LM1291 Datasheet, PDF (7/12 Pages) National Semiconductor (TI) – Video PLL System for Continuous-Sync Monitors

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 f₁ to f₂ The pulse width is controlled by the time
constants set up through capacitors C_FVC2 and C_FVC1 at
pins 25 and 26 respectively For C_FVC2 ^t 3 c C_FVC1 the
pulse width is approximately
 J ^{l l 40 c 103 c CFVC2 c ln}
f₂ b f₁
2 13 c 10³
seconds
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 mF cap to ground is used in
conjunction with a 100 kX pull-up resistor See Figure 16
The resulting pulse has a slow rise time at the trailing edge
which extends the effective mute duration slightly
FIGURE 4
TL H 12323 – 4
FIGURE 5
TL H 12323 – 5
FIGURE 6
TL H 12323 – 6
7