Datasheet

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CIRCUIT LAYOUT CONSIDERATIONS
GENERAL PowerPAD™ DESIGN CONSIDERATIONS
TLV2470 , , TLV2471
TLV2472 , TLV2473
TLV2474 , TLV2475 , TLV247xA
SLOS232E – JUNE 1999 – REVISED JULY 2007
APPLICATION INFORMATION (continued)
Figure 35 and Figure 36 show the amplifier forward and reverse isolation in shutdown. The operational amplifier
is powered by ± 1.35V supplies and configured as a voltage follower (A
V
= 1). The isolation performance is plotted
across frequency using 0.1V
PP
, 1.5V
PP
, and 2.5V
PP
input signals. During normal operation, the amplifier would
not be able to handle a 2.5V
PP
input signal with a supply voltage of ± 1.35V since it exceeds the common-mode
input voltage range (V
ICR
). However, this curve illustrates that the amplifier remains in shutdown even under a
worst case scenario.
To achieve the levels of high performance of the TLV247x, follow proper printed circuit board (PCB) design
techniques. A general set of guidelines is given below:
• 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.
• Proper power supply decoupling—Use a 6.8 μ F tantalum capacitor in parallel with a 0.1 μ F 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 μ F ceramic capacitor should always be used on the supply terminal of every amplifier. In
addition, the 0.1 μ F 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.
• Sockets—Sockets can be used but are not recommended. 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.
• Short trace runs/compact part placements—Optimum high 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.
• Surface-mount passive components—Using surface-mount passive components is recommended for
high-performance 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.
The TLV247x 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 46 a and Figure 46 b). This
arrangement results in the lead frame being exposed as a thermal pad on the underside of the package (see
Figure 46 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 must 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.
Soldering the PowerPAD to the PCB is always recommended, even with applications that have low power
dissipation. It provides the necessary mechanical and thermal connection between the lead frame die pad and
the PCB.
The PowerPAD package represents a breakthrough in combining the small area and ease of assembly of
surface mount with previously awkward mechanical methods of heatsinking.
17
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