AN993 Datasheet, PDF(25/27 Page) STMicroelectronics – Electronic ballast with PFC using L6574 and L6561

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AN993 Datasheet, PDF (25/27 Pages) STMicroelectronics – Electronic ballast with PFC using L6574 and L6561

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AN993

6.1

Dimming the lamp

Dimming level and lamp turn-on

During the start-up sequence the frequency always goes from fmax to fmin, regardless of the

dimming level, and it is only after this that the lampâs turn-on frequency moves towards

higher frequencies. The delay during which f = fmin allows the lamp to turn on, but has the

drawback of causing a "flash" that can be unpleasant. As the minimum length of this delay

depends on the type of lamp, it is better to set it as low as possible, finding the best

compromise.

The easiest way to set this delay is to act on the amplifierâs compensation, that is, on C9: the

higher C9, the longer the delay and the easier the lamp turn-on. Values up to 200 nF and

more are commonly used.

Doc ID 5656 Rev 10

25/27

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