THS4271 Datasheet, PDF (26/38 Pages) Texas Instruments – LOW NOISE, HIGH SLEW RATE, UNITY GAIN STABLE VOLTAGE FREEBACK AMPLIFIER

THS4271
THS4275
SLOS397E − JULY 2002 − REVISED JANUARY 2004
parallel combination of the shunt resistor and the input
impedance of the destination device: this total
effective impedance should be set to match the trace
impedance. If the 6-dB attenuation of a doubly
terminated transmission line is unacceptable, a long
trace can be series-terminated at the source end only.
Treat the trace as a capacitive load in this case and set
the series resistor value as shown in the plot of R_(ISO)
vs capacitive load. This does not preserve signal
integrity or a doubly-terminated line. If the input
impedance of the destination device is low, there is
some signal attenuation due to the voltage divider
formed by the series output into the terminating
impedance.
5. Socketing a high speed part like the THS4271 is
not recommended. The additional lead length and
pin-to-pin capacitance introduced by the socket can
create a troublesome parasitic network which can
make it almost impossible to achieve a smooth, stable
frequency response. Best results are obtained by
soldering the THS4271 onto the board.
PowerPAD DESIGN CONSIDERATIONS
The THS4271 and THS4275 are available in a
thermally-enhanced PowerPAD family of packages.
These packages are constructed using a downset
leadframe upon which the die is mounted [see
Figure 89(a) and Figure 89(b)]. This arrangement results
in the lead frame being exposed as a thermal pad on the
underside of the package [see Figure 89(c)]. Because this
thermal pad has direct thermal contact with the die,
excellent thermal performance can be achieved by
providing a good thermal path away from the thermal pad.
The PowerPAD package allows both assembly and
thermal management in one manufacturing operation.
During the surface-mount solder operation (when the
leads are being soldered), the thermal pad can also be
soldered to a copper area underneath the package.
Through the use of thermal paths within this copper area,
heat can be conducted away from the package into either
a ground plane or other heat dissipating device.
The PowerPAD package represents a breakthrough in
combining the small area and ease of assembly of surface
mount with the heretofore awkward mechanical methods
of heatsinking.
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DIE
Side View (a)
DIE
End View (b)
Thermal
Pad
Bottom View (c)
Figure 89. Views of Thermally Enhanced
Package
Although there are many ways to properly heatsink the
PowerPAD package, the following steps illustrate the
recommended approach.
ÓÓÓÓÓÓÓÓÓÓÓÓÓÓÓÓÓÓÓÓÓÓÓÓÓÓ ^{Single or Dual}
68 Mils x 70 Mils
(Via diameter = 13mils)
Figure 90. PowerPAD PCB Etch and Via
Pattern
PowerPAD PCB LAYOUT CONSIDERATIONS
1. Prepare the PCB with a top side etch pattern as shown
in Figure 90. 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. The
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 THS4271
and THS4275 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 THS4271
and THS4275 PowerPAD package should make their
connection to the internal ground plane, with a
complete connection around the entire circumference
of the plated-through hole.
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