LM4782TABD/NOPB National Semiconductor, LM4782TABD/NOPB Datasheet - Page 17

• Manufacturer Part Number: LM4782TABD/NOPB
• Manufacturer: National Semiconductor

Specifications of LM4782TABD/NOPB

• Lead Free Status / Rohs Status: Compliant

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
connected to a logic-low voltage, the amplifiers will be in a
non-muted state. There are three 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 Characteristics section
for curves concerning Mute Attenuation vs Mute Pin Voltage.
STANDBY MODE
The standby mode of the LM4782 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
total for all amplifiers. The current drawn from the V
is typically 8mA. Clearly, there is a significant reduction in
idle power consumption when using the standby mode.
There are three 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 LM4782. Since the supplies have essentially settled
to their final value, no DC output spikes occur. At power
down, the outputs of the LM4782 are forced to ground before
the power supply voltages fully decay preventing transients
on the output.
OVER-VOLTAGE PROTECTION
The LM4782 contains over-voltage protection circuitry that
limits the output current while also providing voltage clamp-
ing. The clamp does not, however, use internal clamping
diodes. The clamping effect is quite the same as diodes
because the output transistors are designed to work alter-
nately by sinking large current spikes.
SPiKe PROTECTION
The LM4782 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 SPiKe Pro-
tection Response waveform graph shows the waveform dis-
tortion when SPiKe is enabled. Please refer to AN-898 for
more detailed information.
THERMAL PROTECTION
The LM4782 has a sophisticated thermal protection scheme
to prevent long-term thermal stress of the device. When the
temperature on the die exceeds 150˚C, the LM4782 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-
CC
supply is typically less than 30µA
EE
supply
17
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 so that
thermal shutdown is not activated 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, 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) shows the theoretical maximum power dissipa-
tion point for each amplifier in a single-ended configuration
where V
Thus by knowing the total supply voltage and rated output
load, the maximum power dissipation point can be calcu-
lated. The package dissipation is three times the number
which results from Equation (1) since there are three ampli-
fiers in each LM4782. Refer to the graphs of Power Dissipa-
tion versus Output Power in the Typical Performance Char-
acteristics 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 is not activated under
normal circumstances.
The thermal resistance from the die to the outside air, θ
(junction to ambient), is a combination of three thermal re-
sistances, θ
(sink to ambient). The thermal resistance, θ
case), of the LM4782TA is 1.0˚C/W. Using Thermalloy Ther-
macote thermal compound, the thermal resistance, θ
(case to sink), is about 0.2˚C/W. Since convection heat flow
(power dissipation) is analogous to current flow, thermal
resistance is analogous to electrical resistance, and tem-
perature drops are analogous to voltage drops, the power
dissipation out of the LM4782 is equal to the following:
where T
ture and θ
Once the maximum package power dissipation has been
calculated using Equation 2, the maximum thermal resis-
JMAX
CC
JA
is the total supply voltage.
JC
= θ
= 150˚C, T
(junction to case), θ
P
JC
DMAX
P
DMAX
+ θ
CS
= (T
AMB
= (V
+ θ
JMAX
is the system ambient tempera-
SA
CC
.
−T
)
2
CS
AMB
/ 2π
(case to sink), and θ
) / θ
2
200811B8
R
L
JA
JC
(junction to
www.national.com
(1)
(2)
SA
CS
JA