Datasheet

P = V x 2I
= 24 x 2 x (6.5 x 10 )
= 312mW
Q S q
-3
LM7372
SNOS926E –MAY 1999–REVISED MARCH 2013 www.ti.com
APPLICATION INFORMATION
The LM7372 is a high speed dual operational amplifier with a very high slew rate and very low distortion, yet like
many other op amps, it is used in conventional voltage feedback amplifier applications. Also, again like many op
amps, it has a class AB output stage in order to be able to deliver high currents to low impedance loads, yet
draw a low quiescent supply current in most situations (the supply current increases when necessary to keep up
with large output swing and/or high frequency. See HIGH FREQUENCY/LARGE SIGNAL SWING
CONSIDERATIONS section below). For most op amps in typical applications, this topology means that internal
power dissipation is rarely an issue, even with the trend to smaller surface mount packages. However, the
LM7372 has been designed for applications where significant levels of power dissipation will be encountered,
and an effective means of removing the internal heat generated by this power dissipation is needed to maintain
the semiconductor junction temperature at acceptable levels, particularly in environments with elevated ambient
temperatures.
Several factors contribute to power dissipation and consequently higher semiconductor junction temperatures,
and these factors need to be well understood if the LM7372 is to perform to the desired specifications in a given
application. Since different applications will have different dissipation levels and different compromises can be
made between the ways these factors will contribute to the total junction temperature, this section will examine
the typical application shown on the front page of this data sheet as an example, and offer suggestions for
solutions where excessive junction temperatures are encountered.
There are two major contributors to the internal power dissipation; the product of the supply voltage and the
LM7372 quiescent current when no signal is being delivered to the external load, and the additional power
dissipated while delivering power to the external load. For low frequency (<1MHz) applications, the LM7372
supply current specification will suffice to come up with the quiescent power dissipation (see HIGH
FREQUENCY/LARGE SIGNAL SWING CONSIDERATIONS section for cases where the frequency range
exceeds 1MHz and the LM7372 supply current increases) . The LM7372 quiescent supply current is given as
6.5mA per amplifier, so with a 24Volt supply the power dissipation is
(1)
where (V
S
= V
+
- V
−
)
This is already a high level of internal power dissipation, and in a small surface mount package with a thermal
resistance (θ
JA
= 140°C/Watt (a not unreasonable value for an 8-Pin SOIC package) would result in a junction
temperature 140°C/W x 0.312W = 43.7°C above the ambient temperature. A similar calculation using the worst
case maximum supply current specification of 8.5mA per amplifier at an 85°C ambient will yield a power
dissipation of 456mW with a junction temperature of 149°C, perilously close to the maximum permitted junction
temperature of 150°C!
The second contributor to high junction temperature is the additional power dissipated internally when power is
being delivered to the external load. This cause of temperature rise can be less amenable to calculation, even
when the actual operating conditions are known.
For a Class B output stage, one transistor of the output pair will conduct the load current as the output voltage
swings positive, with the other transistor drawing no current, and hence dissipating no power. During the other
half of the signal swing this situation is reversed, with the lower transistor sinking the load current and the upper
transistor is cut off. The current in each transistor will be a half wave rectified version of the total load current.
Ideally neither transistor will dissipate power when there is no signal swing, but will dissipate increasing power as
the output current increases. However, as the signal voltage across the load increases with load current, the
voltage across the output transistor (which is the difference voltage between the supply voltage and the
instantaneous voltage across the load) will decrease and a point will be reached where the dissipation in the
transistor will begin to decrease again. If the signal is driven into a square wave, ideally the transistor dissipation
will fall all the way back to zero.
For each amplifier then, with an effective load each of R
L
and a sine wave source, integration over the half cycle
with a supply voltage V
S
and a load voltage V
L
yields the average power dissipation
P
D
= V
S
V
L
/πR
L
- V
L
2
/2R
L
(2)
where V
S
is the supply voltage and V
L
is the peak signal swing across the load R
L
.
12 Submit Documentation Feedback Copyright © 1999–2013, Texas Instruments Incorporated
Product Folder Links: LM7372