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, SVideo, 480i/525i & 576i/625i
standards. All the 3 channels feature the same specifications
and similar behaviors guaranteed by a high channelto
channel crosstalk isolation (down to 60 dB at 1 MHz). Each
channel provides an internal voltagetovoltage 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 standalone 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 DCcoupled output to the 150
W video load. In
addition, the NCS2553 integrates a 6th order Butterworth
filter per channel with a 3 dB frequency bandwidth of 8
MHz. This allows rejecting out the aliases or unwanted
oversampling effects produced by the video DAC.
Similarly, in the case of DVD recorders using ADC, this
antialiasing filter (reconstruction filter) will avoid picture
quality issues and will help to filter out parasitic signals
caused by EMI interference.
A builtin diodelike clamp is used in the chip for each
channel to support ACcoupled mode of operation. The
clamp is active when the input signal goes below 0 V.
Figure 13. ACCoupled Inputs and Outputs
DAC
0V
1V
2.28V
0.28V
75W
1VPP
0.7VPP
ZO = 75W
75W
220mF
OUT1
OUT2
OUT3
1.68V
0.28V
0.1mF
RS
IN1
IN2
IN3
Y, R’, G’, B’
Pb, Pr
Clamp
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 ACcoupled at the input and
output. But thanks to the builtin 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
DCCoupled 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 ACcoupled if for example the inputsignal
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 autobias signals by the addition of a
pullup and pulldown resistor or only pullup resistor
(Typical 7.5 M
W combined with the internal 800 kW
pulldown) making the clamp inactive.
The output ACcoupling 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.