Datasheet

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SLOS289E − DECEMBER 1999 − REVISED SEPTEMBER 2006
13
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
_
+
R
null
R
L
C
L
Figure 29
general power design considerations
When driving heavy loads at high junction temperatures there is an increased probability of electromigration
affecting the long term reliability of ICs. Therefore for this not to be an issue either:
D The output current must be limited (at these high junction temperatures).
or
D The junction temperature must be limited.
The maximum continuous output current at a die temperature 150°C will be 1/3 of the current at 105°C.
The junction temperature will be dependent on the ambient temperature around the IC, thermal impedance from
the die to the ambient and power dissipated within the IC.
T
J
= T
A
+ θ
JA
× P
DIS
Where:
P
DIS
is the IC power dissipation and is equal to the output current multiplied by the voltage dropped across the
output of the IC.
θ
JA
is the thermal impedance between the junction and the ambient temperature of the IC.
T
J
is the junction temperature.
T
A
is the ambient temperature.
Reducing one or more of these factors results in a reduced die temperature. The 8-pin SOIC (small outline
integrated circuit) has a thermal impedance from junction to ambient of 176°C/W. For this reason it is
recommended that the maximum power dissipation of the 8-pin SOIC package be limited to 350 mW, with peak
dissipation of 700 mW as long as the RMS value is less than 350 mW.
The use of the MSOP PowerPAD dramatically reduces the thermal impedance from junction to case. And with
correct mounting, the reduced thermal impedance greatly increases the IC’s permissible power dissipation and
output current handling capability. For example, the power dissipation of the PowerPAD is increased to above
1 W. Sinusoidal and pulse-width modulated output signals also increase the output current capability. The
equivalent dc current is proportional to the square-root of the duty cycle:
I
DC(EQ)
+ I
Cont
(
duty cycle
)
Ǹ
CURRENT DUTY CYCLE
AT PEAK RATED CURRENT
EQUIVALENT DC CURRENT
AS A PERCENTAGE OF PEAK
100 100
70 84
50 71
Note that with an operational amplifier, a duty cycle of 70% would often result in the op amp sourcing current
70% of the time and sinking current 30%, therefore, the equivalent dc current would still be 0.84 times the
continuous current rating at a particular junction temperature.