LM7131ACM5X National Semiconductor, LM7131ACM5X Datasheet - Page 18

• Manufacturer Part Number: LM7131ACM5X
• Description: IC OP AMP TINY HI SPEED SOT23-5
• Manufacturer: National Semiconductor

Specifications of LM7131ACM5X

• Amplifier Type: Voltage Feedback
• Number Of Circuits: 1
• Slew Rate: 150 V/µs
• Gain Bandwidth Product: 70MHz
• -3db Bandwidth: 90MHz
• Current - Input Bias: 20µA
• Voltage - Input Offset: 20µV
• Current - Supply: 7.5mA
• Current - Output / Channel: 65mA
• Voltage - Supply, Single/dual (±): 2.7 V ~ 12 V, ± 2.5 V ~ 6 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: SOT-23-5, SC-74A, SOT-25
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Output Type: -
• Other names: LM7131ACM5XTR

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Using the LM7131
The 325˚/W value is based on still air and the pc board land
pattern shown in this datasheet. Actual power dissipation is
sensitive to PC board connections and airflow.
SOT23-5 power dissipation may be increased by airflow or
by increasing the metal connected to the pads, especially
the center pin (pin number 2, V−) on the left side of the
SOT23-5. This pin forms the mounting paddle for the die
inside the SOT23-5, and can be used to conduct heat away
from the die. The land pad for pin 2 can be made larger
and/or connected to power planes in a multilayer board.
Additionally, it should be noted that difficulty in meeting
performance specifications for the LM7131 is most common
at cold temperatures. While excessively high junction tem-
peratures will degrade LM7131 performance, testing has
confirmed that most specifications are met at a junction
temperature of 85˚C.
See “Understanding Integrated Circuit Package Power Ca-
pabilities”, Application Note AN-336, which may be found in
the appendix of the Operational Amplifier Databook.
Layout and Power Supply Bypassing
Since the LM7131 is a high speed (over 50 MHz) device,
good high speed circuit layout practices should be followed.
This should include the use of ground planes, adequate
power supply bypassing, removing metal from around the
input pins to reduce capacitance, and careful routing of the
output signal lines to keep them away from the input pins.
The power supply pins should be bypassed on both the
negative and positive supply inputs with capacitors placed
close to the pins. Surface mount capacitors should be used
for best performance, and should be placed as close to the
pins as possible. It is generally advisable to use two capaci-
tors at each supply voltage pin. A small surface mount
capacitor with a value of around 0.01 microfarad (10 nF),
usually a ceramic type with good RF performance, should be
placed closest to the pin. A larger capacitor, in usually in the
range of 1.0 µF to 4.7 µF, should also be placed near the pin.
The larger capacitor should be a device with good RF char-
acteristics and low ESR (equivalent series resistance) for
best results. Ceramic and tantalum capacitors generally
work well as the larger capacitor.
For single supply operation, if continuous low impedance
ground planes are available, it may be possible to use by-
pass capacitors between the +5V supply and ground only,
and reduce or eliminate the bypass capacitors on the V− pin.
Capacitive Load Driving
The phase margin of the LM7131 is reduced by driving large
capacitive loads. This can result in ringing and slower set-
tling of pulse signals. This ringing can be reduced by placing
a small value resistor (typically in the range of 22Ω–100Ω)
between the LM7131 output and the load. This resistor
should be placed as close as practical to the LM7131 output.
When driving cables, a resistor with the same value as the
characteristic impedance of the cable may be used to isolate
the cable capacitance from the output. This resistor will
reduce reflections on the cable.
Temperature Rise = 0.085 W x 325˚/W = 27.625 degrees
Junction temperature at 40˚ ambient = 40 + 27.625 =
67.6225˚.
This device is within the 0˚ to 70˚ specification limits.
(Continued)
18
Input Current
The LM7131 has typical input bias currents in the 15 µA to
25 µA range. This will not present a problem with the low
input impedances frequently used in high frequency and
video circuits. For a typical 75Ω input termination, 20 µA of
input current will produce a voltage across the termination
resistor of only 1.5 mV. An input impedance of 10 kΩ, how-
ever, would produce a voltage of 200 mV, which may be
large compared to the signal of interest. Using lower input
impedances is recommended to reduce this error source.
Feedback Resistor Values and Feedback
Compensation
Using large values of feedback resistances (roughly 2k) with
low gains (such gains of 2) will result in degraded pulse
response and ringing. The large resistance will form a pole
with the input capacitance of the inverting input, delaying
feedback to the amplifier. This will produce overshoot and
ringing. To avoid this, the gain setting resistors should be
scaled to lower values (below 1k) At higher gains (
values of feedback resistors can be used.
Overshoot and ringing of the LM7131 can be reduced by
adding a small compensation capacitor across the feed back
resistor. For the LM7131 values in pF to tens of pF range are
useful initial values. Too large a value will reduce the circuit
bandwidth and degrade pulse response.
Since the small stray capacitance from the circuit layout,
other components, and specific circuit bandwidth require-
ments will vary, it is often useful to select final values based
on prototypes which are similar in layout to the production
circuit boards.
Reflections
The output slew rate of the LM7131 is fast enough to pro-
duce reflected signals in many cables and long circuit traces.
For best pulse performance, it may be necessary to termi-
nate cables and long circuit traces with their characteristic
impedance to reduce reflected signals.
Reflections should not be confused with overshoot. Reflec-
tions will depend on cable length, while overshoot will de-
pend on load and feedback resistance and capacitance.
When determining the type of problem, often removing or
drastically shortening the cable will reduce or eliminate re-
flections. Overshoot can exist without a cable attached to the
op amp output.
Driving Flash A/D Converters (Video Converters)
The LM7131 has been optimized to drive flash analog to
digital converters in a +5V only system. Different flash A/D
converters have different voltage input ranges. The LM7131
has enough gain-bandwidth product to amplify standard
video level signals to voltages which match the optimum
input range of many types of A/D converters.
For example, the popular 1175 type 8-bit flash A/D converter
has a preferred input range from 0.6V to 2.6V. If the input
signal has an active video range (excluding sync levels) of
approximately 700 mV, a circuit like the one in Figure 13 can
be used to amplify and drive an A/D. The 10 µF capacitor
blocks the DC components, and allows the + input of the
LM7131 to be biased through R clamp so that the minimum
output is equal to V
circuit is determined as follows:
Output Signal Range = 2.6V (V top) = 0.6V (V bottom) =
2.0V
RB
of the A/D converter. The gain of the
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