LM1291 National Semiconductor, LM1291 Datasheet - Page 7

• Manufacturer Part Number: LM1291
• Description: Video PLL System for Continuous-Sync Monitors
• Manufacturer: National Semiconductor

Application Hints
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
constants set up through capacitors C
pins 25 and 26 respectively For C
pulse width is approximately
Input Output Schematics
40
c
1
to f
10
3 c
2
The pulse width is controlled by the time
C
FVC2 c
ln
(Continued)
2 13
l
f
2
FVC2 t
b
c
FVC2
f
10
1
l
3
3
and C
seconds
c
C
FVC1
FVC1
the
at
FIGURE 4
FIGURE 5
FIGURE 6
7
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
conjunction with a 100 k
The resulting pulse has a slow rise time at the trailing edge
which extends the effective mute duration slightly
TL H 12323 – 6
TL H 12323– 5
TL H 12323– 4
pull-up resistor See Figure 16