STV6889 datasheet(45/56 Pages) STMICROELECTRONICS

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9.8 Miscellaneous

9.8.1

Safety functions

The safety functions comprise supply voltage

monitoring with appropriate actions, soft start and

soft stop features on H-drive and B-drive signals

on HOut and BOut outputs, B-drive cut-off at unlock

condition and X-ray protection.

For supply voltage supervision, refer to subsection

9.1.1

and Figure 1. A schematic diagram putting to-

gether all safety functions and composite PLL1

lock and V-blanking indication is in Figure 17.

9.8.1.1 Soft start and soft stop function

For soft start and soft stop features for H-drive and

B-drive signal, refer to subsection 9.3.7 and subsec-

tion 9.7

, respectively. See also the Figure 10 and

Figure 11

. Regardless why the H-drive or B-drive

signal are switched on or off (I²C-bus command,

power up or down, X-ray protection), the signals

always phase-in and phase-out in the way drawn

in the figures, the first to phase-in and last to

phase-out being the H-drive signal, which is to bet-

ter protect the power stages at abrupt changes like

switch-on and off. The timing of phase-in and

phase-out depends on the capacitance connected

to HPosF pin which is virtually unlimited for this

function. However, as it has a dual function (see

subsection 9.3.2

), a compromise thereof is to be

found.

The soft stop at power down condition can be con-

sidered as a special case. As at this condition the

thresholds VHOn, VBOn and VHBNorm depend on the

momentary level of supply voltage (marked VHOn’,

VBOn’, VHBNorm’ in Figure 11), the timing of soft stop

mechanism depends, apart from the capacitance

on HPosF, also on the falling speed of supply volt-

age. The device is capable of performing a correct

soft stop sequence providing that, at the moment

the supply voltage reaches VCCStop, the voltage on

HPosF

has already fallen below VHOn (Section 9.8).

9.8.1.2 B-drive cut-off at unlock condition

This function is described in subsection 9.7.2 .

9.8.1.3 X-ray protection

The X-ray protection is activated if the voltage lev-

el on XRay input exceeds VThrXRay threshold and if

the VCC is higher than the voltage level VCCXRayEn.

As a consequence, the H-drive and B-drive signals

on HOut and BOut outputs are inhibited (switched

off) after a 2-horizontal deflection line delay provid-

ed to avoid erratic excessive X-ray condition de-

tection at short parasitic spikes. The XRayAlarm

I²C-bus flag is set to 1 to inform the MCU.

This protection is latched; it may be reset either by

VCC drop or by I²C-bus bit XRayReset

(see Chapter 8 - page 25).

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