APPLICATION NOTES CON'T

Rev. A 5/02

4

OUTPUT CONNECTIONS

In applying the MSK 1901 in a system, two challenges

present themselves. The first challenge is to minimize

any stray capacitance from the output pin to ground.

Since the output connection is extremely susceptible to

capacitance loading, the elimination of ground planes

adjacent to the output and resistive load are important or

the rise and fall times will be limited. Keep output con-

nections as short as possible and insure that any ground

plane is at least one inch from the output signal.

The second challenge is to provide a very low

impedence connection between two sets of ground pins

(1, 2, 3, 4 and 15, 16, 17, 18). If mounting permits, the

best solution is to run a board ground track under the

MSK 1901 connecting the adjacent ground pins. How-

ever, the standard practice of heat sinking the MSK 1901

directly to the CRT chassis usually precludes this. A cut-

out is usually provided in the PC board where the MSK

1901 is surface mounted on the opposite side from the

other components. Two suggestions for this surface

mounting technique to improve performance are directed

at functionality or speed.

A functional solution is to run a ground trace on the

output pin side of the hybrid on the back side of the PC

board. The trace should be 0.1 to 0.2 inch necking down

to 0.1 inch as it perpendicularly crosses the output trace

on the other side of the board. This results in added

capacitance of only 0.1 to 0.4 pF.

A high speed solution is to have the ground cross the

input pin side of the hybrid. To counter the signal ground

disruption, the signal ground (pin 11) is internally con-

nected to the (15, 16, 17, 18) grounds. Use as broad a

ground trace as possible to improve stability.

A third suggestion is to buffer the MSK 1901 using a

differential follower stage. This configuration as shown

in Figure 1 below allows an easier layout which mini-

mizes stray capacitance. The rise time is essentially lim-

ited by the capacitance of the output transistor and that

of Q1 and Q2.

Figure 1

POWER SUPPLIES

A +10V and a -10.5V power supply are required for

proper operation. These supplies can be set at ±12V for

convenience but this will increase the internal power dis-

achieve maximum performance good high frequency

grounding practices and PC board layout are essential.

Proper power supply decoupling is also essential for

stability and good video performance. Place bypass ca-

pacitors as close to power supply pins as possible. Refer

to the typical connection circuit for recommended con-

nections.

POWER SUPPLY SEQUENCING

Power supply sequencing is necessary to avoid inter-

nal latch-up of the hybrid. External diodes should be placed

possible, all supplies should be applied to the hybrid within

5 mS of each other.

POWER DISSIPATION

The MSK 1901 power dissipation will vary depending

on load requirements and speed. The quad flat pack of

the hybrid is designed to provide a low thermal resis-

tance path from the hybrid circuit to an external heat

sink. Mounting flanges provide for excellent mechanical

and thermal attachment of the package to the heat sink.

In addition, the package is electrically isolated so that

mounting insulators are not needed and the heat sink

can be at any convenient potential. Refer to the follow-

ing table for typical power levels for selected video con-

ditions:

POWER DISSIPATION TABLE

Duty

Cycle %

Watts

Watts

TOTAL

0

100

80

80

0

35

35

50

1.6

7.8

6.5

5.6

0

6.1

4.9

10

1.6

13.9

11.4

15.6

When using multiple MSK 1901's, attach all devices

to a common heat sink (e.g. in a RGB system). This al-

lows close thermal tracking between hybrids and improves

color balance with varying input drive and ambient tem-

perature conditions. Common thermal tracking of the

devices reduces timing and other errors found in RGB

systems.