Datasheet

THS3122
THS3125
www.ti.com SLOS382D –SEPTEMBER 2001– REVISED FEBRUARY 2011
Power-Down Reference Pin Operation Printed-Circuit Board Layout Techniques for
Optimal Performance
In addition to the shutdown pin, the THS3125
features a reference pin (REF) which allows the user Achieving optimum performance with high-frequency
to control the enable or disable power-down voltage amplifiers such as the THS3125 and THS3122
levels applied to the SHUTDOWN pin. In most requires careful attention to board layout parasitic and
split-supply applications, the reference pin is external component types. Recommendations that
connected to ground. In either case, the user must be optimize performance include:
aware of voltage-level thresholds that apply to the
• Minimize parasitic capacitance to any ac ground
shutdown pin. Table 2 shows examples and illustrate
for all of the signal I/O pins. Parasitic capacitance
the relationship between the reference voltage and
on the output and input pins can cause instability.
the power-down thresholds. In the table, the threshold
To reduce unwanted capacitance, a window
levels are derived by the following equations:
around the signal I/O pins should be opened in all
of the ground and power planes around those
SHUTDOWN ≤ REF + 0.8 V for enable
pins. Otherwise, ground and power planes should
SHUTDOWN ≥ REF + 2V for disable
be unbroken elsewhere on the board.
Where the usable range at the REF pin is:
• Minimize the distance [0.25 inch, (6,4 mm)] from
the power-supply pins to high-frequency 0.1-µF
V
CC–
≤ V
REF
≤ (V
CC+
– 4V)
and 100-pF decoupling capacitors. At the device
The recommended mode of operation is to tie the
pins, the ground and power plane layout should
REF pin to midrail, therefore setting the
not be in close proximity to the signal I/O pins.
enable/disable thresholds to V
(midrail)
+ 0.8 V and
Avoid narrow power and ground traces to
V
(midrail)
= 2 V, respectively.
minimize inductance between the pins and the
decoupling capacitors. The power-supply
Table 2. Shutdown Threshold Voltage Levels
connections should always be decoupled with
these capacitors. Larger (6.8 µF or more)
REFERENCE
tantalum decoupling capacitors, effective at lower
SUPPLY PIN ENABLE DISABLE
frequencies, should also be used on the main
VOLTAGE (V) VOLTAGE (V) LEVEL (V) LEVEL (V)
supply pins. These capacitors may be placed
±15, ±5 0 0.8 2.0
somewhat farther from the device and may be
±15 2.0 2.8 4.0
shared among several devices in the same area
±15 –2.0 –1.2 0
of the printed circuit board (PCB).
±5 1.0 1.8 3.0
• Careful selection and placement of external
±5 –1.0 –0.2 1.0
components preserve the high-frequency
+30 15.0 15.8 17 performance of the THS3125 and THS3122.
Resistors should be a very low reactance type.
+10 5.0 5.8 7.0
Surface-mount resistors work best and allow a
tighter overall layout. Again, keep the leads and
Note that if the REF pin is left unterminated, it floats
PCB trace length as short as possible. Never use
to the positive rail and falls outside of the
wirebound type resistors in a high-frequency
recommended operating range given above V
CC–
≤
application. Because the output pin and inverting
V
REF
≤ (V
CC+
– 4V). As a result, it no longer serves as
input pins are the most sensitive to parasitic
a reliable reference for the SHUTDOWN pin, and the
capacitance, always position the feedback and
enable/disable thresholds given above no longer
series output resistors, if any, as close as possible
apply. If the SHUTDOWN pin is also left
to the inverting input pins and output pins. Other
unterminated, it floats to the positive rail and the
network components, such as input termination
device is disabled. If balanced, split supplies are used
resistors, should be placed close to the
(±V
S
) and the REF and SHUTDOWN pins are
gain-setting resistors. Even with a low parasitic
grounded, the device is enabled.
capacitance that shunts the external resistors,
space
excessively high resistor values can create
significant time constants that can degrade
space
performance. Good axial metal-film or
space
surface-mount resistors have approximately 0.2
pF in shunt with the resistor. For resistor values
greater than 2.0 kΩ, this parasitic capacitance can
add a pole and/or a zero that can affect circuit
operation. Keep resistor values as low as
possible, consistent with load driving
considerations.
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Product Folder Link(s): THS3122 THS3125