NCS2553

http://onsemi.com

7

APPLICATIONS INFORMATION

The NCS2553 triple video driver has been optimized for

Standard Definition video applications covering the

requirements of the CVBS, S−Video, 480i/525i & 576i/625i

standards. All the 3 channels feature the same specifications

and similar behaviors guaranteed by a high channel−to−

channel crosstalk isolation (down to 60 dB at 1 MHz). Each

channel provides an internal voltage−to−voltage gain of 2

from its input to its output reducing the number of external

components usually needed in the case of some discrete

approaches (using stand−alone op amps). An internal level

shifter is employed shifting up the output voltage by adding

an offset of about 280 mV. This avoids sync pulse clipping

and allows DC−coupled output to the 150

W video load. In

filter per channel with a 3 dB frequency bandwidth of 8

MHz. This allows rejecting out the aliases or unwanted

over−sampling effects produced by the video DAC.

Similarly, in the case of DVD recorders using ADC, this

anti−aliasing filter (reconstruction filter) will avoid picture

quality issues and will help to filter out parasitic signals

caused by EMI interference.

A built−in diode−like clamp is used in the chip for each

channel to support AC−coupled mode of operation. The

clamp is active when the input signal goes below 0 V.

Figure 13. AC−Coupled Inputs and Outputs

DAC

0V

1V

2.28V

0.28V

75W

75W

75W

75W

75W

75W

220mF

220mF

220mF

OUT1

OUT2

OUT3

1.68V

0.28V

0.1mF

0.1mF

0.1mF

IN1

IN2

IN3

Y, R’, G’, B’

Pb, Pr

6db

6db

6db

0.7V

0V

8MHz

LS

280mV

800k

8MHz

LS

280mV

800k

8MHz

LS

280mV

800k

Figure 13 shows an example for which the external video

source coming from the DAC is AC−coupled at the input and

output. But thanks to the built−in transparent clamp and

level shifter the device can operate in different configuration

modes depending essentially on the DAC output signal level

High and Low and how it fits the input common mode

voltage of the video driver. When the configuration is

DC−Coupled at the Inputs and Outputs the 0.1

mF and

220

mF coupling capacitors are no longer used, the clamps

are in that case inactive; this configuration has the big

advantage of being relatively low cost with the use of less

external components.

The input is AC−coupled if for example the input−signal

amplitude goes over the range 0 to 1.4 V or if the video

source requires such a coupling. In some circumstances it

may be necessary to auto−bias signals by the addition of a

pull−up and pull−down resistor or only pull−up resistor

(Typical 7.5 M

W combined with the internal 800 kW

pull−down) making the clamp inactive.

The output AC−coupling configuration has the advantage

of eliminating DC ground loop with the drawback of making

the device more sensitive to video line or field tilt issues in

the case of a too low output coupling capacitor. In some

cases it may be necessary to increase the nominal 220

mF

capacitor value.