L6567 Datasheet, PDF (9/15 Pages) STMicroelectronics – HIGH VOLTAGE DRIVER FOR CFL

L6567
Other easy formulas fit rather well:
T_DT ≅ 46.75 · 10^{^-12} · R_REF
T_PRE ≅ 224 · C_P · R_REF
As far as df/dt is concerned, there are no easy formulas that fit the relation between C_F, R_F, and C_I. C_I is charged
and discharged by three different currents that are derived from different mirroring ratios by the current flowing
on R_REF. The voltage variations on C_I are proportional to the current that charges C_F, that is to say they are
proportional to df/dt.
The values obtained in the testing conditions (C_I = 100nF) are:
during preheating and working conditions the typical frequency increase is ~ 20KHz/ms, the typical decrease is
~-10Khz/ms;
During ignition the frequency variation is ~ -200Hz/ms.
If slower variations are needed, CI has to be increased.
Due to these tight relationships, it is recommended to follow a precise procedure: first R_HV has to be chosen
looking at startup current needs and dissipation problems. Then the feed forward frequency range has to be
determined, and so C_F is set.
Given a certain C_F, R_REF is set in order to fix F_MIN. Now C_P can be chosed to set the desired T_PRE and T_IGN.
The other external parameters (R_SHUNT and C_I) can be chosen at the end because they are just related to a
single circuit parameters.
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