TDA4885 Datasheet, PDF (7/56 Pages) NXP Semiconductors – 150 MHz video controller with I2C-bus

Philips Semiconductors
150 MHz video controller with I²C-bus
Product specification
TDA4885
7 FUNCTIONAL DESCRIPTION
See block diagram (Fig.1) and definition of levels and
output signals (Chapter “Characteristics” notes 1 to 3;
Figs 3 to 6).
7.1 Signal input stage
(input clamping, blanking and clipping)
The RGB input signals with nominal signal amplitude of
0.7V_b-w are capacitively coupled into the TDA4885 from a
low-ohmic source (75 Ω recommended) and actively
clamped to an internal DC voltage during signal black
level. Because of the high-ohmic input impedance of the
TDA4885 the coupling capacitor (which also functions as
storage capacitor during clamping pulses) can be
relatively small (10 nF recommended). The internal
leakage currents will discharge the coupling capacitor
resulting in black output signals for missing input
clamping pulses.
Composite signals will not disturb normal operations
because a clipping circuit cuts all signal parts below
black level.
A fast signal blanking stage belongs to the input stage
which is driven by several blanking pulses (see Section
“Clamping and blanking pulses”) and control bit DISV = 1.
During the off condition the internal reference black level
will be inserted instead of the input signals.
7.2 Electronic potentiometer stages
7.2.1
CONTRAST CONTROL (DRIVEN BY I²C-BUS, 6-BIT
DAC)
The input signals related to the internal reference black
level can be simultaneously adjusted by contrast control
with a control range of typically 32 dB. The nominal
contrast setting is defined for 26H (4 dB below maximum).
7.2.2
BRIGHTNESS CONTROL (DRIVEN BY I²C-BUS, 6-BIT
DAC)
With brightness control the video black level will be shifted
in relation to the reference black level simultaneously for
all three channels. With a negative setting (maximum 10%
of nominal signal amplitude) dark signal parts will be lost in
ultra black while for positive settings (maximum 30% of
nominal signal amplitude) the background will alter from
black to grey. The nominal brightness setting (10H) is no
shift. The brightness setting is also valid for OSD signals.
During blanking and output clamping the video black level
will be blanked to reference black level (brightness
blanking).
7.2.3
GAIN CONTROL (DRIVEN BY I²C-BUS, 6-BIT DAC)
AND GREY SCALE TRACKING
Gain control is used for white point adjustment (correction
for different voltage to light amplification of the three colour
channels) and therefore individual for the three channels.
The video signals related to the reference black level can
be gain controlled within a range of typical 7 dB.
The nominal setting is maximum gain. The video signal is
the addition of the contrast controlled input signal and the
brightness shift. The gain setting is also valid for OSD
signals, thus the complete ‘grey scale’ is effected by gain
control.
7.3 Output stage
In the output stage the nominal input signal will be
amplified to 2.8V_b-w output colour signal at nominal
contrast and maximum gain. The maximum input-output
amplification at maximum contrast and gain settings is
16 dB. By output clamping the reference black level can
be adjusted. In order to achieve very fast rise and fall times
of the output signals with minimum crosstalk between the
channels, each output stage has its own supply voltage
and ground pin.
7.4 Pedestal blanking
For the video portion the reference black level should
correspond to the ‘extended cut-off voltage’ at the
cathode. During vertical flyback nevertheless retrace lines
may be visible, though blanking to spot cut-off is useful.
With control bit PEDST = 1 the pedestal black level will be
adjusted by output clamping instead of the reference black
level (see Fig.5). The pedestal black level is more negative
than the video black level at minimum brightness setting
and the voltage difference to reference black level is
independent of any user control.
7.5 Output clamping, feedback references and
DAC outputs
Aim of the output clamping (pins FB₁, FB₂ and FB₃) is to
set the reference black level of the signal outputs to a
value which corresponds to the ‘extended cut-off voltage’
of the CRT cathodes. At lack of output clamping pulses the
integrated storage capacitors will discharge resulting in
output signals going to switch-off voltage. Feedback
references are driven by the I²C-bus.
1997 Nov 25
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