THS4021, THS4022

350-MHz LOW-NOISE HIGH-SPEED AMPLIFIERS

SLOS265B – SEPTEMBER 1999 – REVISED FEBRUARY 2000

14

POST OFFICE BOX 655303

APPLICATION INFORMATION

circuit layout considerations

To achieve the levels of high frequency performance of the THS402x, follow proper printed-circuit board high

frequency design techniques. A general set of guidelines is given below. In addition, a THS402x evaluation

board is available to use as a guide for layout or for evaluating the device performance.

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.

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.

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.

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.

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 THS402x 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 37(a) and Figure 37(b)]. This arrangement results in the lead frame being exposed as a

thermal pad on the underside of the package [see Figure 37(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.