turn-on MUTE or soft start circuit may also be used during

power up. A simple circuit like the one shown below may be

used.

200586A3

The RC combination of C

M and RM1 may cause the voltage

at point A to change more slowly than the -V

EE supply

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

LM4780 will enter play mode when the voltage at point A is

-17.6V. 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 LM4780. Since the supplies have essentially settled

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

down, the outputs of the LM4780 are forced to ground before

the power supply voltages fully decay preventing transients

on the output.

OVER-VOLTAGE PROTECTION

The LM4780 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.

SPiKe PROTECTION

The

LM4780

protected

from

instantaneous

peak-

temperature stressing of the power transistor array. The Safe

Operating graph in the Typical Performance Characteris-

tics section shows the area of device operation where

SPiKe Protection Circuitry is not enabled. The SPiKe Pro-

tection Response waveform graph shows the waveform dis-

tortion when SPiKe is enabled. Please refer to AN-898 for

more detailed information.

THERMAL PROTECTION

The LM4780 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 LM4780 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 of 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 twice the number which

results from Equation (2) since there are two amplifiers in

each LM4780. Refer to the graphs of Power Dissipation

versus Output Power in the Typical Performance Charac-

teristics section which show the actual full range of power

dissipation not just the maximum theoretical point that re-

sults from Equation (2).

LM4780 Datasheet(PDF) 15 Page - National Semiconductor (TI)

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