THS4601

SLOS388B – OCTOBER 2001 – REVISED JUNE 2002

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APPLICATION INFORMATION

PC board layout guidelines

Achieving optimum performance with a high frequency amplifier requires careful attention to board layout

parasitics and external component selection. Recommendations that optimize performance include the

following.

components with a low impedance connection to ground. However, the ground plane should be cleared

around the amplifier inputs and outputs to minimize parasitic capacitance. A solid ground plane is

recommended wherever possible.

used on each power supply node. Good performance is possible if the 6.8

µF capacitor is shared among

several amplifiers, but each amplifier should have a dedicated 0.1

µF capacitor for each supply. The 0.1

µF capacitor should be placed as close to the power supply pins as possible. As the distance from the device

increases, the trace inductance rises and decreases the effectiveness of the capacitor. A good design has

less than 2.5 mm separating the ceramic capacitor and the power supply pin. The tantalum capacitors can

be placed significantly further away from the device.

with the socket pins often leads to stability problems. Direct soldering to a printed-circuit board yields the

best performance.

design and lead to better high-frequency performance.

to the extremely low lead inductance and the small component footprint. These characteristics minimize

problems with stray series inductance and allow for a more compact circuit layout. Compact layout reduces

both parasitic inductance and capacitance in the design.

output pins can degrade high frequency behavior or cause instability in the circuit. Capacitance on the

inverting input or the output is a common cause of instability in high performance amplifiers, and

capacitance on the noninverting input can react with the source impedance to cause unintentional

band-limiting. To reduce unwanted capacitance around these pins, a window should be opened up in the

signal/power layers that are underneath those pins. Power and ground planes should otherwise be

unbroken.

PowerPAD design considerations

The THS4601 is available in a thermally-enhanced PowerPAD package. This package is constructed using a

downset leadframe upon which the die is mounted (see Figure 39). This arrangement results in the lead frame

exposed as a thermal pad on the underside of the package. 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 is electrically insulated from the amplifier circuitry, but connection to the ground plane is

recommended due to the high thermal mass typically associated with a ground plane.

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.