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LME49811 Datasheet(PDF) 11 Page - Texas Instruments

Part # LME49811
Description  Audio Power Amplifier Series High Fidelity 200 Volt Power Amplifier Input Stage with Shutdown
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Manufacturer  TI1 [Texas Instruments]
Direct Link  http://www.ti.com
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LME49811
www.ti.com
SNAS394C – DECEMBER 2007 – REVISED APRIL 2013
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.
POWER DISSIPATION AND HEAT SINKING
When in “play” mode, the LME49811 draws a constant amount of current, regardless of the input signal
amplitude. Consequently, the power dissipation is constant for a given supply voltage and can be computed with
the equation PDMAX = ICC* (VCC– VEE).
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,
θJA (junction to ambient), is a combination of three thermal
resistances,
θJC (junction to case), θCS (case to sink), and θSA (sink to ambient). The thermal resistance, θJC
(junction to case), of the LME49811 is 0.4 °C/W. Using Thermalloy Thermacote thermal compound, the thermal
resistance,
θCS (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 LME49811 is equal to the following:
PDMAX = (TJMAX−TAMB) / θJA
where
TJMAX = 150°C
TAMB is the system ambient temperature
θJA = θJC + θCS + θSA
(1)
Once the maximum package power dissipation has been calculated using Equation (1), the maximum thermal
resistance,
θSA, (heat sink to ambient) in °C/W for a heat sink can be calculated. This calculation is made using
Equation (2) which is derived by solving for
θSA in Equation (1).
θSA = [(TJMAX−TAMB)−PDMAXJC CS)] / PDMAX
(2)
Again it must be noted that the value of
θSA 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.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components is required to meet the design targets of an application. The choice of
external component values that will affect gain and low frequency response are discussed below.
The gain of each amplifier is set by resistors RF and Ri for the non-inverting configuration shown in Figure 1. The
gain is found by Equation (3) below:
AV = RF / Ri (V/V)
(3)
For best noise performance, lower values of resistors are used. A value of 1k
Ω is commonly used for Ri and then
setting the value of RF for the desired gain. For the LME49811 the gain should be set no lower than 26dB. Gain
settings below 26dB may experience instability.
The combination of Ri with Ci (see Figure 1) creates a high pass filter. The low frequency response is determined
by these two components. The -3dB point can be found from Equation (4) shown below:
fi = 1 / (2πRiCi) (Hz)
(4)
Copyright © 2007–2013, Texas Instruments Incorporated
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