LM1247 datasheet(11/48 Pages) NSC

11 / 48 page

Pin Descriptions and Application Information (Continued)

No.

Pin Name

Schematic

Description

Green Output

Red Output

Blue Output

These are the three video output pins. They are

intended to drive the LM246x family of cathode

drivers. Nominally, about 2V peak to peak will

produce 40V peak to peak of cathode drive.

ABL

The Automatic Beam Limiter input is biased to

the desired beam current limit by R

ABL and VBB

and normally keeps D

INT forward biased. When

the current resupplying the CRT capacitance

(averaged by C

ABL) exceeds this limit, then DINT

begins to turn off and the voltage at pin 22

begins to drop. The LM1247 then lowers the

gain of the three video channels until the beam

current reaches an equilibrium value.

CLAMP

This pin accepts either TTL or CMOS logic

levels. The internal switching threshold is

approximately one-half of V

CC. An external

series resistor, R

31, of about 1K is

recommended to avoid overdriving the input

devices. In any event, R

EXT must be large

enough to prevent the voltage at pin 23 from

going higher than V

CC or below GND.

H Flyback

Proper operation requires current reversal. R

should be large enough to limit the peak current

at pin 24 to about +4 ma during blanking, and

−500 µA during scan. C

17 is usually needed for

logic level inputs and should be large enough to

make the time constant, R

HC17 significantly

larger than the horizontal period. R

34 and C8 are

typically 300

Ω and 330 pF when the flyback

waveform has ringing and needs filtering. C

may be needed to filter extraneous noise and

can be up to 100 pF.

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