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參數(shù)資料
| 型號： | LM1881N-X/NOPB |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 信號分離 |
| 英文描述： | SYNC SEPARATOR IC, PDIP8 |
| 封裝： | PLASTIC, DIP-8 |
| 文件頁數(shù)： | 8/12頁 |
| 文件大?。?/td> | 688K |
| 代理商： | LM1881N-X/NOPB |
Application Notes (Continued)
ence called V
2 going to one of its inputs. Both comparators
have a common input at their noninverting input coming from
the internal integrator. The internal integrator is used for
integrating the composite sync signal. This signal comes
from the input side of the composite sync buffer and are
positive going sync pulses. The capacitor to the integrator is
internal to the LM1881. The capacitor charge current is set
by the value of the external resistor R
SET. The output of the
integrator is going to be at a low voltage during the normal
horizontal lines because the integrator has a very short time
to charge the capacitor, which is during the horizontal sync
period. The equalization pulses will keep the output voltage
of the integrator at about the same level, below the V
1.
During the vertical sync period the narrow going positive
pulses shown in Figure 1 is called the serration pulse. The
wide negative portion of the vertical sync period is called the
vertical sync pulse. At the start of the vertical sync period,
before the first Serration pulse occurs, the integrator now
charges the capacitor to a much higher voltage. At the first
serration pulse the integrator output should be between V
1
and V
2. This would give a high level at the output of the
comparator with V
1 as one of its inputs. This high is clocked
into the “D” flip-flop by the falling edge of the serration pulse
(remember the sync signal is inverted in this section of the
LM1881). The “Q” output of the “D” flip-flop goes through the
OR gate, and sets the R/S flip-flop. The output of the R/S
flip-flop enables the internal oscillator and also clocks the
ODD/EVEN “D” flip-flop. The ODD/EVEN field pulse opera-
tion is covered in the next section. The output of the oscilla-
tor goes to a divide by 8 circuit, thus resetting the R/S
flip-flop after 8 cycles of the oscillator. The frequency of the
oscillator is established by the internal capacitor going to the
oscillator and the external R
SET. The “Q” output of the R/S
flip-flop goes to pin 3 and is the actual vertical sync output of
the LM1881. By clocking the “D” flip-flop at the start of the
first serration pulse means that the vertical sync output pulse
starts at this point in time and lasts for eight cycles of the
internal oscillator as shown in Figure 1.
How R
SET affects the integrator and the internal oscillator is
shown under the Typical Performance Characteristics. The
first graph is “R
SET Value Selection vs Vertical Serration
Pulse Separation”. For this graph to be valid, the vertical
sync pulse should last for at least 85% of the horizontal half
line (47% of a full horizontal line). A vertical sync pulse from
any standard should meet this requirement; both NTSC and
PAL do meet this requirement (the serration pulse is the
remainder of the period, 10% to 15% of the horizontal half
line). Remember this pulse is a positive pulse at the integra-
tor but negative in Figure 1. This graph shows how long it
takes the integrator to charge its internal capacitor above V
1.
With R
SET too large the charging current of the integrator will
be too small to charge the capacitor above V
1, thus there will
be no vertical synch output pulse. As mentioned above, R
SET
also sets the frequency of the internal oscillator. If the oscil-
lator runs too fast its eight cycles will be shorter than the
vertical sync portion of the composite sync. Under this con-
dition another vertical sync pulse can be generated on one of
the later serration pulse after the divide by 8 circuit resets the
R/S flip-flop. The first graph also shows the minimum R
SET
necessary to prevent a double vertical pulse, assuming that
the serration pulses last for only three full horizontal line
periods (six serration pulses for NTSC). The actual pulse
width of the vertical sync pulse is shown in the “Vertical
Pulse Width vs R
SET” graph. Using NTSC as an example,
lets see how these two graphs relate to each other. The
Horizontal line is 64 s long, or 32 s for a horizontal half
line. Now round this off to 30 s. In the “R
SET Value Selection
vs Vertical Serration Pulse Separation” graph the minimum
resistor value for 30 s serration pulse separation is about
550 k
. Going to the “Vertical Pulse Width vs R
SET” graph
one can see that 550 k
gives a vertical pulse width of about
180 s, the total time for the vertical sync period of NTSC (3
horizontal lines). A 550 k
will set the internal oscillator to a
frequency such that eight cycles gives a time of 180 s, just
long enough to prevent a double vertical sync pulse at the
vertical sync output of the LM1881.
The LM1881 also generates a default vertical sync pulse
when the vertical sync period is unusually long and has no
serration pulses. With a very long vertical sync time the
integrator has time to charge its internal capacitor above the
voltage level V
2. Since there is no falling edge at the end of
a serration pulse to clock the “D” flip-flop, the only high signal
going to the OR gate is from the default comparator when
output of the integrator reaches V
2. At this time the R/S
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 k
. The
vertical default time delay is about 50 s, much longer than
the 30 s 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 s long. The vertical sync
period is two horizontal lines long, or 64 s. The vertical
default sync delay time must be longer than the vertical
sync period of 64 s. In this case R
SET must be larger than
680 k
.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 s in this example.
This graph is linear, meaning that a value as large as 2.7 M
can be used for R
SET (twice the value as the maximum at
30 s). Due to leakage currents it is advisable to keep the
value of R
SET under 2.0 M
. In this example a value of 1.0
M
is selected, well above the minimum of 680 k. With this
value for R
SET the pulse width of the vertical sync output
pulse of the LM1881 is about 340 s.
LM1881
www.national.com
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM1881-X | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:LM1881 Video Sync Separator |
| LM1882 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Programmable Video Sync Generator |
| LM1882CM | 制造商:Rochester Electronics LLC 功能描述:- Bulk |
| LM1882CN | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Programmable Video Sync Generator |
| LM1882-R | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Programmable Video Sync Generator |
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