LM1876TF National Semiconductor, LM1876TF Datasheet - Page 16

• Manufacturer Part Number: LM1876TF
• Description: Audio Power Amplifier IC
• Manufacturer: National Semiconductor

Specifications of LM1876TF

• No. Of Channels: 2
• Amplifier Case Style: TO-220
• No. Of Pins: 15
• Termination Type: Through Hole
• Mounting Type: Through Hole
• Peak Reflow Compatible (260 C): No
• Supply Voltage Max: 22V
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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Application Information
MUTE MODE
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.
STANDBY MODE
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
current drawn from the V
µA total for both amplifiers. The current drawn from the V
supply 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.
UNDER-VOLTAGE PROTECTION
Upon system power-up, the under-voltage protection cir-
cuitry allows the power supplies and their corresponding
capacitors 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
before the power supplies such that no transients occur at
power-down.
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.
SPiKe PROTECTION
The LM1876 is protected from instantaneous peak-
temperature stressing of the power transistor array. The Safe
Operating graph in the Typical Performance Characteris-
tics section shows the area of device operation where
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 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
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
CC
supply is typically less than 10
EE
16
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 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.
DETERMlNlNG MAXIMUM POWER DISSIPATION
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
limiting the output power.
Equation (1) exemplifies the theoretical maximum power
dissipation point of each amplifier where V
supply voltage.
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
results 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 Charac-
teristics section which show the actual full range of power
dissipation not just the maximum theoretical point that re-
sults from equation (1).
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.
The thermal resistance from the die (junction) to the outside
air (ambient) is a combination of three thermal resistances,
θ
(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, θ
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:
where T
ture and θ
Once the maximum package power dissipation has been
calculated using equation (1), the maximum thermal resis-
tance, θ
be calculated. This calculation is made using equation (3)
which is derived by solving for θ
Again it must be noted that the value of θ
upon the system designer’s amplifier requirements. If the
ambient temperature that the audio amplifier is to be working
under is higher than 25˚C, then the thermal resistance for the
heat sink, given all other things are equal, will need to be
smaller.
JC
, θ
CS
θ
SA
SA
JMAX
, and θ
JA
, (heat sink to ambient) in ˚C/W for a heat sink can
= [(T
= θ
= 150˚C, T
JMAX
SA
P
JC
DMAX
. In addition, the thermal resistance, θ
P
+ θ
−T
DMAX
CS
AMB
= (T
AMB
+ θ
= V
)−P
JMAX
is the system ambient tempera-
SA
CC
DMAX
.
SA
−T
2/2π
AMB
(θ
in equation (2).
2
JC
R
CS
)/θ
L
+θ
JA
(case to sink), is
CS
SA
CC
)]/P
is dependent
is the total
DMAX
(1)
(2)
(3)
JC