Datasheet

LM4780, LM4780TABD
www.ti.com SNAS193B –JULY 2003–REVISED APRIL 2013
SPiKe PROTECTION
The LM4780 is protected from instantaneous peak-temperature stressing of the power transistor array. The Safe
Operating graph in the Typical Performance Characteristics section shows the area of device operation where
SPiKe Protection Circuitry is not enabled. The SPiKe Protection Response waveform graph shows the waveform
distortion when SPiKe is enabled.
THERMAL PROTECTION
The LM4780 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 LM4780 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 between 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
understanding if optimum power output is to be obtained. An incorrect maximum power dissipation calculation
may result in inadequate heat sinking causing thermal shutdown and thus limiting the output power.
Equation 3 shows the theoretical maximum power dissipation point of each amplifier in a single-ended
configuration where V
CC
is the total supply voltage.
P
DMAX
= (V
CC
)
2
/ 2π
2
R
L
(3)
Thus by knowing the total supply voltage and rated output load, the maximum power dissipation point can be
calculated. The package dissipation is twice the number which results from Equation 3 since there are two
amplifiers in each LM4780. 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 3.
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
JA
(junction to ambient), is a combination of three thermal
resistances, θ
JC
(junction to case), θ
CS
(case to sink), and θ
SA
(sink to ambient). The thermal resistance, θ
JC
(junction to case), of the LM4780T is 0.8°C/W. Using Thermalloy 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 temperature drops are analogous to
voltage drops, the power dissipation out of the LM4780 is equal to the following:
P
DMAX
= (T
JMAX
−T
AMB
) / θ
JA
where
• T
JMAX
= 150°C
• T
AMB
is the system ambient temperature
• θ
JA
= θ
JC
+ θ
CS
+ θ
SA
(4)
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