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STMicroelectronics Confidential
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AN1290 Horizontal
Section
Hex code 00 corresponds to a 65% duty factor. This is the default value at start-up, in order to
minimize the stress on the scanning transistor.
4.1.11 Soft-start
The TDA9112 is equipped with a self-contained soft-start. When switching ON, HOut will remain
inhibited until the supply voltage ramps up to more than 8.5V. Afterwards, the initial duty factor will
progressively decrease from 85% to 65% (the scanning transistor will be controlled OFF for 65% of
the period), then to the value that has been set through I²C programming. The same events will take
place in the reverse order when the supply ramps down through a 6.5V threshold (thresholds for
TDA9112A: 8.0V and 6.8V).
The time constant that controls the soft-start is the charge rate of H-position capacitor on pin 10.
The soft-start is effective when switching ON and when resetting the HOut inhibition. It also controls
the duty factor of the DC/DC converter (except in the so-called “internal sawtooth, voltage mode
configuration”). For more information, refer to
Section 7: DC/DC Converter Section
4.1.12 Output Stage
The drive signal for the H scanning transistor is available on pin 26, which is connected to the
collector of the output NPN transistor. The transistor is conductive (pin 26 LOW) when the H
scanning transistor is to be ON and vice-versa. Pin 26 should be connected to the supply through a
pull-up resistor. Of course, some kind of driver stage is mandatory to control the scanning transistor
base.
The saturation voltage on pin 26 (less than 0.4V for 30mA) is low enough to allow direct control of a
bipolar driver. Nevertheless and for other reasons we recommend keeping an AC coupling between
pin 26 and the driver. For more information, refer to
.
The output transistor is forced ON during the negative slope part of the sawtooth. It is forced OFF
during flyback (as seen on pin 12), and this safety function has the priority over any other control.
The goal is to prevent the scanning transistor from turning ON again, while it still sustains the high
flyback voltage.
4.1.13 X-ray Protection
Pin 25 is a comparator with 8V (typical) threshold for X-ray detection. A voltage higher than 8V on
pin 25 will stop H scanning (and the DC/DC converter as well). This situation will last until some
Reset takes place, for instance by switching the supply voltage on pin 29, OFF and ON again
(resetting threshold: 6.5V typ at supply fall-down).
In fact, once the voltage on pin 25 exceeds 8V, it must remain at this value for 2 lines before the X-
ray safety is triggered. This digital filtering provides a protection against very short parasitic
voltage, affecting pin 25, due for instance, to an arcing of the tube.
It is possible to read the current state of X-ray protection through the I²C bus. It is also possible to
reset it in the same way (Sad16h/d7).
4.1.14 Lock-Unlock Detection
Remembering how the PLL1 works with two current generators (one sink and one source), it
appears that when PLL1 is locked both generators are OFF almost all the time; when out of phase,
one generator will be ON for part of the period. When frequencies differ, one generator or the other
will be ON for approximately half the time (mean value).
This is used to build a lock-unlock detector (
) where a gate output is LOW whenever one of
the current sources is ON. It controls very low charge and discharge currents to an internal
capacitor. The capacitor voltage will take a value of approximately 8V when PLL1 is locked, and
