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STMicroelectronics Confidential
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AN1290
DC/DC Converter Section
Considering that the BOut control pulse will have constant duration (Open loop), or nearly constant
(Current mode, Step-up or Step-down), you can choose to trigger conduction:
●
at low frequency, with HOut down,
●
at high frequency, after flyback,
and obtain that the switching always takes place inside the first half of H positive ramp, which is the
least critical period regarding parasitic voltage.
In Step-up, Current mode, any timing is convenient; nevertheless, usually disturbances will be
minimized when triggering with HOut up.
7.2.2
Frame-top Distortion related to Keystone Correction
This distortion mainly appears in monitors with separated EHV and scanning (though it can also
take place with diode modulators, in relation with the modulator amplifier).
In the monitors of this category, E/W correction is obtained by modulating the value of the B+
scanning stage with a parabolic waveform.
If there is no Keystone correction, the E/W waveform is continuous, and B+ scanning can easily
follow.
But as soon as Keystone correction is introduced there appears a fast transition (positive or
negative) between the end of a frame and the beginning of the next one as shown in
. To
get the B+ scanning to follow this waveform, it would be necessary to charge or discharge at a very
high rate the Cb capacitor which proves difficult because the available current is limited.
Consequently, it is mandatory to maintain the Cb value as low as possible (possibly less than 1µF).
A limiting factor of course is the increasing B+ ripple at the horizontal frequency. Regarding this
aspect, the architectures that do not need Cb have a distinctive advantage (step-down, current
mode or open-loop).
Once Cb is made small enough to charge or discharge within the vertical blanking time, there may
remain a distortion of the frame top which corresponds to the transient response of the global
DC/DC feedback loop to the fast transition. The response may be damped or oscillatory depending
on horizontal frequency. Again, the transient phenomena should be finished within the vertical
blanking time. For that purpose, the loop response must be fast enough. Once Cb has been made
minimum the only possible action is on the frequency compensation of the op-amp between pins 14
and 15. A good starting value is given by Ri x Cfb = 15 µs. The op-amp itself has a unity gain of
6MHz and is not a limiting factor.
Figure 22: E/W Waveforms
Without Keystone
With Keystone
Fast
Transition
