THS4051 Datasheet, PDF (20/36 Pages) Texas Instruments – 70-MHz HIGH-SPEED AMPLIFIERS

THS4051
THS4052
SLOS238D − MAY 1999 − REVISED AUGUST 2008
APPLICATION INFORMATION
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CIRCUIT LAYOUT CONSIDERATIONS
To achieve the levels of high frequency performance of the THS405x, follow proper printed-circuit board high frequency
design techniques. A general set of guidelines is given below. In addition, a THS405x evaluation board is available to use
as a guide for layout or for evaluating the device performance.
D Ground planes − It is highly recommended that a ground plane be used on the board to provide all components
with a low inductive ground connection. However, in the areas of the amplifier inputs and output, the ground plane
can be removed to minimize the stray capacitance.
D Proper power supply decoupling − Use a 6.8-µF tantalum capacitor in parallel with a 0.1-µF ceramic capacitor
on each supply terminal. It may be possible to share the tantalum among several amplifiers depending on the
application, but a 0.1-µF ceramic capacitor should always be used on the supply terminal of every amplifier. In
addition, the 0.1-µF capacitor should be placed as close as possible to the supply terminal. As this distance
increases, the inductance in the connecting trace makes the capacitor less effective. The designer should strive
for distances of less than 0.1 inches between the device power terminals and the ceramic capacitors.
D Sockets − Sockets are not recommended for high-speed operational amplifiers. The additional lead inductance
in the socket pins will often lead to stability problems. Surface-mount packages soldered directly to the
printed-circuit board is the best implementation.
D Short trace runs/compact part placements − Optimum high frequency performance is achieved when stray series
inductance has been minimized. To realize this, the circuit layout should be made as compact as possible,
thereby minimizing the length of all trace runs. Particular attention should be paid to the inverting input of the
amplifier. Its length should be kept as short as possible. This will help to minimize stray capacitance at the input
of the amplifier.
D Surface-mount passive components − Using surface-mount passive components is recommended for high
frequency amplifier circuits for several reasons. First, because of the extremely low lead inductance of
surface-mount components, the problem with stray series inductance is greatly reduced. Second, the small size
of surface-mount components naturally leads to a more compact layout thereby minimizing both stray inductance
and capacitance. If leaded components are used, it is recommended that the lead lengths be kept as short as
possible.
GENERAL POWERPAD DESIGN CONSIDERATIONS
The THS405x is available packaged in a thermally-enhanced DGN package, which is a member of the PowerPAD family
of packages. This package is constructed using a downset leadframe upon which the die is mounted [see Figure 51(a) and
Figure 51(b)]. This arrangement results in the lead frame being exposed as a thermal pad on the underside of the package
[see Figure 51(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 the surface
mount with the, heretofore, awkward mechanical methods of heatsinking.
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