Datasheet

100 1k 10k 1M 10M
FREQUENCY (Hz)
1
10
100
100k
VOLTAGE NOISE (nV/
Hz)
i
n
e
n
1
10
100
CURRENT NOISE
(
pA
/
Hz)
LMH6672
SNOS957G –APRIL 2001–REVISED MARCH 2013 www.ti.com
Typical Performance Characteristics (continued)
e
n
& i
n
vs. Frequency, V
S
= 5V and 12V
Figure 44.
Application Notes
THERMAL MANAGEMENT
The LMH6672 is a high-speed, high power, dual operational amplifier with a very high slew rate and very low
distortion. For ease of use, it uses conventional voltage feedback. These characteristics make the LMH6672
ideal for applications where driving low impedances of 25-100Ω such as xDSL and active filters.
A class AB output stage allows the LMH6672 to deliver high currents to low impedance loads with low distortion
while consuming low quiescent supply current. For most op-amps, class AB topology means that internal power
dissipation is rarely an issue, even with the trend to smaller surface mount packages. However, the LMH6672
has been designed for applications where high levels of power dissipation may be encountered.
Several factors contribute to power dissipation and consequently higher junction temperatures. These factors
need to be well understood if the LMH6672 is to perform to specifications in all applications. This section will
examine the typical application shown in Figure 2, as an example. Because both amplifiers are in a single
package, the calculations are for the total power dissipated by both amplifiers.
There are two separate contributors to the internal power dissipation:
1. The product of the supply voltage and the quiescent current when no signal is being delivered to the external
load.
2. The additional power dissipated while delivering power to the external load.
The first of these components appears easy to calculate simply by inspecting the data sheet. The typical
quiescent supply current for this part is 7.2 mA per amplifier, therefore, with a ±6 volt supply, the total power
dissipation is:
P
D
= V
S
× 2 × l
Q
= 12 × (14.4×10
-3
) = 173 mW
(V
S
= V
CC
+ V
EE
)
With a thermal resistance of 172°C/W for the SOIC package, this level of internal power dissipation will result in a
junction temperature (T
J
) of 30°C above ambient.
Using the worst-case maximum supply current of 18 mA and an ambient of 85°C, a similar calculation results in a
power dissipation of 216 mW, or a T
J
of 122°C.
This is approaching the maximum allowed T
J
of 150°C before a signal is applied. Fortunately, in normal
operation, this term is reduced, for reasons that will soon be explained.
The second contributor to high T
J
is the power dissipated internally when power is delivered to the external load.
This cause of temperature rise is more difficult to calculate, even when the actual operating conditions are
known.
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