APPLICATION NOTES

BLACK LEVEL CONTROL

Unlike many currently available video amplifiers, the MSK 645

is a D.C. coupled device. D.C. coupling affords the user direct

black level control.

A video input voltage of approximately

0.85 volts will set the output voltage to 50VDC (black level for

in the application of the amplifier. For example, the user could

apply a 0.85VDC level to the video input to bias the output at

input video information swing from 0.85 to 1.5V causing the

output to swing from the black level towards white (zero).

This configuration would dissipate the least amount of power

and is most common. Another possible circuit configuration

could be to D.C. bias the video input pin so that the output is

must be paid to device power dissipation in this configuration

since it will be very high.

VBIAS INPUT

sistor in the equivalent schematic. The purpose of the cascode

transistor is to isolate the input transistor from the high volt-

age supply. The input transistor must have a very high transi-

tion frequency specification and this is difficult to find in high

voltage transistors. By using the cascode transistor to relieve

the input transistor of its high BVceo requirement, high speed,

low breakdown transistors can be used. The voltage applied

to pin three minus a base to emitter voltage drop of approxi-

mately 0.6 volts is the voltage present at the collector of the

input transistor that acts as the voltage to current convertor.

of 10.0 volts. Above 10.0 volts the device may not be able to

reach white level without going into cutoff. The practical lower

limit for this pin is approximately 2.0 volts. Below 2.0 volts

the input transistor will be dangerously close to cutoff. The

OUTPUT CONSIDERATIONS

The output of the MSK 645 is driven by a complimentary

push-pull buffer. The output stage isolates the capacitive load

from the amplifier thereby making rising and falling edges rela-

tively load independent. The bandwidth of the MSK 645 is

limited by the RC time constant made up of the resistance

from the NPN buffer base to ground. The coils in the equiva-

lent schematic are chosen at the factory to moderately peak

the amplifiers response (10%). For application specific user

adjustable peaking, see the Typical Connection Circuit page.

OUTPUT PROTECTION

High voltage arcing can occur in the CRT being driven and

cause severe damage to the MSK 645 output unless certain

precautions are taken. The clamp diodes D1 and D2 (see fig-

ure 1) will keep the voltage at the output at a safe level. These

diodes should have a low series resistance and shunt capaci-

tance as well as a high surge rating (FDH400 is recommended).

In the event of an arcover, Rb limits the current flowing through

the clamp diode and Ra limits the current into the MSK 645

output. The recommended values shown in figure 1 should

not be deviated from without checking the monitor performance

since increasing these values will adversely affect transistion

times.

HEAT SINK SELECTION

To calculate what size heat sink is needed for a particular ap-

plication, the following formula must be used:

Tj = Pd (R

θjc + Rθsa) + Ta

where:

Tj=junction temperature = 150°C max.

R

θjc=30°C/W max.

Ta is the ambient temperature and Pd is the device power dis-

sipation. R

θsa is the heat sink thermal resistance.

EXAMPLE:

In an application an MSK 645 is dissipating 4 watts of power

and the ambient temperature is +25°C. Plugging in all the

known variables and rearranging the equation it can be seen

that:

R

θsa = ((150°C - 25°C)/4W) - 30°C/W

= 1.2°C/W

A heat sink with a thermal resistance of no more than 1.2°C/W

must be used to maintain a junction temperature of 150°C max.

POWER DISSIPATION

The most efficient method to reduce device power dissipa-

tion when using the MSK 645 is to fix the black level at a point

video output voltage to as small an excursion as possible. The

case of the MSK 645 is electrically isolated from internal cir-

cuitry and therefore the user should attach the heat sink di-

rectly to the case of the device.

POWER SUPPLY DECOUPLING

nally with 0.1µF capacitors to contain line noise. However it is

good practice to decouple the MSK 645 externally with at least

a 4.7µF electrolytic capacitor placed as close as possible to

the associated device pins.

Rev. D 10/05

3