LME49830 Datasheet, PDF (12/23 Pages) National Semiconductor (TI) – Mono High Fidelity 200 Volt MOSFET Power Amplifier Input Stage with Mute

LME49830
SNAS396D – JANUARY 2008 – REVISED APRIL 2013
APPLICATION INFORMATION
www.ti.com
MUTE FUNCTION
The mute function of the LME49830 is controlled by the amount of current that flows into the MUTE pin. If there
is less than 100μA of current flowing into the MUTE pin, the part will be in mute mode. This can be achieved by
shorting the MUTE pin to ground. It is recommended to connect a capacitor C_M (its value not less than 47μF)
between the MUTE pin and ground for reducing voltage fluctuation when switching between ‘play’ and ‘mute’
mode. If there is between 130μA and 2mA of current flowing into the MUTE pin, the part will be in ‘play’ mode.
This can be done by connecting a power supply, V_MUTE, to the MUTE pin through a resister, R_M. The current into
the MUTE pin can be determined by the equation I_MUTE = (V_MUTE – V_BE) / (1kΩ +R_M) (A), where V_BE ≅ 0.7V. For
example, if a 5V power supply is connected through a 27kΩ resistor to the MUTE pin, then the mute current will
be 154μA, at the center of the specified range. It is also possible to use V_CC as the power supply for the MUTE
pin, though R_M will have to be recalculated accordingly. It is not recommended to flow more than 2mA of current
into the MUTE pin because damage to the LME49830 may occur.
THERMAL PROTECTION
When the temperature on the die exceeds 150°C, the LME49830 shuts down. It starts operating again when the
die temperature drops to about 145°C. When in thermal shutdown, the current supply internal to the LME49830
will be cut-off. There will be no signal generated to the output while in thermal shutdown. After the die
temperature decreases, the LME49830 will power up again and resume normal operation. If the fault conditions
continue, thermal protection will be activated and repeat the cycle preventing the LME49830 from over heating.
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. It is recommended to use a separate heat
sink from the output stage heat sink for the LME49830. A heat sink may not be needed if the supply voltages are
low.
POWER DISSIPATION AND HEAT SINKING
When in “play” mode, the LME49830 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 x (V_CC – V_EE) (W). For a quick calculation of P_DMAX, approximate the current to be
20mA 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 any power IC 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 LME49830TB 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 LME49830 is equal to the following:
P_DMAX = (T_JMAX−T_AMB) / θ_JA (W)
where
• T_JMAX = 150°C
• T_AMB is the system ambient temperature
• θ_JA = θ_JC + θ_CS + θ_SA
(1)
12
Submit Documentation Feedback
Product Folder Links: LME49830
Copyright © 2008–2013, Texas Instruments Incorporated