LM2876 datasheet(14/22 Pages) NSC

14 / 22 page

Application Information

GENERAL FEATURES

Mute Function

The muting function of the LM2876 allows the user to mute

the music going into the amplifier by drawing less than 0.5

mA out of pin 8 of the device. This is accomplished as shown

in the Typical Application Circuit where the resistor R

M is

chosen with reference to your negative supply voltage and is

used in conjuction with a switch. The switch (when opened)

cuts off the current flow from pin 8 to V

−, thus placing the

LM2876 into mute mode. Refer to the Mute Attenuation vs

Mute Current curves in the Typical Performance Charac-

teristics section for values of attenuation per current out of

pin 8. The resistance R

M is calculated by the following

equation:

M (|VEE| − 2.6V)/I8

where I8

≥ 0.5 mA.

Under-Voltage Protection

Upon system power-up the under-voltage protection circuitry

allows the power supplies and their corresponding caps to

come up close to their full values before turning on the

LM2876 such that no DC output spikes occur. Upon turn-off,

the output of the LM2876 is brought to ground before the

power supplies such that no transients occur at power-down.

Over-Voltage Protection

The LM2876 contains overvoltage protection circuitry that

limits the output current to approximately 4Apeak 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

LM2876

protected

from

instantaneous

peak-

temperature stressing by the power transistor array. The

Safe Operating Area graph in the Typical Performance

Characteristics section shows the area of device operation

where the SPiKe Protection Circuitry is not enabled. The

waveform to the right of the SOA graph exemplifies how the

dynamic protection will cause waveform distortion when en-

abled.

Thermal Protection

The LM2876 has a sophisticated thermal protection scheme

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

temperature on the die reaches 165˚C, the LM2876 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

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 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, the heat sink should be chosen as discussed in

the Thermal Considerations section, such that thermal

shutdown will not be reached during normal operation. Using

the best heat sink possible within the cost and space con-

straints of the system will improve the long-term reliability of

any power semiconductor device.

THERMAL CONSIDERATIONS

Heat Sinking

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 heat sink should be chosen to

dissipate the maximum IC power for a given supply voltage

and rated load.

With high-power pulses of longer duration than 100 ms, the

case temperature will heat up drastically without the use of a

heat sink. Therefore the case temperature, as measured at

the center of the package bottom, is entirely dependent on

heat sink design and the mounting of the IC to the heat sink.

For the design of a heat sink for your audio amplifier appli-

cation refer to the Determining The Correct Heat Sink

section.

Since a semiconductor manufacturer has no control over

which heat sink is used in a particular amplifier design, we

can only inform the system designer of the parameters and

the method needed in the determination of a heat sink. With

this in mind, the system designer must choose his supply

voltages, a rated load, a desired output power level, and

know the ambient temperature surrounding the device.

These parameters are in addition to knowing the maximum

junction temperature and the thermal resistance of the IC,

both of which are provided by National Semiconductor.

As a benefit to the system designer we have provided Maxi-

mum Power Dissipation vs Supply Voltages curves for vari-

ous loads in the Typical Performance Characteristics sec-

tion, giving an accurate figure for the maximum thermal

resistance required for a particular amplifier design. This

data was based on

JC = 1˚C/W and

CS = 0.2˚C/W. We also

provide a section regarding heat sink determination for any

audio amplifier design where

CS may be a different value. It

should be noted that the idea behind dissipating the maxi-

mum power within the IC is to provide the device with a low

resistance to convection heat transfer such as a heat sink.

Therefore, it is necessary for the system designer to be

conservative in his heat sink calculations. As a rule, the

lower the thermal resistance of the heat sink the higher the

amount of power that may be dissipated. This is of course

guided by the cost and size requirements of the system.

Convection cooling heat sinks are available commercially,

and their manufacturers should be consulted for ratings.

Proper mounting of the IC is required to minimize the thermal

drop between the package and the heat sink. The heat sink

must also have enough metal under the package to conduct

heat from the center of the package bottom to the fins

without excessive temperature drop.

A thermal grease such as Wakefield type 120 or Thermalloy

Thermacote should be used when mounting the package to

the heat sink. Without this compound, thermal resistance will

be no better than 0.5˚C/W, and probably much worse. With

the compound, thermal resistance will be 0.2˚C/W or less,

assuming under 0.005 inch combined flatness runout for the

package and heat sink. Proper torquing of the mounting

bolts is important and can be determined from heat sink

manufacturer’s specification sheets.

Should it be necessary to isolate V

− from the heat sink, an

insulating washer is required. Hard washers like beryluum

oxide, anodized aluminum and mica require the use of ther-

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