LM1238 datasheet(10/24 Pages) NSC

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Pin Descriptions and Application Information (Continued)

No.

Pin Name

Schematic

Description

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 LM1238 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 ohms 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.

LM1238 Datasheet(PDF) 10 Page - National Semiconductor (TI)

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