AN2708 datasheet(17/42 Pages) STMICROELECTRONICS

Part #	AN2708

Description	2X36 W digital dimmable ballast with L6574 and ST7FDALI

PDF	42 Pages

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Manufacturer	STMICROELECTRONICS [STMicroelectronics]

Direct Link	http://www.st.com

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17 / 42 page

AN2708

High-frequency ballast

17/42

Figure 7.

DALI protocol brightness values

To avoid the presence of stationary waves along the tubes at minimum dimming level, a

resistor of 100 k

Ω / 2 W has been placed in parallel to the battery capacitor of each lamp.

The resistance value ensures an additional current of 2 mA on the cathodes without

affecting the ballast efficiency.

Finally, during the startup sequence the frequency always goes from fmax to fmin,

independently of the set dimming level. Only after lamp turn-on does the frequency move

towards higher values.

2.2.2

Supply section

To supply the DALI slave microcontroller an AC-DC buck converter based on the

VIPer12A-E and L78L05 has been implemented on the ballast board. It converts the

rectified and filtered mains to a 5 V regulated output voltage dedicated to the

microcontroller. The converter works in discontinuous current mode adjusting the duty cycle

AN2708 Datasheet(PDF) 17 Page - STMicroelectronics