THS3202_16 Datasheet, PDF (21/37 Pages) Texas Instruments – 2-GHz, LOW DISTORTION, DUAL CURRENT-FEEDBACK AMPLIFIERS

THS3202
www.ti.com
SLOS242F – SEPTEMBER 2002 – REVISED JANUARY 2010
transmission line is acceptable, implement a matched impedance transmission line using microstrip or
stripline techniques (consult an ECL design handbook for microstrip and stripline layout techniques).
A 50-Ω environment is not necessary onboard, and in fact, a higher impedance environment improves
distortion as shown in the distortion versus load plots. With a characteristic board trace impedance based on
board material and trace dimensions, a matching series resistor into the trace from the output of the THS320x
is used as well as a terminating shunt resistor at the input of the destination device.
Remember also that the terminating impedance is the 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. 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 THS320x 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 THS320x family devices directly onto the board.
PowerPAD DESIGN CONSIDERATIONS
The THS320x family is 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 79(a) and Figure 79(b)]. This
arrangement results in the lead frame being exposed as a thermal pad on the underside of the package [see
Figure 79(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 for 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.
DIE
Side View (a)
DIE
End View (b)
Thermal
Pad
Bottom View (c)
Figure 79. Views of Thermally-Enhanced Package
Although there are many ways to properly heatsink the PowerPAD package, the following steps illustrate the
recommended approach.
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