EL1881CS Intersil, EL1881CS Datasheet - Page 9

• Manufacturer Part Number: EL1881CS
• Description: IC VIDEO SYNC SEPARATOR 8-SOIC
• Manufacturer: Intersil
• Type: Synchronous Separatorr

Specifications of EL1881CS

• Applications: Drivers, Communications, PCMCIA
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Applications Information
Video In
A simplified block diagram is shown following page.
An AC coupled video signal is input to Video In pin 2 via
nominally 0.1µF. Clamp charge current will prevent the
signal on pin 2 from going any more negative than Sync Tip
Ref, about 1.5V. This charge current is nominally about
1mA. A clamp discharge current of about 10µA is always
attempting to discharge C
lost between sync pulses that must be replaced during sync
pulses. The droop voltage that will occur can be calculated
from IT = CV, where V is the droop voltage, I is the discharge
current, T is the time between sync pulses (sync period -
sync tip width), and C is C
An NTSC video signal has a horizontal frequency of
15.73kHz, and a sync tip width of 4.7µs. This gives a period
of 63.6µs and a time T = 58.9µs. The droop voltage will then
1
1
.
to Sync Tip Ref, thus charge is
9
FIGURE 4. STANDARD (NTSC INPUT) H. SYNC DETAIL
,
C
EL1881C
1
be V = 5.9mV. This is less than 2% of a nominal sync tip
amplitude of 286mV. The charge represented by this droop
is replaced in a time given by T = CV/I, where I = clamp
charge current = 1mA. Here T = 590ns, about 12% of the
sync pulse width of 4.7µs. It is important to choose C
enough so that the droop voltage does not approach the
switching threshold of the internal comparator.
Fixed Gain Buffer
The clamped video signal then passes to the fixed gain
buffer which places the sync slice level at the equivalent
level of 70mV above sync tip. The output of this buffer is
presented to the comparator, along with the slice reference.
The comparator output is level shifted and buffered to TTL
levels, and sent out as Composite Sync to pin 1.
Burst
A low-going Burst pulse follows each rising edge of sync,
and lasts approximately 3.5µs for an R
SET
of 681kΩ.
1
FN7018.1
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