SNAS091C – MAY 1999 – REVISED MARCH 2013
APPLICATION INFORMATION
GENERAL FEATURES
Mute Function:
The muting function of the LM3886 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 LM3886 into mute mode. Refer to
and
in
for values of attenuation per current out of pin 8. The resistance R
M
is calculated by the
following equation:
R
M
(|V
EE
|
−
2.6V)/I8
where
•
I8
≥
0.5 mA.
(1)
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 LM3886
such that no DC output spikes occur. Upon turn-off, the output of the LM3886 is brought to ground before
the power supplies such that no transients occur at power-down.
Over-Voltage Protection:
The LM3886 contains overvoltage protection circuitry that limits the output current to
approximately 11Apeak 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 LM3886 is protected from instantaneous peak-temperature stressing by the power
transistor array.
in
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 LM3886 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 LM3886 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
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
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.
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