Application Information
MUTE MODE
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.
By placing a logic-high voltage on the mute pins, the signal
going into the amplifiers will be muted. If the mute pins are
left floating or connected to a logic-low voltage, the amplifi-
ers will be in a non-muted state. There are two mute pins,
one for each amplifier, so that one channel can be muted
without muting the other if the application requires such a
configuration. Refer to the Typical Performance Character-
istics section for curves concerning Mute Attenuation vs
Mute Pin Voltage.
Since the die temperature is directly dependent upon the
heat sink used, the heat sink should be chosen such that
thermal shutdown will not be reached during normal opera-
tion. 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.
STANDBY MODE
DETERMlNlNG MAXIMUM POWER DISSIPATION
The standby mode of the LM1876 allows the user to drasti-
cally reduce power consumption when the amplifiers are
idle. By placing a logic-high voltage on the standby pins, the
amplifiers will go into Standby Mode. In this mode, the cur-
rent drawn from the VCC supply is typically less than 10 µA
total for both amplifiers. The current drawn from the VEE sup-
ply is typically 4.2 mA. Clearly, there is a significant reduction
in idle power consumption when using the standby mode.
There are two Standby pins, so that one channel can be put
in standby mode without putting the other amplifier in
standby if the application requires such flexibility. Refer to
the Typical Performance Characteristics section for
curves showing Supply Current vs. Standby Pin Voltage for
both supplies.
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 lim-
iting the output power.
Equation (1) exemplifies the theoretical maximum power dis-
sipation point of each amplifier where VCC is the total supply
voltage.
PDMAX VCC2/2π2RL
(1)
=
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 re-
sults from equation (1) since there are two amplifiers in each
LM1876. Refer to the graphs of Power Dissipation versus
Output Power in the Typical Performance Characteristics
section which show the actual full range of power dissipation
not just the maximum theoretical point that results from
equation (1).
UNDER-VOLTAGE PROTECTION
Upon system power-up, the under-voltage protection cir-
cuitry allows the power supplies and their corresponding ca-
pacitors to come up close to their full values before turning
on the LM1876 such that no DC output spikes occur. Upon
turn-off, the output of the LM1876 is brought to ground be-
fore the power supplies such that no transients occur at
power-down.
DETERMINING THE CORRECT HEAT SINK
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.
OVER-VOLTAGE PROTECTION
The LM1876 contains over-voltage protection circuitry that
limits the output current to approximately 3.5 Apk 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.
The thermal resistance from the die (junction) to the outside
air (ambient) is a combination of three thermal resistances,
θJC, θCS, and θSA. In addition, the thermal resistance, θJC
(junction to case), of the LM1876TF is 2˚C/W and the
LM1876T is 1˚C/W. Using Thermalloy Thermacote thermal
compound, the thermal resistance, θCS (case to sink), is
about 0.2˚C/W. Since convection heat flow (power dissipa-
tion) is analogous to current flow, thermal resistance is
analogous to electrical resistance, and temperature drops
are analogous to voltage drops, the power dissipation out of
the LM1876 is equal to the following:
SPiKe PROTECTION
The
LM1876
is
protected
from
instantaneous
peak-temperature stressing of the power transistor array.
The Safe Operating graph in the Typical Performance
Characteristics section shows the area of device operation
where SPiKe Protection Circuitry is not enabled. The wave-
form to the right of the SOA graph exemplifies how the dy-
namic protection will cause waveform distortion when en-
abled.
=
PDMAX (TJMAX−TAMB)/θJA
(2)
=
where TJMAX 150˚C, TAMB is the system ambient tempera-
=
ture and θJA θJC + θCS + θSA
.
Once the maximum package power dissipation has been
calculated using equation (1), the maximum thermal resis-
tance, θSA, (heat sink to ambient) in ˚C/W for a heat sink can
be calculated. This calculation is made using equation (3)
which is derived by solving for θSA in equation (2).
THERMAL PROTECTION
The LM1876 has a sophisticated thermal protection scheme
to prevent long-term thermal stress of the device. When the
temperature on the die reaches 165˚C, the LM1876 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 de-
vice 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-
=
θSA [(TJMAX−TAMB)−PDMAX(θJC +θCS)]/PDMAX (3)
Again it must be noted that the value of θSA is dependent
upon the system designer’s amplifier requirements. If the
ambient temperature that the audio amplifier is to be working
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