LM4731TA/NOPB National Semiconductor, LM4731TA/NOPB Datasheet - Page 10

• Manufacturer Part Number: LM4731TA/NOPB
• Description: IC AMP AUDIO PWR 25W AB TO220-15
• Manufacturer: National Semiconductor
• Type: Class ABr

Specifications of LM4731TA/NOPB

• Output Type: 2-Channel (Stereo)
• Max Output Power X Channels @ Load: 25W x 2 @ 8 Ohm
• Voltage - Supply: 20 V ~ 56 V, ±10V ~ 28 V
• Features: Depop, Mute, Short-Circuit and Thermal Protection
• Mounting Type: Through Hole
• Package / Case: TO-220-15 (Bent and Staggered Leads)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *LM4731TA; *LM4731TA/NOPB; LM4731TA

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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 LM4731 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 4.8mA. Clearly, there is a significant re-
duction 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 LM4731 such that no DC output spikes occur. Upon
turn-off, the output of the LM4731 is brought to ground
before the power supplies such that no transients occur at
power-down.
OVER-VOLTAGE PROTECTION
The LM4731 contains over-voltage protection circuitry that
limits the output current while also providing voltage clamp-
ing, though not through internal clamping diodes. The clamp-
ing effect is quite the same, however, the output transistors
are designed to work alternately by sinking large current
spikes.
THERMAL PROTECTION
The LM4731 has a sophisticated thermal protection scheme
to prevent long-term thermal stress of the device. When the
temperature on the die exceeds150˚C, the LM4731 shuts
down. It starts operating again when the die temperature
drops to about 145˚C, but if the temperature again begins to
rise, shutdown will occur again above 150˚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 150˚C and
145˚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
CC
supply is typically less than 10
EE
10
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 LM4731. 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 LM4731TA is 1.5˚C/W. Using Ther-
malloy Thermacote thermal compound, the thermal resis-
tance, θ
heat flow (power dissipation) 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 LM4731 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.
SUPPLY BYPASSING
The LM4731 has excellent power supply rejection and does
not require a regulated supply. However, to improve system
performance as well as eliminate possible oscillations, the
LM4731 should have its supply leads bypassed with low-
inductance capacitors having short leads that are located
close to the package terminals. Inadequate power supply
bypassing will manifest itself by a low frequency oscillation
known as “motorboating” or by high frequency instabilities.
JC
, θ
CS
θ
SA
CS
SA
JMAX
, and θ
JA
, (heat sink to ambient) in ˚C/W for a heat sink can
= [(T
(case to sink), is about 0.2˚C/W. Since convection
= 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
)/θ
L
+θ
JA
CS
SA
CC
)]/P
is dependent
is the total
DMAX
(1)
(2)
(3)
JC