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AN1290 Horizontal
Section
●
Any ripple, non synchronous to horizontal, on the transformer supply capacitor will influence
the H-scanning transistor storage time and cause supplementary jitter.
●
If a MOS transistor is used as driver, its Gate-Source capacitance must be charged or
discharged at each transition. This gives rise to large current spikes. The corresponding dI/dt
may induce electromagnetic voltages in the H circuitry and disturb H sync or the oscillator; the
effect will be a localized jitter (taking place for one precise setting of Horizontal position or one
precise amplitude etc.). In order to avoid this problem, the disturbing loops must be minimized:
— charging loop: supply capacitor
→
NPN
→
gate/source capacitance and back to supply
capacitor.
— discharging loop: gate/source capacitance
→
PNP and back to source.
●
Be aware that during fast transitions, the base current may be high because the transistor gain
may fall to low values. A series resistor connected to the bases will help to limit the current
spike in pin 26.
●
Since very fast transitions take place on pin 26, any capacitance tied to this pin should be
avoided because it would give rise to high current spikes which may cause localized jitter.
By the way, please notice in this schematic the AC coupling between IC output and driver stage. The
reason for this is that when the H output is inhibited (for instance, when the X-ray protection is
activated), the driver stage is supposed to remain fully conductive. This would lead to high current in
the driver transformer and resistor. AC coupling prevents remaining in this dangerous state: if a
permanent inhibition takes place, the driver stage will soon be OFF and the scanning transistor also
remains OFF because of the transformer coupling.
4.2.3
Enlarging the Frequency Range
If the frequency range is considered too tight, a very simple modification can enlarge it.
In a given application, the frequency is proportional to the voltage across Ro. Voltage on pin 8 may
vary between 1.33V and 6.2V.
If Ro is referenced to a Vext voltage source instead of GND, the relative frequency range, previously
(6.2 / 1.33), becomes (6.2-Vext) / (1.33-Vext), which might prove much higher.
For practical implementation, just follow the schematic diagram shown in
, and use the
equivalent resistance and voltage values to determine the frequency range.
Figure 5: Enlarging the Frequency Range
8
Ra
Rb
R
Proposed Schematic
13
8
Ra
Rb
R
Equivalent Circuit
Vext = Href x Rb/(Ra+Rb)
Vext
