THS4271 Datasheet, PDF (27/38 Pages) Texas Instruments – LOW NOISE, HIGH SLEW RATE, UNITY GAIN STABLE VOLTAGE FREEBACK AMPLIFIER

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6. The top-side solder mask should leave the terminals
of the package and the thermal pad area with its five
holes exposed. The bottom-side solder mask should
cover the five holes of the thermal pad area. This
prevents solder from being pulled away from the
thermal pad area during the reflow process.
7. Apply solder paste to the exposed thermal pad area
and all of the IC terminals.
8. With these preparatory steps in place, the IC is simply
placed in position and run through the solder reflow
operation as any standard surface-mount
component. This results in a part that is properly
installed.
For a given θ_JA , the maximum power dissipation is shown
in Figure 91 and is calculated by the equation 5:
P_D
+
T_max *
q_JA
T_A
(6)
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).
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
THS4271
THS4275
SLOS397E − JULY 2002 − REVISED JANUARY 2004
maximum junction temperature of the device. Failure may
result if the absolute maximum junction temperature of
150_ C is exceeded.
The thermal characteristics of the device are dictated by
the package and the PC board. Maximum power
dissipation for a given package can be calculated using the
following formula.
P _Dmax
+
T_max–T_A
q_JA
(7)
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).
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
TA − Ambient Temperature − °C
θ_JA = 170°C/W for 8-Pin SOIC (D)
θ_JA = 58.4°C/W for 8-Pin MSOP (DGN)
TJ = 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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