LME49810 Datasheet, PDF (12/21 Pages) National Semiconductor (TI) – 200V Audio Power Amplifier Driver with Baker Clamp

LME49810
SNAS391C – MAY 2007 – REVISED APRIL 2013
50V-100V
R_Z
22 k:
R_M
470:
IN4733
TO CLPFLAG PIN
www.ti.com
THERMAL PROTECTION
The LME49810 has a thermal protection scheme to prevent long-term thermal stress of the device. When the
temperature on the die exceeds 150°C, the LME49810 goes into thermal shutdown. The LME49810 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 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.
POWER DISSIPATION
When in “play” mode, the LME49810 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 P_DMAX = I_CC * (V_CC – V_EE). For a quick calculation of P_DMAX, approximate the current to be 11mA
and multiply it by the total supply voltage (the current varies slightly from this value over the operating range).
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 LME49810 is 4°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 LME49810 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)
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