THS3115CPWP Datasheet, PDF (16/35 Pages) Texas Instruments – LOW-NOISE, HIGH-SPEED, CURRENT FEEDBACK AMPLIFIERS

THS3112
THS3115
SLOS385C – SEPTEMBER 2001 – REVISED SEPTEMBER 2010
• Connections to other wideband devices on the
board may be made with short direct traces or
through onboard transmission lines. For short
connections, consider the trace and the input to
the next device as a lumped capacitive load.
Relatively wide traces [0.05 inch (1,3 mm) to 0.1
inch (2,54 mm)] should be used, preferably with
ground and power planes opened up around
them. Estimate the total capacitive load and
determine if isolation resistors on the outputs are
necessary. Low parasitic capacitive loads (less
than 4 pF) may not need an R_S because the
THS3115 and THS3112 are nominally
compensated to operate with a 2-pF parasitic
load. Higher parasitic capacitive loads without an
R_S are allowed as the signal gain increases (thus
increasing the unloaded phase margin). If a long
trace is required, and the 6-dB signal loss intrinsic
to a doubly-terminated 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 THS3115/THS3112 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
configuration 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 as a result of the
voltage divider formed by the series output into
the terminating impedance.
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• Socketing a high-speed device such as the
THS3115 and THS3112 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 THS3115/THS3112 amplifiers
directly onto the board.
PowerPAD™ Design Considerations
The THS3115 and THS3112 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 50(a) and Figure 50(b)]. This arrangement
results in the lead frame being exposed as a thermal
pad on the underside of the package [see
Figure 50(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. Note that
devices such as the THS311x have no electrical
connection between the PowerPAD and the die.
DIE
(a) Side View
DIE
(b) End View
Thermal
Pad
(c) Bottom View
Figure 50. Views of Thermally-Enhanced Package
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.
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