lm7131 National Semiconductor Corporation, lm7131 Datasheet - Page 16

• Manufacturer Part Number: lm7131
• Description: Tiny High Speed Single Supply Operational Amplifier
• Manufacturer: National Semiconductor Corporation

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Using the LM7131
Output Short Circuits
The LM7131 has output short circuit protection, however, it is
not designed to withstand continuous short circuits, very fast
high energy transient voltage or current spikes, or shorts to
any voltage beyond the power supply rails. Designs should
reduce the number and energy level of any possible output
shorts, especially when used with
A resistor in series with the output, such as the 75
used to back terminate 75 cables, will reduce the effects of
shorts. For outputs which will send signals off the PC board
additional protection devices, such as diodes to the power
rails, zener-type surge suppressors, and varistors may be
useful.
Thermal Management
Note that the SOT23-5 (Tiny) package has less power dissi-
pation capability (325˚/W) than the S0-8 package (115˚/W).
This may cause overheating with
loads at high ambient temps. This is less of a problem when
using +5V single supplies.
Example:
Driving a 150 load to 2.0V at a 40˚C (104 ˚F) ambient tem-
perature. (This is common external maximum temperature
for office environments. Temperatures inside equipment may
be higher.)
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
No load power-
No load LM7131 supply current - 9.0 mA
Supply voltage is 5.0V
No load LM7131 power - 9.0 mA x 5.0V = 45 mW
Power with load-
Current out is 2.0V/150
Voltage drop in LM7131 is 5.0V (supply) − 2.0V (output) =
3.0V
Power dissipation 13.33 mA x 3.0V = 40 mW
Total Power = 45 mW + 40 mW = 85 mW = 0.085
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.
FIGURE 12.
Gain of +2
= 13.33 mA
(Continued)
±
±
5V supplies.
5 supplies and heavy
DS012313-20
resistor
16
the center pin (pin number 2, V−) on the left side of the
SOT23-5. This pin forms the mounting paddle for the die in-
side 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 per-
formance specifications for the LM7131 is most common at
cold temperatures. While excessively high junction tempera-
tures will degrade LM7131 performance, testing has con-
firmed that most specifications are met at a junction tem-
perature 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 in-
put 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 ca-
pacitor with a value of around 0.01 microfarad (10 nF), usu-
ally 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 22 –100 )
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 re-
duce reflections on the cable.
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 in-
put impedances frequently used in high frequency and video
circuits. For a typical 75
current will produce a voltage across the termination resistor
of only 1.5 mV. An input impedance of 10 k , however,
would produce a voltage of 200 mV, which may be large
compared to the signal of interest. Using lower input imped-
ances is recommended to reduce this error source.
input termination, 20 µA of input