K04RLM12 Datasheet, PDF (17/23 Pages) Texas Instruments – Operational Amplifier

OBSOLETE
K04RLM12, LM12CL
www.ti.com
SNOSBY8D – MAY 1999 – REVISED APRIL 2013
DETERMINING MAXIMUM DISSIPATION
It is a simple matter to establish power requirements for an op amp driving a resistive load at frequencies well
below 10 Hz. Maximum dissipation occurs when the output is at one-half the supply voltage with high-line
conditions. The individual output transistors must be rated to handle this power continuously at the maximum
expected case temperature. The power rating is limited by the maximum junction temperature as determined by
T_J = T_C + P_DISS θ_JC
where
• T_C is the case temperature as measured at the center of the package bottom
• P_DISS is the maximum power dissipation
• θ_JC is the thermal resistance at the operating voltage of the output transistor
(1)
Recommended maximum junction temperatures are 200°C within the power transistor and 150°C for the control
circuitry.
If there is ripple on the supply bus, it is valid to use the average value in worst-case calculations as long as the
peak rating of the power transistor is not exceeded at the ripple peak. With 120 Hz ripple, this is 1.5 times the
continuous power rating.
Dissipation requirements are not so easily established with time varying output signals, especially with reactive
loads. Both peak and continuous dissipation ratings must be taken into account, and these depend on the signal
waveform as well as load characteristics.
With a sine wave output, analysis is fairly straightforward. With supply voltages of ±V_S, the maximum average
power dissipation of both output transistors is
where
• Z_L is the magnitude of the load impedance
• θ its phase angle
(2)
Maximum average dissipation occurs below maximum output swing for θ < 40°.
The instantaneous power dissipation over the conducting half cycle of one output transistor is shown here. Power
dissipation is near zero on the other half cycle. The output level is that resulting in maximum peak and average
dissipation. Plots are given for a resistive and a series RL load. The latter is representative of a 4Ω loudspeaker
operating below resonance and would be the worst case condition in most audio applications. The peak
dissipation of each transistor is about four times average. In ac applications, power capability is often limited by
the peak ratings of the power transistor.
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