LM1290 National Semiconductor, LM1290 Datasheet - Page 7

• Manufacturer Part Number: LM1290
• Description: Autosync Horizontal Deflection Processor
• Manufacturer: National Semiconductor

Application Hints
1. Phase Control for Geometry Correction: Pin 10
2. Programmable Frequency Ramping: H frequency
(PHASE) is designed to control static phase (picture
horizontal position) as well as dynamic phase for geom-
etry correction. Complete control of static and dynamic
phase can be achieved by superposing a correction
waveform (Sawtooth and/or parabola) on the DC control
voltage at pin 10 (see Figure 12 ).
transitions from high to low present a special problem for
deflection output stages without current limiting. If, dur-
ing such a transition, the output transistor on-time in-
creases excessively before the B+ voltage has de-
creased to its final level, then the deflection inductor
current ramps too high and the induced flyback pulse
can exceed the breakdown voltage, BV
transistor. To prevent this, the rate of change of the VCO
frequency must be limited.
Recommended Phase Voltage for 640 x 480
CEX
, of the output
FIGURE 11.
7
3. Phase Voltage Range vs Delay Time: The recom-
Consider a scanning mode transition at t = 0 from f
The VCO frequency as a function of time, f
scribed by the equation,
f
where
The above equation can be used to predict VCO behav-
ior during frequency transitions, but in practice the value
of C
large values minimize the chance of exceeding BV
but generate long PLL capture times.
mended phase voltage range to use on pin 10 (PHASE)
depends on the delay time of the deflection output stage.
Delay time is defined as the time from the rising edge of
H Drive to the center of flyback. For best performance
the phase voltage range should be in the unshaded area
of Figure 11 .
VCO
FVC
(t)
is most easily determined empirically. In general,
= 40 x 10
f
1
@
+ ( f
60 Hz
2
− f
3
x C
1
) (1 – exp (− t / )),
FVC
.
DS012917-11
VCO
www.national.com
(t), is de-
1
to f
CEX
2
.
,