tda4855 NXP Semiconductors, tda4855 Datasheet - Page 11

• Manufacturer Part Number: tda4855
• Description: Autosync Deflection Controller Asdc
• Manufacturer: NXP Semiconductors

Philips Semiconductors
Supply voltage stabilizer, references and protection
The ASDC provides an internal supply voltage stabilizer
for excellent stabilization of all internal references.
An internal gap reference especially designed for
low-noise is the reference for the internal horizontal and
vertical supply voltages. All internal reference currents and
drive current for the vertical output stage are derived from
this voltage via external resistors.
A special protection mode has been implemented in order
to protect the deflection stages and the picture tube during
start-up, shut-down and fault conditions. This protection
mode can be activated as shown in Table 4.
Table 4 Activation of protection mode
When protection mode is active, several pins of the ASDC
are forced into a defined state:
If the protection mode is activated via the supply voltage at
pin 9, all these actions will be performed in a well defined
sequence (see Fig.14). For activation via X-ray protection
or HPLL2 all actions will occur simultaneously.
1996 Jul 18
Low supply voltage at pin 9
X-ray protection XRAY (pin 2)
triggered
HPLL2 (pin 31) pulled to
ground
HDRV (horizontal driver output) is floating
BDRV (B+ control driver output) is floating
VOUT1 and VOUT2 (vertical outputs) are floating
CLBL provides a continuous blanking signal
The capacitor at HPLL2 is discharged.
Autosync Deflection Controller (ASDC)
ACTIVATION
increase supply voltage
remove supply voltage
release pin 31
RESET
11
The return to normal operation is performed in accordance
with the start-up sequence in Fig.14a, if the reset was
caused by the supply voltage at pin 9. The first action with
increasing supply voltage is the activation of continuous
blanking at CLBL. When the threshold for activation of
HDRV is passed, an internal current begins to charge the
external capacitor at HPLL2 and a PLL2 soft start
sequence is performed (see Fig.15). In the beginning of
this phase 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.
Then the PLL2 voltage passes the threshold for activation
of BDRV, VOUT1 and VOUT2.
For activation of these pins not only the PLL2 voltage, but
also the supply voltage must have passed the appropriate
threshold. A last pair of thresholds has to be passed by
PLL2 voltage and supply voltage before the continuous
blanking is finally removed, and the operation of PLL2 and
frequency-locked loop is enabled.
A return to the normal operation by releasing the voltage
at HPLL2 will lead to a slightly different sequence. Here the
activation of all functions is influenced only by the voltage
at HPLL2 (see Fig.15).
Application hint: Internal discharge of the capacitor at
HPLL2 will only be performed, if the protection mode was
activated via the supply voltage or X-ray protection.
Preliminary specification
TDA4855