LM3875 Datasheet, PDF (16/29 Pages) National Semiconductor (TI) – OverturTM Audio Power Amplifier Series High-Performance 56W Audio Power Amplifier

LM3875
SNAS083D – JUNE 1999 – REVISED APRIL 2013
www.ti.com
Proper mounting of the IC is required to minimize the thermal drop between the package and the heat sink. The
heat sink must also have enough metal under the package to conduct heat from the center of the package
bottom to the fins without excessive temperature drop.
A thermal grease such as Wakefield type 120 or Thermalloy Thermacote should be used when mounting the
package to the heat sink. Without this compound, the thermal resistance will be no better than 0.5°C/W, and
probably much worse. With the compound, thermal resistance will be 0.2°C/W or less, assuming under 0.005
inch combined flatness runout for the package and heat sink. Proper torquing of the mounting bolts is important
and can be determined from heat sink manufacturer's specification sheets.
Should it be necessary to isolate V^− from the heat sink, an insulating washer is required. Hard washers like
berylum oxide, anodized aluminum and mica require the use of thermal compound on both faces. Two-mil mica
washers are most common, giving about 0.4°C/W interface resistance with the compound.
Silicone-rubber washers are also available. A 0.5°C/W thermal resistance is claimed without thermal compound.
Experience has shown that these rubber washers deteriorate and must be replaced should the IC be
dismounted.
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 heatsinking, 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_CC2/2π² 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
• V_Opk = V_CC/π
(2)
P_DAVE = V_CC V_Opk/π R_L − V_Opk2/2 R_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 5 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 LM3875 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 LM3875 is
equal to the following:
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