Philips Semiconductors
Product specification
I2C-bus autosync deflection controller for
PC monitors
TDA4856
The formulae for RHBUF also takes into account the voltage
swing across this resistor:
An external modulation of the PLL2 phase is not allowed,
because this would disturb the pre-correction of the
horizontal focus parabola.
R
HREF × R
RHBUF
=
HBUFpar × 0.8 = 805 Ω
---------------------------------------------
R
HREF – RHBUFpar
Soft start and standby
If HPLL2 is pulled to ground, either by an external DC
current or by resetting register SOFTST, the horizontal
output pulses and B+ control driver pulses will be inhibited.
This means that HDRV (pin 8) and BDRV (pin 6) are
floating in this state. In both cases PLL2 and the
frequency-locked loop are disabled, and CLBL (pin 16)
provides a continuous blanking signal and HUNLOCK
(pin 17) is floating.
PLL1 phase detector
The phase detector is a standard type using switched
current sources, which are independent of horizontal
frequency. It compares the middle of horizontal sync with
a fixed point on the oscillator sawtooth voltage. The PLL1
loop filter is connected to HPLL1 (pin 26).
See also Section “Horizontal position adjustment and
corrections”.
This option can be used for soft start, protection and
power-down modes. When pin HPLL2 is released again,
an automatic soft start sequence on the horizontal drive as
well as on the B-drive output will be performed
(see Fig.24).
Horizontal position adjustment and corrections
A linear adjustment of the relative phase between the
horizontal sync and the oscillator sawtooth (in PLL1 loop)
is achieved via register HPOS. Once adjusted, the relative
phase remains constant over the whole frequency range.
A soft start can only be performed if the supply voltage for
the IC is a minimum of 8.6 V.
Correction of pin unbalance and parallelogram is achieved
by modulating the phase between oscillator sawtooth and
horizontal flyback (in loop PLL2) via registers HPARAL
and HPINBAL. If those asymmetric EW corrections are
performed in the deflection stage, both registers can be
disconnected from the horizontal phase via control
bit ACD. This does not change the output at pin ASCOR.
The soft start timing is determined by the filter capacitor at
HPLL2 (pin 30), which is charged with a constant current
during soft start. In the beginning the horizontal driver
stage generates very small output pulses. The width of
these pulses increases with the voltage at HPLL2 until the
final duty cycle is reached. The voltage at HPLL2
increases further and performs a soft start at BDRV (pin 6)
as well. After BDRV has reached full duty cycle, the
voltage at HPLL2 continues to rise until HPLL2 enters its
normal operating range. The internal charge current is now
disabled. Finally PLL2 and the frequency-locked loop are
activated. If both functions reach normal operation,
HUNLOCK (pin 17) switches from the floating status to
normal vertical blanking, and continuous blanking at CLBL
(pin 16) is removed.
Horizontal moire cancellation
To achieve a cancellation of horizontal moire (also known
as ‘video moire’), the horizontal frequency is
divided-by-two to achieve a modulation of the horizontal
phase via PLL2. The amplitude is controlled by
register HMOIRE. To avoid a visible structure on screen
the polarity changes with half of the vertical frequency.
Control bit MOD disables the moire cancellation function.
Output stage for line drive pulses [HDRV (pin 8)]
PLL2 phase detector
An open-collector output stage allows direct drive of an
inverting driver transistor because of a low saturation
voltage of 0.3 V at 20 mA. To protect the line deflection
transistor, the output stage is disabled (floating) for a low
supply voltage at VCC (see Fig.23).
The PLL2 phase detector is similar to the PLL1 detector
and compares the line flyback pulse at HFLB (pin 1) with
the oscillator sawtooth voltage. The control currents are
independent of the horizontal frequency. The PLL2
detector thus compensates for the delay in the external
horizontal deflection circuit by adjusting the phase of the
HDRV (pin 8) output pulse.
The duty cycle of line drive pulses is slightly dependent on
the actual horizontal frequency. This ensures optimum
drive conditions over the whole frequency range.
1999 Jul 13
8
NXP [ NXP ]
