lm6313 National Semiconductor Corporation, lm6313 Datasheet - Page 8

• Manufacturer Part Number: lm6313
• Description: High Power Operational Amplifier
• Manufacturer: National Semiconductor Corporation

Application Hints
The LM6313 is a high-speed high power operational ampli-
fier that is designed for driving low-impedance loads such
as 50X and 75X cables Available in the standard low cost
16-pin DIP this amplifier will drive back terminated video
cables with up to 10 Vp-p The ability to add additional com-
pensation allows the LM6313 to drive capacitive loads of
any size at bandwidths previously possible only with very
expensive hybrid devices
The LM6313 is excellent for driving high-speed flash A-to-D
converters that require low-impedance drive at high fre-
quencies At 1 MHz when used as a buffer the LM6313
output impedance is below 0 1X This very low output im-
pedance also means that cables can be accurately back-
terminated by just placing the characteristic impedance in
series with the LM6313 output
OVER-VOLTAGE PROTECTION
If the LM6313 is being operated on supply voltages of great-
er than
transistors exists At higher supply voltages if the output is
shorted or excessive power dissipation causes the output
stage to shut down the maximum A2 input-to-output volt-
age can be exceeded This occurs when the input stage
tries to drive the output while the output is at ground To
prevent this from happening an easy solution is to place
diodes around the output stage (See Figure 4 ) This will
limit the maximum differential voltage to about 1 3V Any
signal diode such as the 1N914 or the 1N4148 will work
fine
HEAT SINKING
When driving a low impedance load such as 50X and oper-
ating from
the LM6313 can rise above 3W To prevent overheating of
the chip which would cause the thermal protection circuitry
to shut the system down the following guidelines should be
followed
1 Reduce the supply voltage The LM6313 will operate
2 Solder pins 4 5 12 and 13 to copper traces which are at
with little change in performance except output voltage
swing on
to the level where no precautions against overheating
are necessary for loads of 10X or more
least 0 100 inch wide and have a total area of at least 2
square inches to obtain a i
pins are connected to the back of the chip and will be at
V
less pin 3 is also connected to this same point
b
They should not be used as a V
g
5V the possibility of damaging the output stage
g
15V supplies the internal power dissipation of
g
5V supplies This will reduce the dissipation
FIGURE 4
JA
of 40 C W These four
b
connection un-
TL H 10521 – 11
8
SUPPLY BYPASSING
Because of the large currents required to drive low-imped-
ance loads supply bypassing as close as possible to the
I C is important At 50 MHz a few inches of wire or circuit
trace can have 20X or 30X of inductive reactance This
inductance in series with a 0 1 mF bypass capacitor can
resonate at 1 MHz to 2 MHz and just appear as an inductor
at higher frequencies A 0 1 mF and a 10mF to 15 mF capac-
itor connected in parallel and as close as possible to the
LM6313 supply pins from each supply to ground will give
best performance
SELECTION OF COMPENSATION CAPACITOR
The compensation pin pin 15 makes it possible to drive
any load at any closed loop gain without stability problems
In most cases where the gain is
is resistive no compensation capacitor is required When
used at unity gain or when driving reactive loads a small
capacitor of 5 pF to 20 pF will insure optimum performance
The easiest way to determine the best value of compensa-
tion capacitor is to temporarily connect a trimmer capacitor
(typical range of 2 pF to 15 pF) between pin 15 and ground
and adjust it for little or no overshoot at the output while
driving the input with a square wave
If the actual load capacitance is known the typical graphs
‘‘Gain-Bandwidth and Phase Margin vs Load Capacitance’’
can be used to select a value
VIDEO CABLE DRIVER
The LM6313 is ideally suited for driving 50X or 75X cables
Unlike a buffer that requires a separate gain stage to make
up for the losses involved in termination the LM6313 gain
can be set to 1 plus the line losses when the transmission
line is end-terminated If back-termination is needed adding
the line impedance in series with the output and raising the
gain to 2 plus the expected line losses will provide a 0 dB
loss system Figure 5 illustrates the back and end terminat-
ed video system including compensation for line losses The
excellent stability of the LM6313 with changes in supply
voltages allow running the amplifier on unregulated sup-
plies The typical change in phase shift when the supplies
are changed from
g
5V to
FIGURE 5
g
15V is less than 3 at 10 MHz
b
1 or greater and the load
TL H 10521 – 12