couple the preamp output to the cathode drivers. Therefore

only one cutoff adjustment is provided, this is used primarily

to optimize the operation of the cathode drivers.

Note that the Blank and Clamp Gates are active low. These

pins are normally controlled by standard TTL signals. For

video applications the Clamp Gate must be used. There are

designs where the blank function may not be required. When

the Blank Gate is not used, it must be tied high by a pullup

resistor. A resistor value of 47k is acceptable, going to either

4V or 12V.

Gain of −A2 is controlled by the Drive Adjust pins. These are

also 0V to 4V control voltages. 4V results in no attenuation at

−A2, and 0V results in a −12 dB attenuation. The 12 dB ad-

justment range should provide more than enough adjust-

ment for setting the white level. Note that a 12 dB range

givesa4to1 range in the output levels between the three

channels.

Applications of the LM1282

A schematic for a demonstration board is shown in

Figure 7.

This board was used for the characterization of the LM1282.

Note that a 33

Ω resistor is in series with all inputs to the IC

that receive external signals. These resistors are recom-

mended to protect the IC from any sudden voltage surges

that may result during the power up and power down modes,

or when connecting the monitor to other equipment. The

monitor designer should include these resistors in his de-

sign. If additional protection against ESD at the video inputs

is necessary, then adding clamp diodes on the IC side of the

33

Ω resistor is recommended; one to V

ground. Normally a designer may want to increase the value

of the 33

Ω resistor at pins 5, 8, and 11 for additional ESD

protection at the video inputs. Remember that the input ca-

pacitor to the video inputs is also part of the DC restoration

circuit. This circuit is depending on a maximum circuit resis-

tance of about 110

Ω. The 33Ω resistors should not be in-

creased in value. The internal ESD protection and the exter-

nal clamp diodes, one to +12V and the other to ground, will

provide excellent ESD protection.

Interfacing to the OSD inputs is quite easy since the signal

processing necessary to match the OSD signals to the video

levels is done internally by the LM1282. However, proper de-

sign techniques must be followed in assuring that a good

TTL signal is received at the LM1282. Ground bounce in the

TTL signal can cause improper switching times, possibly

with multiple switching. Such affects will result in degradation

in the quality of the displayed OSD window. The final TTL

stage needs to be located near the LM1282 to assure clean

TTL signals. Propagation delay is another source capable of

degrading the OSD display. The optimum condition is to

have all OSD signals originate from one register, keeping the

variation in the propagation delays under 5 ns. If the OSD

feature is not used, or the lines may be disconnected for

some testing operations, then the Video/OSD Switch pin (pin

4) must have a pull down resistor to ground to insure opera-

tion in the video mode. Using a 47k pull down resistor will

keep this pin low, and provide enough resistance to where

the pin can still be driven directly by a TTL signal. Pins 1

through 3 should also be terminated the same way, eliminat-

ing the potential to switch logic levels just from the noise at

the open pins.

Figures 2, 3, 4 show the timing diagrams for the LM1282.

When measuring propagation delays all TTL signals are

measured at the time they cross 1.3V. The video output is set

to4V

half way in its transition (changed by 2V). Rise and fall times

of the video output are measured between the 10% and 90%

points of the transitions.

Board layout is always critical in a high frequency application

such as using the LM1282. A poor layout can result in ringing

of the video waveform after sudden transitions, or the part

could actually oscillate. A good ground plane and proper

routing of the +12V are important steps to a good PCB lay-

out. The LM1282 can operate on a single sided board with a

good layout. A ground plane is recommended and it is best to

isolate the output stage grounds from the rest of the circuit.

Also the two grounds should be connected together only at

one point, ideally where the ground cable is connected to the

board ground. Yes, all grounds are connected internally, but

trace resistance can still allow for ground bounce, giving

enough feedback for oscillations. The output stage power

supply pin, pin 22, does not have an internal connection to

the other power supply pins. This pin must be connected to

the +12V supply, preferably with high frequency isolation.

This is easily done with a ferrite bead between pin 22 and the

+12V supply.

Figure 8 and Figure 9 show the waveform ob-

tained with the LM1282 using the single sided demo board

designed for this part.

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