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LM1881N 参数 Datasheet PDF下载

LM1881N图片预览
型号: LM1881N
PDF下载: 下载PDF文件 查看货源
内容描述: LM1881视频同步分离 [LM1881 Video Sync Separator]
分类和应用:
文件页数/大小: 12 页 / 302 K
品牌: NSC [ National Semiconductor ]
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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 RSET must be larger than  
680 k. RSET must still be small enough for the output of the  
integrator to reach V1 before the end of the vertical period of  
the input pulse. The first graph can be used to confirm that  
RSET 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 RSET (twice the value as the maximum at  
30 µs). Due to leakage currents it is advisable to keep the  
value of RSET under 2.0 M. In this example a value of 1.0  
Mis selected, well above the minimum of 680 k. With this  
value for RSET the pulse width of the vertical sync output  
pulse of the LM1881 is about 340 µs.  
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 V1 as a reference input. This will retrig-  
ger the oscillator, generating a second vertical sync output  
pulse. The “Vertical Default Sync Delay Time vs RSET” graph  
shows the relationship between the RSET 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 RSET is 500 k. 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  
RSET 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 RSET” graph to select the nec-  
essary RSET 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 RSET” graph to make sure  
00915003  
FIGURE 1. (a) Composite Video; (b) Composite Sync; (c) Vertical Output Pulse;  
(d) Odd/Even Field Index; (e) Burst Gate/Back Porch Clamp  
www.national.com  
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