LM3875 Datasheet, PDF (15/29 Pages) National Semiconductor (TI) – OverturTM Audio Power Amplifier Series High-Performance 56W Audio Power Amplifier

LM3875
www.ti.com
APPLICATION INFORMATION
SNAS083D – JUNE 1999 – REVISED APRIL 2013
GENERAL FEATURES
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 LM3875 such
that no DC output spikes occur. Upon turn-off, the output of the LM3875 is brought to ground before the power
supplies such that no transients occur at power-down.
Over-Voltage Protection: The LM3875 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 alternately by sinking
large current spikes.
SPiKe Protection: The LM3875 is 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 enabled.
Thermal Protection: The LM3875 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 LM3875 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 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 constraints 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 application 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
Texas Instruments.
As a benefit to the system designer we have provided Maximum Power Dissipation vs Supply Voltages curves
for various loads in the Typical Performance Characteristics section, giving an accurate figure for the maximum
thermal resistance required for a particular amplifier design. This data was based on θ_JC = 1°C/W and θ_CS =
0.2°C/W. We also provide a section regarding heat sink determination for any audio amplifier design where θ_CS
may be a different value. It should be noted that the idea behind dissipating the maximum 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.
Copyright © 1999–2013, Texas Instruments Incorporated
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