LM1291N

Manufacturer Part NumberLM1291N
DescriptionVideo PLL System for Continuous Sync Monitors
ManufacturerNational Semiconductor
LM1291N datasheet
 
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Input Output Schematics

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Application Hints
(Continued)
3 Video mute
Numerous designs require video blanking during scanning
mode transitions The LM1291 provides an active-low pulse
at pin 3 when triggered by a step change of H sync frequen-
cy from f
to f
The pulse width is controlled by the time
1
2
constants set up through capacitors C
FVC2
pins 25 and 26 respectively For C
FVC2 t
pulse width is approximately
l
l
f
f
b
2
1
3 c
40
10
C
ln
c
FVC2 c
3
2 13
10
c

Input Output Schematics

Many sync sources fail to exhibit a clean step change of H
sync frequency during scanning mode transitions For this
reason in most applications a pulse smoothing circuit is
needed at pin 3 Typically a 2 2 F cap to ground is used in
and C
at
conjunction with a 100 k
FVC1
3
C
the
The resulting pulse has a slow rise time at the trailing edge
c
FVC1
which extends the effective mute duration slightly
seconds
FIGURE 4
TL H 12323– 5
FIGURE 5
TL H 12323 – 6
FIGURE 6
7
pull-up resistor See Figure 16
TL H 12323– 4