AN_UBA2024T_15 Datasheet, PDF (9/18 Pages) NXP Semiconductors – CFL applications

Philips Semiconductors
CFL applications with the UBA2024T
APPLICATION NOTE
5.3 Ignition frequency and preheating
The IC starts at an output frequency of about 2¹/2 times the nominal output frequency, and gradually decreases this
until the nominal output frequency is reached. The lamp inductor LLA and the lamp capacitor CLA will boost the lamp
voltage gradually higher as the output frequency gets closer to their resonance frequency, until it is sufficient to ignite
the lamp. In the mean time the current in the resonance circuit flows through the filaments thereby providing some
preheating. The UBA2024 has a circuit that stops the frequency sweep at the resonance frequency if the lamp has not
ignited yet (see UBA2024 specifications for details). This ensures maximum effort to ignite the lamp.
The ignition frequency fign is higher than or equal to the resonance frequency of LLA and CLA (fres=1/(2π√(LLACLA)) ).
The resonance frequency should be choosen so that 1.6⋅fout≤fres≤1.8⋅fout. The time needed to sweep down (set by
CSW) from the start frequency to fres can be used as an approximation for the ignition time. It’s about 0.5s/100nF. For
large values the ignition time is shorter, because the lamp ignites before the resonance frequency is reached. Typical
ignition time is 1 s when CSW=330nF.
CSW determines the sweep time. The larger CSW, the longer the sweep time and better the preheating of the electrodes.
However, the rise of the pre-ignition lamp voltage is also slower. Both a too short preheat as well as a too slow
voltage rise increase the glow time of the lamp (that’s when the lamp is not yet fully ignited, but it’s not off anymore
either), which decreases lamp life time. The best preheat time strongly depends on the lamp. Typical values for CSW
are 33nF to 330nF.
5.4 Choosing the other components
• For D1..D4 plain low cost 1N4007 diodes can be used.
• For lamp current ≥150mA CDV=220pF, for lower currents CDV=100pF.
• The values for CVDD and CFS are CFS=CVDD=10nF.
• Advised half bridge capacitors (CHB1 and CHB2) are >47nF when fout= 40–50kHz and >68nF when fout= 25–30
kHz.
• The resonance frequency of the input filter, consisting of LFILT and CHB (CHB being de effective capacitor as seen
on the HV pin of the IC, i.e. the series capacitance of CHB1 and CHB2), has to be at least two times lower than the
nominal output frequency.
Note: Performance and lifetime can not be guaranteed by using the values given in this chapter only. Lamp and UBA2024 performance
strongly interact with each other and need to be qualified together as a combination.
5.5 About component tolerances
For all components, generally used tolerances can be used (20% for electrolytic capacitors, 10% for other capacitors
(foil or ceramic) and 5% for resistors and inductors). Since ROSC, COSC and LLA determine the lamp current, their
tolerance also determines the spread in the lamp current. Therefore, the required lamp current accuracy may require
closer tolerance ROSC , COSC and LLA.
Example 1: ROSC ±5%, COSC ±10%, LLA ±5%, CLA ±10% and the IC’s internal frequency ±3% then lamp current
tolerance is 12.6% effective³.
3 _{Valid for component values with normal distribution.}
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