Datasheet

DIE
Side View (a)
DIE
End View (b)
Thermal
Pad
Bottom View (c)
0.060
0.040
0.075 0.025
0.205
0.010
vias
Pin 1
Top View
0.017
0.035
0.094
0.030
0.013
THS4502
THS4503
www.ti.com SLOS352E –APRIL 2002–REVISED OCTOBER 2011
effect circuit operation. Keep resistor values as PowerPAD DESIGN CONSIDERATIONS
low as possible, consistent with load driving
The THS4500 family is available in a
considerations.
thermally-enhanced PowerPAD family of packages.
• Connections to other wideband devices on the
These packages are constructed using a downset
board may be made with short direct traces or
leadframe upon which the die is mounted [see
through onboard transmission lines. For short
Figure 112(a) and Figure 112(b)]. This arrangement
connections, consider the trace and the input to
results in the lead frame being exposed as a thermal
the next device as a lumped capacitive load.
pad on the underside of the package [see
Relatively wide traces (50 mils to 100 mils) should
Figure 112(c)]. Because this thermal pad has direct
be used, preferably with ground and power planes
thermal contact with the die, excellent thermal
opened up around them. Estimate the total
performance can be achieved by providing a good
capacitive load and determine if isolation resistors
thermal path away from the thermal pad.
on the outputs are necessary. Low parasitic
The PowerPAD package allows for both assembly
capacitive loads (< 4 pF) may not need an R
S
and thermal management in one manufacturing
since the THS4500 family is nominally
operation. During the surface-mount solder operation
compensated to operate with a 2-pF parasitic
(when the leads are being soldered), the thermal pad
load. Higher parasitic capacitive loads without an
can also be soldered to a copper area underneath the
R
S
are allowed as the signal gain increases
package. Through the use of thermal paths within this
(increasing the unloaded phase margin). If a long
copper area, heat can be conducted away from the
trace is required, and the 6-dB signal loss intrinsic
package into either a ground plane or other heat
to a doubly-terminated transmission line is
dissipating device.
acceptable, implement a matched impedance
transmission line using microstrip or stripline
The PowerPAD package represents a breakthrough
techniques (consult an ECL design handbook for
in combining the small area and ease of assembly of
microstrip and stripline layout techniques).
surface mount with the, heretofore, awkward
• A 50-Ω environment is normally not necessary
mechanical methods of heatsinking.
onboard, and in fact, a higher impedance
environment improves distortion as shown in the
distortion versus load plots. With a characteristic
board trace impedance defined based on board
material and trace dimensions, a matching series
resistor into the trace from the output of the
THS4500 family is used as well as a terminating
shunt resistor at the input of the destination
device.
Figure 112. Views of Thermally
• Remember also that the terminating impedance is
Enhanced Package
the parallel combination of the shunt resistor and
the input impedance of the destination device: this
Although there are many ways to properly heatsink
total effective impedance should be set to match
the PowerPAD package, the following steps illustrate
the trace impedance. If the 6-dB attenuation of a
the recommended approach.
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. This
does not preserve signal integrity as well as 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.
• Socketing a high speed part like the THS4500
family is not recommended. The additional lead
length and pin-to-pin capacitance introduced by
the socket can create an extremely troublesome
parasitic network which can make it almost
impossible to achieve a smooth, stable frequency
response. Best results are obtained by soldering
the THS4500 family parts directly onto the board.
Figure 113. PowerPAD PCB Etch and Via Pattern
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