Datasheet

LM2876
www.ti.com SNAS088C –AUGUST 1995–REVISED MARCH 2013
Determining 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 (P
D
)
calculation may result in inadequate heat sinking, causing thermal shutdown circuitry to operate and limit the
output power.
The following equations can be used to accurately calculate the maximum and average integrated circuit power
dissipation for your amplifier design, given the supply voltage, rated load, and output power. These equations
can be directly applied to the Power Dissipation vs Output Power curves in the TYPICAL PERFORMANCE
CHARACTERISTICS section.
Equation 1 exemplifies the maximum power dissipation of the IC and Equation 2 and Equation 3 exemplify the
average IC power dissipation expressed in different forms.
P
DMAX
= V
CC
2/2π
2
R
L
where
• V
CC
is the total supply voltage (1)
P
DAVE
= (V
Opk
/R
L
)[V
CC
/π − V
Opk
/2]
where
• V
CC
is the total supply voltage and V
Opk
= V
CC
/π (2)
P
DAVE
= V
CC
V
Opk
/πR
L
− V
Opk
2
/2R
L
where
• V
CC
is the total supply voltage (3)
Determining the Correct Heat Sink
Once the maximum IC power dissipation is known for a given supply voltage, rated load, and the desired rated
output power the maximum thermal resistance (in °C/W) of a heat sink can be calculated. This calculation is
made using Equation 4equation (4) and is based on the fact that thermal heat flow parameters are analogous to
electrical current flow properties.
It is also known that typically the thermal resistance, θ
JC
(junction to case), of the LM2876 is 1°C/W and that
using Thermalloy Thermacote thermal compound provides a thermal resistance, θ
CS
(case to heat sink), of about
0.2°C/W as explained in the Heat Sinking section.
Referring to the figure below, it is seen that the thermal resistance from the die (junction) to the outside air
(ambient) is a combination of three thermal resistances, two of which are known, θ
JC
and θ
CS
. 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 LM2876 is
equal to the following:
P
DMAX
= (T
Jmax
− T
Amb
)/θ
JA
where
• θ
JA
= θ
JC
+ θ
CS
+ θ
SA
But since we know P
DMAX
, θ
JC
, and θ
SC
for the application and we are looking for θ
SA
, we have the following:
θ
SA
= [(T
Jmax
− T
Amb
) − P
DMAX
(θ
JC
+ θ
CS
)]/P
DMAX
(4)
Again it must be noted that the value of θ
SA
is dependent upon the system designer's amplifier application and its
corresponding parameters as described previously. If the ambient temperature that the audio amplifier is to be
working under is higher than the normal 25°C, then the thermal resistance for the heat sink, given all other things
are equal, will need to be smaller.
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