Datasheet

P
D
+
T
max
* T
A
q
JA
where:
P
D
= Maximum power dissipation of THS4271 (watts)
T
MAX
= Absolute maximum junction temperature (150°C)
T
A
= Free-ambient temperature (°C)
θ
JA
= θ
JC
+ θ
CA
θ
JC
= Thermal coefficient from junction to the case
θ
CA
= Thermal coefficient from the case to ambient air
(°C/W).
P
Dmax
+
T
max
–T
A
q
JA
where:
P
Dmax
is the maximum power dissipation in the amplifier (W).
T
max
is the absolute maximum junction temperature (°C).
T
A
is the ambient temperature (°C).
θ
JA
= θ
JC
+ θ
CA
θ
JC
is the thermal coefficient from the silicon junctions to the
case (°C/W).
θ
CA
is the thermal coefficient from the case to ambient air
(°C/W).
2
1.5
1
0
−40 −20 0 20
− Maximum Power Dissipation − W
2.5
3
3.5
40 60 80
T
A
− Ambient Temperature − °C
P
D
8-Pin DGN Package
θ
JA
= 170°C/W for 8-Pin SOIC (D)
θ
JA
= 58.4°C/W for 8-Pin MSOP (DGN)
T
J
= 150°C, No Airflow
0.5
8-Pin D Package
THS4271
THS4275
SLOS397F –JULY 2002–REVISED OCTOBER 2009 www.ti.com
For a given θ
JA
, the maximum power dissipation is The thermal characteristics of the device are dictated
shown in Figure 91 and is calculated by the by the package and the PCB. Maximum power
Equation 6: dissipation for a given package can be calculated
using the following formula.
(6)
The next consideration is the package constraints.
The two sources of heat within an amplifier are
(7)
quiescent power and output power. The designer
should never forget about the quiescent heat For systems where heat dissipation is more critical,
generated within the device, especially multi-amplifier the THS4271 is offered in an 8-pin MSOP with
devices. Because these devices have linear output PowerPAD. The thermal coefficient for the MSOP
stages (Class AB), most of the heat dissipation is at PowerPAD package is substantially improved over
low output voltages with high output currents. the traditional SOIC. Maximum power dissipation
levels are depicted in the graph for the two packages.
The other key factor when dealing with power
The data for the DGN package assumes a board
dissipation is how the devices are mounted on the
layout that follows the PowerPAD layout guidelines
PCB. The PowerPAD devices are extremely useful
referenced above and detailed in the PowerPAD
for heat dissipation. But, the device should always be
application notes in the Additional Reference Material
soldered to a copper plane to fully use the heat
section at the end of the data sheet.
dissipation properties of the PowerPAD. The SOIC
package, on the other hand, is highly dependent on
how it is mounted on the PCB. As more trace and
copper area is placed around the device,θ
JA
decreases and the heat dissipation capability
increases. For a single package, the sum of the RMS
output currents and voltages should be used to
choose the proper package.
THERMAL ANALYSIS
The THS4271 device does not incorporate automatic
thermal shutoff protection, so the designer must take
care to ensure that the design does not violate the
absolute maximum junction temperature of the
device. Failure may result if the absolute maximum
junction temperature of +150° C is exceeded.
Figure 91. Maximum Power Dissipation
vs Ambient Temperature
When determining whether or not the device satisfies
the maximum power dissipation requirement, it is
important to consider not only quiescent power
dissipation, but also dynamic power dissipation. Often
maximum power is difficult to quantify because the
signal pattern is inconsistent, but an estimate of the
RMS power dissipation can provide visibility into a
possible problem.
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