AN993 Datasheet, PDF(9/27 Page) STMicroelectronics – Electronic ballast with PFC using L6574 and L6561

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AN993 Datasheet, PDF (9/27 Pages) STMicroelectronics – Electronic ballast with PFC using L6574 and L6561

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AN993

That is to say:

Equation 2

Figure 5. Timing block

Ifs

Device block description

Ipre

Cpre

Figure 6. Timing oscillator block

DISCHARGE

AM01313v1

TIMING

Imax

Imin

Cpre

Iosc

OSC

AM01314v1

After the preheating time, the capacitor Cpre is first quickly discharged and then recharged

by the current Ifs, generating a second voltage ramp which feeds a transconductance

amplifier, as shown in Figure 6 (the switch is closed). Thus, this voltage signal is converted

into a growing current which is subtracted from Imax to produce the frequency shifting from

fmax to fmin. The current that drives the oscillator to set the frequency during this shifting is

equal to:

Equation 3

Doc ID 5656 Rev 10

9/27

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