THS4502CD datasheet(29/40 Pages) TI

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THS4502

THS4503

SLOS352D − APRIL 2002 − REVISED JANUARY 2004

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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 RS

since the THS4500 family is nominally compensated

to operate with a 2-pF parasitic load. Higher parasitic

capacitive loads without an RS 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

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.

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

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.

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

Figure 112. PowerPAD PCB Etch and Via Pattern

PowerPAD PCB LAYOUT CONSIDERATIONS

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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