THS4271_16 Datasheet, PDF (30/50 Pages) Texas Instruments – LOW NOISE, HIGH SLEW RATE, UNITY GAIN STABLE VOLTAGE FEEDBACK AMPLIFIER

THS4271
THS4275
SLOS397F – JULY 2002 – REVISED OCTOBER 2009
For a given θ_JA, the maximum power dissipation is
shown in Figure 91 and is calculated by the
Equation 6:
P_D
+
T_max *
q_JA
T_A
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).
(6)
The next consideration is the package constraints.
The two sources of heat within an amplifier are
quiescent power and output power. The designer
should never forget about the quiescent heat
generated within the device, especially multi-amplifier
devices. Because these devices have linear output
stages (Class AB), most of the heat dissipation is at
low output voltages with high output currents.
The other key factor when dealing with power
dissipation is how the devices are mounted on the
PCB. The PowerPAD devices are extremely useful
for heat dissipation. But, the device should always be
soldered to a copper plane to fully use the heat
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.
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The thermal characteristics of the device are dictated
by the package and the PCB. Maximum power
dissipation for a given package can be calculated
using the following formula.
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).
(7)
For systems where heat dissipation is more critical,
the THS4271 is offered in an 8-pin MSOP with
PowerPAD. The thermal coefficient for the MSOP
PowerPAD package is substantially improved over
the traditional SOIC. Maximum power dissipation
levels are depicted in the graph for the two packages.
The data for the DGN package assumes a board
layout that follows the PowerPAD layout guidelines
referenced above and detailed in the PowerPAD
application notes in the Additional Reference Material
section at the end of the data sheet.
3.5
8-Pin DGN Package
3
2.5
2
8-Pin D Package
1.5
1
0.5
0
−40 −20 0
20 40 60 80
T_A − Ambient Temperature − °C
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
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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