TDA4850 Datasheet, PDF (5/20 Pages) NXP Semiconductors – Horizontal and vertical deflection controller for VGA/XGA and multi-frequency monitors

Philips Semiconductors
Horizontal and vertical deflection controller
for VGA/XGA and multi-frequency monitors
Product specification
TDA4850
FUNCTIONAL DESCRIPTION
Horizontal sync separator and polarity correction
An AC-coupled video signal or a DC-coupled TTL sync
signal (H only or composite sync) is input on pin 9. Video
signals are clamped with top sync on 1.28 V, and are
sliced at 1.4 V. This results in a fixed absolute slicing level
of 120 mV related to top sync.
DC-coupled TTL sync signals are also sliced at 1.4 V,
however with the clamping circuit in current limitation.
The polarity of the separated sync is detected by internal
integration of the signal, then the polarity is corrected.
The polarity information is fed to the VGA mode detector.
The corrected sync is input signal for the vertical sync
integrator and the PLL1 stage.
Vertical sync separator, polarity correction and
vertical sync integrator
DC-coupled vertical TTL sync signals may be applied to
pin 10. They are sliced at 1.4 V. The polarity of the
separated sync is detected by internal integration, then
polarity is corrected. The polarity information is fed to the
VGA mode detector. If pin 10 is not used, it must be
connected to ground.
The separated V_i(sync) signal from pin 10, or the integrated
composite sync signal from pin 9 (TTL or video) triggers
directly the vertical oscillator.
VGA mode detector and mode output
The three standard VGA modes and a 4th not fixed mode
are decoded by the polarities of the horizontal and the
vertical sync input signals. An external resistor (from V_P to
pin 7) is necessary to match this function. In all three VGA
modes the correct amplitudes are activated. The presence
of the 4th mode is indicated by HIGH on pin 7. This signal
can be used externally to switch any horizontal or vertical
parameters.
VGA mode detector input
For multi-frequency operation the voltage on pin 7 must be
externally forced to a level of <50 mV. Vertical amplitude
pre-settings for VGA are then inhibited. The delay time
between vertical trigger pulse and the start of vertical
deflection changes from 575 to 300 µs (575 µs is needed
for VGA). The vertical amplitude then remains constant in
a frequency range from 50 to 110 Hz.
Clamping and blanking generator
A combined clamping and blanking pulse is available on
pin 8 (suitable for the video preamplifier TDA4880).
The lower level of 2.1 V can be the blanking signal derived
from line flyback, or the vertical blanking pulse from the
internal vertical oscillator.
Vertical blanking equals to the delay between vertical sync
and start of vertical scan. By this, an optimum blanking is
achieved for VGA/XGA as well as for multi-frequency
operation (selectable via pin 7).
The upper level of 3.9 V is the horizontal clamping pulse
with internally fixed pulse width of 1 µs. A mono flop, which
is triggered by the trailing edge of the horizontal sync
pulse, generates this pulse.
PLL1 phase detector
The phase detector is a standard one using switched
current sources. The middle of the sync is compared with
a fixed point of the oscillator sawtooth voltage. The PLL
filter is connected to pin 17.
Horizontal oscillator
This oscillator is a relaxation type oscillator. Its frequency
is determined mainly by the capacitor on pin 19.
A frequency range of one octave is achieved by the current
on pin 18. The ϕ1 control voltage from pin 17 is fed via a
buffer amplifier and an attenuator to the current reference
pin 18 to achieve a high DC loop gain. Therefore, changes
in frequency will not affect the phase relationship between
horizontal sync pulses and line flyback pulses.
PLL2 phase detector
This phase detector is similar to the PLL1 phase detector.
Line flyback signals (pin 2) are compared with a fixed point
of the oscillator sawtooth voltage. Delays in the horizontal
deflection circuit are compensated by adjusting the phase
relationship between horizontal sync and horizontal output
pulses.
A certain amount of phase adjustments is possible by
injecting a DC current from an external source into the
PLL2 filter capacitor on pin 20.
1997 Jun 05
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