LM2876 datasheet(11/19 Pages) NSC

11 / 19 page

Typical Performance Characteristics (Continued)

Application Information

GENERAL FEATURES

Mute Function: The muting function of the LM2876 allows

the user to mute the music going into the amplifier by draw-

ing less than 0.5 mA out of pin 8 of the device. This is accom-

plished 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 At-

tenuation vs Mute Current curves in the Typical Perfor-

mance Characteristics 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 overvolt-

age protection circuitry that limits the output current to ap-

proximately 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 alternately by sinking large current spikes.

SPiKe Protection: The LM2876 is protected from instanta-

neous peak-temperature stressing by the power transistor

array. The Safe Operating Area graph in the Typical Perfor-

mance Characteristics section shows the area of device

operation where the SPiKe Protection Circuitry is not en-

abled. The waveform to the right of the SOA graph exempli-

fies how the dynamic protection will cause waveform distor-

tion when enabled.

Thermal Protection: The LM2876 has a sophisticated ther-

mal 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 between the thermal shutdown tem-

perature 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 applica-

tion refer to the Determining The Correct Heat Sink sec-

tion.

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 θCS = 0.2˚C/W. We also

provide a section regarding heat sink determination for any

Power Supply

Rejection Ratio

DS011775-47

Common-Mode

Rejection Ratio

DS011775-48

Open Loop

Frequency Response

DS011775-49

LM2876 Datasheet(PDF) 11 Page - National Semiconductor (TI)

LM2876 Datasheet(HTML) 11 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Datasheet Download

Link URL