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AN1290
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STMicroelectronics Confidential
4.2.4
X-Ray Protection
The X-ray protection is latched; once triggered by a voltage higher than 8V, it stops the IC operation
until it is reset by switching the monitor OFF and ON again, or through I²C programming
(Sad16h/d7).
The voltage on pin 25 is always filtered, which makes it rather insensitive to parasitic voltage.
Nevertheless, an arcing inside the display tube neck may happen to trigger the protection and stop
monitor operation, which is most unpleasant to the user.
In the TDA9112, the risk has been minimized, since the latch will be triggered not earlier than two
lines after an overvoltage has been detected. Nevertheless, it is preferable to maintain the layout
rules:
●
connect the filter capacitor very close to the IC; do not create a loop which may be influenced
by a huge dI/dt,
●
establish a path for the arcing current that does not cross the main chassis. A well-accepted
disposition is as follows:
— The ground connection from the outer conductive paint of the display tube should only go to
the ground near the tube socket: this way, the charges stored inside the tube will not flow
through the main chassis.
— From there, a ground connection should reach the chassis ground, preferably near the EHV
transformer, because this transformer usually includes an EHV capacitor, which will also
discharge during arcing.
Warning! If the X-ray filtering capacitor has a high value, when switching OFF the monitor it can
keep a high voltage while Vref decreases. This will unduly trigger and latch the X-ray protection if
not reset by Vsupply (in case of fast ON-OFF-ON).
This problem is not present in the TDA9112A, where the X-ray protection is disabled as soon as
Vcc drops below 10.4 V (typical).
