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-

pulse width is approximately



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 22 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

InputOutput Schematics

TLH12323 – 4

FIGURE 4

TLH12323 – 5

FIGURE 5

TLH12323 – 6

FIGURE 6

7