Datasheet

LM4765
SNAS030C –AUGUST 1998–REVISED APRIL 2013 www.ti.com
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 1 exemplifies the theoretical maximum power dissipation point of each amplifier where V
CC
is the total
supply voltage.
P
DMAX
= V
CC
2/2π
2
R
L
(1)
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 1 since there are two
amplifiers in each LM4765. 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 LM4765 is 1°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
LM4765 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
(2)
Once the maximum package power dissipation has been calculated using Equation 1, the maximum thermal
resistance, 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 LM4765 has excellent power supply rejection and does not require a regulated supply. However, to improve
system performance as well as eliminate possible oscillations, the LM4765 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. 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.
If adequate bypassing is not provided, the current in the supply leads which is a rectified component of the load
current may be fed back into internal circuitry. This signal causes distortion at high frequencies requiring that the
supplies be bypassed at the package terminals with an electrolytic capacitor of 470 μF or more.
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