LM4766TF National Semiconductor, LM4766TF Datasheet - Page 15

• Manufacturer Part Number: LM4766TF
• Description: IC,Audio Amplifier,DUAL,BIPOLAR,ZIP,15PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM4766TF

• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Application Information
MUTE MODE
The muting function of the LM4766 allows the user to mute
the music going into the amplifier by drawing more than
0.5mA out of each mute pin on the device. This is accom-
plished as shown in the Typical Application Circuit where the
resistor R
voltage and is used in conjunction with a switch. The switch
when opened cuts off the current flow from pin 6 or 11 to
−V
Mute Attenuation vs Mute Current curves in the Typical
Performance Characteristics section for values of attenu-
ation per current out of pins 6 or 11. The resistance R
calculated by the following equation:
where I
Both pins 6 and 11 can be tied together so that only one
resistor and capacitor are required for the mute function. The
mute resistance must be chosen such that greater than 1mA
is pulled through the resistor R
pulled out of mute mode. Taking into account supply line
fluctuations, it is a good idea to pull out 1mA per mute pin or
2 mA total if both pins are tied together.
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 LM4766 such that no DC output spikes occur. Upon
turn-off, the output of the LM4766 is brought to ground
before the power supplies such that no transients occur at
power-down.
OVER-VOLTAGE PROTECTION
The LM4766 contains over-voltage protection circuitry that
limits the output current to approximately 4.0A
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 LM4766 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.
Please refer to AN-898 for more detailed information.
THERMAL PROTECTION
The LM4766 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 LM4766 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
EE
, thus placing the LM4766 into mute mode. Refer to the
pin6
M
= I
is chosen with reference to your negative supply
pin11
R
M
≥ 0.5mA.
≤ (|−V
EE
| − 2.6V)/I
M
so that each amplifier is fully
pin6
PK
while also
M
is
15
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 LM4766. 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 LM4766T is 1˚C/W. Using Thermal-
loy Thermacote thermal compound, the thermal resistance,
θ
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 LM4766 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
JC
CS
, θ
(case to sink), is about 0.2˚C/W. Since convection heat
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
)−P
= V
JMAX
is the system ambient tempera-
SA
CC
DMAX
.
SA
−T
2
/2π
AMB
(θ
in Equation (2).
2
JC
R
10092852
)/θ
L
+θ
JA
CS
CC
)]/P
www.national.com
DMAX
is the total
(1)
(2)
(3)
JC