LM1881, LM1881-X
Application Notes
(Continued)
flip-flop is toggled by the default comparator, starting the
vertical sync pulse at pin 3 of the LM1881. If the default
vertical sync period ends before the end of the input vertical
sync period, then the falling edge of the vertical sync (posi-
tive pulse at the “D” flip-flop) will clock the high output from
the comparator with V
1
as a reference input. This will retrig-
ger the oscillator, generating a second vertical sync output
pulse. The “Vertical Default Sync Delay Time vs R
SET
” graph
shows the relationship between the R
SET
value and the
delay time from the start of the vertical sync period before
the default vertical sync pulse is generated. Using the NTSC
example again the smallest resistor for R
SET
is 500 kΩ. The
vertical default time delay is about 50 µs, much longer than
the 30 µs serration pulse spacing.
A common question is how can one calculate the required
R
SET
with a video timing standard that has no serration
pulses during the vertical blanking. If the default vertical sync
is to be used this is a very easy task. Use the “Vertical
Default Sync Delay Time vs R
SET
” graph to select the nec-
essary R
SET
to give the desired delay time for the vertical
sync output signal. If a second pulse is undesirable, then
check the “Vertical Pulse Width vs R
SET
” graph to make sure
the vertical output pulse will extend beyond the end of the
input vertical sync period. In most systems the end of the
vertical sync period may be very accurate. In this case the
preferred design may be to start the vertical sync pulse at the
end of the vertical sync period, similar to starting the vertical
sync pulse after the first serration pulse. A VGA standard is
to be used as an example to show how this is done. In this
standard a horizontal line is 32 µs long. The vertical sync
period is two horizontal lines long, or 64 µs. The vertical
default sync delay time
must be longer
than the vertical
sync period of 64 µs. In this case R
SET
must be larger than
680 kΩ. R
SET
must still be small enough for the output of the
integrator to reach V
1
before the end of the vertical period of
the input pulse. The first graph can be used to confirm that
R
SET
is small enough for the integrator. Instead of using the
vertical serration pulse separation, use the actual pulse
width of the vertical sync period, or 64 µs in this example.
This graph is linear, meaning that a value as large as 2.7 MΩ
can be used for R
SET
(twice the value as the maximum at
30 µs). Due to leakage currents it is advisable to keep the
value of R
SET
under 2.0 MΩ. In this example a value of 1.0
MΩ is selected, well above the minimum of 680 kΩ. With this
value for R
SET
the pulse width of the vertical sync output
pulse of the LM1881 is about 340 µs.
00915003
FIGURE 1. (a) Composite Video; (b) Composite Sync; (c) Vertical Output Pulse;
(d) Odd/Even Field Index; (e) Burst Gate/Back Porch Clamp
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