EL4581CS datasheet(8/12 Pages) ELANTEC

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EL4581C

Sync Separator 50% Slice S-H Filter

Description of Operation

A simplified block schematic is shown in Figure

2 The following description is intended to pro-

vide the user with sufficient information to be

able to understand the effects that the external

components and signal conditions have on the

outputs of the integrated circuit

The video signal is AC coupled to pin 2 via the

capacitor C1 nominally 01 mF The clamp circuit

A1 will prevent the input signal on pin 2 going

any more negative than 15V the value of refer-

ence voltage VR1 Thus the sync tip the most

negative part of the video waveform

will be

clamped at 15V The current source I1 nominal-

ly 10

mA charges the coupling capacitor during

the remaining portion of the H line approxi-

mately 58

ms for a 1575 kHz timebase From

t e C V the video time-constant can be

calculated It is important to note that the charge

taken from the capacitor during video must be

replaced during the sync tip time which is much

shorter (ratio of x 125) The corresponding cur-

rent to restore the charge during sync will there-

fore be an order of magnitude higher and any

resistance in series with CI will cause sync tip

crushing For this reason the internal series re-

sistance has been minimized and external high

resistance values in series with the input cou-

pling capacitor should be avoided The user can

exercise some control over the value of the input

time constant by introducing an external pull-up

resistance from pin 2 to the 5V supply The maxi-

mum voltage across the resistance will be VDD

less 15V for black level For a net discharge cur-

rent greater than zero the resistance should be

greater than 450k This will have the effect of

increasing the time constant and reducing the de-

gree of picture tilt The current source I1 directly

tracks reference current ITR and thus increases

with scan rate adjustment as explained later

The signal is processed through an active 3 pole

filter (F1) designed for minimum ripple with con-

stant phase delay The filter attenuates the color

burst by 24 dB and eliminates fast transient

spikes without sync crushing An external filter

is not necessary The filter also amplifies the

video signal by 6 dB to improve the detection

accuracy Note that the filter cut-off frequency is

a function of RSET through IOT and is propor-

tional to IOT

Internal reference voltages (block VREF) with

high immunity to supply voltage variation are

derived on the chip Reference VR4 with op-amp

A2 forces pin 6 to a reference voltage of 17V

nominal Consequently it can be seen that the

external resistance RSET will determine the val-

ue of the reference current ITR The internal re-

sistance R3 is only about 6 k

X much less than

RSET All the internal timing functions on the

chip are referenced to ITR and have excellent

supply voltage rejection

Comparator C2 on the input to the sample and

hold block (SH) compares the leading and trail-

ing edges of the sync pulse with a threshold volt-

age VR2 which is referenced at a fixed level above

the clamp voltage VR1 The output of C2 initiates

the timing one-shots for gating the sample and

hold circuits The sample of the sync tip is de-

layed by 08

ms to enable the actual sample of

ms to be taken on the optimum section of the

sync pulse tip The acquisition time of the circuit

is about three horizontal lines The double poly

CMOS technology enables long time constants to

be achieved with small high quality on-chip ca-

pacitors The back porch voltage is similarly de-

rived from the trailing edge of sync which also

serves to cut off the tip sample if the gate time

exceeds the tip period Note that the sample and

hold gating times will track RSET through IOT

The 50% level of the sync tip is derived through

the resistor divider R1 and R2 from the sample

and held voltages VTIP and VBP and applied to

the plus input of comparator C1 This compara-

tor has built in hysteresis to avoid false trigger-

ing The output of C2 is a digital 5V signal which

feeds the CS ouput buffer B1 and the other in-

ternal circuit blocks the vertical back porch and

oddeven functions

The vertical circuit senses the CS edges and ini-

tiates an integrator which is reset by the shorter

horizontal sync pulses but times out the longer

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