STV6886 datasheet(27/43 Pages) STMICROELECTRONICS

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STV6886

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2.4 Output Section

The H-drive signal is sent to the output through a

shaping stage which also controls the H-drive duty

cycle (I2C adjustable) (see Figure 11). In order to

secure the scanning power part operation, the out-

put is inhibited in the following cases:

– when VCC or VDD are too low

– when the XRAY protection is activated

– during the Horizontal flyback

– when the HDrive I

2C bit control is off.

The output stage consists of a NPN bipolar tran-

sistor. Only the collector is accessible (see

Figure 13).

Figure 13.

This output stage is intended for “reverse” base

control, where setting the output NPN in off-state

will control the power scanning transistor in off-

state.

The maximum output current is 30mA, and the

corresponding voltage drop of the output VCEsat is

0.4V Max.

Obviously the power scanning transistor cannot be

directly driven by the integrated circuit. An inter-

face has to be added between the circuit and the

power transistor either of bipolar or MOS type.

2.5 X-RAY Protection

The X-Ray protection is activated by application of

a high level on the X-Ray input (more than 8.2V on

Pin 25). It inhibits the H-Drive and B+ outputs.

This activation is internally delayed by 2 lines to

avoid erratic detection when short parasitics are

present .

This protection is latched; it may be reset either by

VCC switch-off or by I

2C (see Figure 14 on

page 28).

2.6 Horizontal and Vertical Dynamic Focus

For dynamic focus adjustment, the STV6886 deliv-

ers the sum of two signals on pin 10:

– a parabolic waveform at horizontal frequency,

– a parabolic waveform at vertical frequency.

The horizontal parabola comes from a sawtooth in

phase advance with flyback pulse middle. The

phase advance versus horizontal flyback middle is

kept constant versus frequency (about 1

µs). Sym-

metry and amplitude are I2C adjustable (see

Figure 15 on page 28).

The vertical parabola is tracked with VPOS and

VAMP. Its amplitude can be adjusted. It is also af-

fected by S and C corrections.

This positive signal once amplified is to be sent to

the CRT focusing grids.

Because the DC/DC converter is triggered by the

HFocus sawtooth, it is recommended to connect a

capacitor to pin 9, even if HFocus is not needed.

The capacitor value is critical only if Focus is used.

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