THS4221D Datasheet, PDF (20/40 Pages) Texas Instruments – LOW-DISTORTION, HIGH-SPEED, RAIL-TO-RAIL OUTPUT OPERATIONAL AMPLIFIERS

THS4221, THS4225
THS4222, THS4226
SLOS399G − AUGUST 2002 − REVISED JANUARY 2004
Pin 1
0.205
0.060
0.013
0.017
0.030
0.075
0.025 0.094
0.010
vias
0.035
0.040
Top View
Figure 37. PowerPAD PCB Etch and Via
Pattern
PowerPAD PCB LAYOUT CONSIDERATIONS
1. Prepare the PCB with a top side etch pattern as shown
in Figure 37. There should be etch for the leads as well
as etch for the thermal pad.
2. Place five holes in the area of the thermal pad. These
holes should be 13 mils in diameter. Keep them small
so that solder wicking through the holes is not a
problem during reflow.
3. Additional vias may be placed anywhere along the
thermal plane outside of the thermal pad area. They
help dissipate the heat generated by the THS4222
family IC. These additional vias may be larger than the
13-mil diameter vias directly under the thermal pad.
They can be larger because they are not in the thermal
pad area to be soldered, so that wicking is not a
problem.
4. Connect all holes to the internal ground plane.
5. When connecting these holes to the ground plane, do
not use the typical web or spoke via connection
methodology. Web connections have a high thermal
resistance connection that is useful for slowing the
heat transfer during soldering operations. This
resistance makes the soldering of vias that have plane
connections easier. In this application, however, low
thermal resistance is desired for the most efficient
heat transfer. Therefore, the holes under the THS4222
family PowerPAD package should make their
connection to the internal ground plane, with a
complete connection around the entire circumference
of the plated-through hole.
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
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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 38 and is calculated by the equation 5:
P_D
+
T_max *
q_JA
T_A
(3)
where:
P_D = Maximum power dissipation of THS4222 (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 THS4222 family of devices 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.
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.