AN_UBA2024T_15 Datasheet, PDF (8/18 Pages) NXP Semiconductors – CFL applications

Philips Semiconductors
CFL applications with the UBA2024T
APPLICATION NOTE
fout can be chosen freely up to 60kHz (the maximum nominal output frequency for the UBA2024, corresponding with
a start-up frequency of 150kHz, see datasheet for start-up sequence description). However, usually fout is chosen
between 25kHz and 30kHz or between 40kHz and 50kHz. This is because below 25kHz there may be audible noise,
operation in the 30kHz to 40kHz band may result in interference with infra-red remote control and above 50kHz the
third harmonic is in the range where conducted noise requirements for most countries have to be met. Since
inductors and capacitors decrease in size and cost with increase in frequency, the 40 to 50kHz range is preferred.
Throughout this application note we will presume the lamp frequency will be in this range.
fout is set by ROSC and COSC according to the following formula:
fout =
1
kOSC ROSC COSC
Practical values for ROSC range from 50kΩ to 400kΩ. Note that the low values of ROSC will cause a larger VDD
output current, thus increasing the total package dissipation. Practical values for COSC range from 100pF to 1nF.
Advised value for COSC is 180pF for 40..50kHz and 270pF for 25..30kHz. The oscillator constant kOSC is shown in
figure 4.
Figure 4: Typical kosc dependency of Rosc and Cosc for UBA2024T.
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