Application Information
(Continued)
pulled out of mute mode. Taking into account supply line fluc-
tuations, it is a good idea to pull out 1 mA 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 ca-
pacitors 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 be-
fore 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 4.0 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 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 pro-
tection 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 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-
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.
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 lim-
iting the output power.
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
LM4766. 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).
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
JC
,
θ
CS
, and
θ
SA
. In addition, the thermal resistance,
θ
JC
(junction to case), of the LM4766T is 1˚C/W. Using Thermal-
loy Thermacote thermal compound, the thermal resistance,
θ
CS
(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 LM4766 is equal to the following:
P
DMAX
= (T
JMAX
−T
AMB
)/θ
JA
(2)
= 150˚C, T
AMB
is the system ambient tempera-
where T
JMAX
ture and
θ
JA
=
θ
JC
+
θ
CS
+
θ
SA
.
DS100928-52
Once the maximum package power dissipation has been
calculated using
Equation (1),
the maximum thermal resis-
tance,
θ
SA
, (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
θ
SA
in
Equation (2).
θ
SA
= [(T
JMAX
−T
AMB
)−P
DMAX
(θ
JC
+θ
CS
)]/P
DMAX
(3)
Again it must be noted that the value of
θ
SA
is dependent
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 LM4766 has excellent power supply rejection and does
not require a regulated supply. However, to improve system
performance as well as eliminate possible oscillations, the
LM4766 should have its supply leads bypassed with
low-inductance capacitors having short leads that are lo-
cated close to the package terminals. Inadequate power
supply bypassing will manifest itself by a low frequency oscil-
lation known as “motorboating” or by high frequency insta-
bilities. These instabilities can be eliminated through multiple
bypassing utilizing a large tantalum or electrolytic capacitor
(10 µF or larger) which is used to absorb low frequency
variations and a small ceramic capacitor (0.1 µF) to prevent
any high frequency feedback through the power supply lines.
Equation (1)
exemplifies the theoretical maximum power dis-
sipation point of each amplifier where V
CC
is the total supply
voltage.
P
DMAX
= V
CC2
/2π
2
R
L
(1)
11
www.national.com
NSC [ NATIONAL SEMICONDUCTOR ]
