THS4502 Datasheet, PDF (29/40 Pages) Texas Instruments – WIDEBAND, LOW-DISTORTION FULLY DIFFERENTIAL AMPLIFIERS

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(50 mils to 100 mils) 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 (< 4 pF) may not need an R_S
since the THS4500 family is 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 (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 normally 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 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.
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.
D 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.
THS4502
THS4503
SLOS352D − APRIL 2002 − REVISED JANUARY 2004
thermal pad on the underside of the package [see
Figure 111(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 111. Views of Thermally Enhanced
Package
Although there are many ways to properly heatsink the
PowerPAD package, the following steps illustrate the
recommended approach.
Pin 1
0.205
0.060
0.013
0.017
0.030
0.075
0.025 0.094
0.010
vias
0.035
Top View
0.040
PowerPAD DESIGN CONSIDERATIONS
The THS4500 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 111(a) and Figure 111(b)]. This
arrangement results in the lead frame being exposed as a
Figure 112. PowerPAD PCB Etch and Via Pattern
PowerPAD PCB LAYOUT CONSIDERATIONS
1. Prepare the PCB with a top side etch pattern as shown
in Figure 112. There should be etch for the leads as
well as etch for the thermal pad.
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