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 01X 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-

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 13V Any

signal diode such as the 1N914 or the 1N4148 will work

fine

TLH10521 – 11

FIGURE 4

HEAT SINKING

When driving a low impedance load such as 50X and oper-

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

with little change in performance except output voltage

to the level where no precautions against overheating

are necessary for loads of 10X or more

2 Solder pins 4 5 12 and 13 to copper traces which are at

least 0100 inch wide and have a total area of at least 2

pins are connected to the back of the chip and will be at

less pin 3 is also connected to this same point

SUPPLY BYPASSING

Because of the large currents required to drive low-imped-

ance loads supply bypassing as close as possible to the

IC 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 01 mF bypass capacitor can

resonate at 1 MHz to 2 MHz and just appear as an inductor

at higher frequencies A 01 mFanda10mFto15 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

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 providea0dB

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

TLH10521 – 12

FIGURE 5

8