TDA4780 Datasheet, PDF (8/44 Pages) NXP Semiconductors – RGB video processor with automatic cut-off control and gamma adjust

Philips Semiconductors
RGB video processor with automatic
cut-off control and gamma adjust
Preliminary specification
TDA4780
Automatic cut-off control
During leakage measurement time the leakage current is
compensated in order to get a reference voltage at the
cut-off measurement info pin. This compensation value is
stored in an external capacitor. During cut-off current
measurement times for the R, G and B channels, the
voltage at this pin is compared with the reference voltage,
which is individually adjustable via I²C-bus for each colour
channel. The control voltages that are derived in this way
are stored in the external feedback capacitors. Shift stages
add these voltages to the corresponding output signals.
The automatic cut-off control may be disabled via the
I²C-bus. In this mode the output voltage is clamped to
2.5 V. Clamping periods are the same as the cut-off
measurement periods.
Signal limiting
The TDA4780 provides two kinds of signal limiting.
First, an average beam limiting, that reduces signal level if
a certain average is exceeded. Second, a peak drive
limiting, that is activated if one of the RGB signals even
shortly exceeds a via I²C-bus adjusted threshold.
The latter can be either referred to the cut-off
measurement level of the outputs or to ground.
When signal limiting occurs, contrast is reduced, and at
minimum contrast brightness is reduced additionally.
Sandcastle decoder and timer
A 3-level detector separates the sandcastle pulse into
combined line and field pulses, line pulses, and clamping
pulses. The timer contains a line counter and controls the
cut-off control measurement.
Application with a 2-level 5 V sandcastle pulse is possible.
Switch on delay circuit
After switch on all signals are blanked and a warm up test
pulse is fed to the outputs during the cut-off measurement
lines. If the voltage at the cut-off measurement input
exceeds an internal level the cut-off control is enabled but
the signal remains still blanked. In the event of output
clamping, the cut-off control is disabled and the switch on
procedure will be skipped.
Y output and hue adjust
The TDA4780 contains a D/A converter for hue adjust.
The analog information can be fed, e.g. to the
multistandard decoder TDA4650 or TDA4655. This output
pin may be switched to a Y output signal, which can be
used for scan velocity modulation (SCAVEM). The Y
output is the Y input signal or the matrixed (RGB) input
signal according to the switch position of the fast switch.
I²C-bus
The TDA4780 contains an I²C-bus receiver for control
function.
ESD protection
The Pins are provided with protection diodes against
ground and supply voltage (see Chapter “Internal pin
configurations”). I²C-bus input pins do not shunt the
I²C-bus signals in the event of missing supply voltage.
EMC
The pins are protected against electromagnetic radiation.
1997 Feb 06
8