Part # Application Note Lighting Control L6574 EVAL6574B AN993 datasheet

Part Manufacturer: ST Microelectronics

ST Microelectronics


Part Details:

AN993 APPLICATION NOTE ELECTRONIC BALLAST WITH PFC USING L6574 AND L6561 The advent of dedicate IC for lamp ballast applications is replacing the old solutions based on bi-polar transistor driven by a saturable pulse transformer. The L6574 is a high performance ballast driver, designed in 600V BCD OFF-LINE technology,which ensures all the features needed to drive and control properly a fluorescent bulb. It is provid-ed with a built-in VCO start up sequence circuitry, Protections, and an OP-AMP, useful to imple-ment a closed loop control of the lamp current. 1 INTRODUCTION 1.1 Half bridge converter for electronic lamp ballast Voltage fed series resonant half bridge inverters are currently used for fluorescent lamps (fig.1). This topologyallows to easily operate in Zero Voltage Switching (ZVS) resonant mode, drammatically reducing the transistorswitching losses and the electromagnetic interference.To design a cost effective, compact and smart electronic lamp ballast it could be used a dedicated IC able todrive directly the power MOSFETs of the half bridge. Such controllers require a high voltage capability for thehigh side floating transistor driver. Figure 1. Half Bridge topology HV DRIVER LRES CRES D98IN819 1.2 Lamp requirements To provide long life and to insure an efficient ignition of the lamp the cathodes must be preheated. In fact thepreheating of the filaments allows an easy strike of the lamp reducing ignition voltage. During preheating timethe lamp is characterised by a high impedance and the current flows only in the filaments. The resistance value May 2004 1/20 AN993 APPLICATION NOTE of the filaments are strictly dependent on the lamp model. Typically these filaments present a initial low value(a few Ohms) that will increase by 4-5 times during the preheating.After the preheating time the lamp must be ignited, by increasing the voltage across it. The ignition voltage valuealso depends on the lamp type, and it increases with the aging. For a typical TL 58W it is not much less than1000V. Using a simple inverter, with a constant switching frequency, external circuitry must be used (e.g. PTCor discrete timer). Instead with the ST L6574 smart controller both the preheating and the ignition functions areachieved by using simple resistors and a capacitor, which set all the start-up procedure. 2 L6574 BALLAST DRIVER The L6574, whose internal block diagram is shown in fig.2, is an IC intended to drive two power MOS or IGBTin half bridge topology, ensuring all the features needed to drive and control properly a fluorescent bulb. More-over, by varying the switching frequency it is possible to modulate the current in the lamp, therefore the outputpower. The device is available in DIP16 and SO16N packages.The most significant features of the L6574 concern the following points: high voltage rail up to 600V; dV/dt immunity ± 50 V/ns in full temperature range; driver current capability (250 mA source and 450 mA sink); switching times 80/40 ns rise fall with 1 nF load; CMOS shut down input; under voltage lock out; preheat and frequency shifting timing; sense OP AMP for closed loop control or protection features; high accuracy current controlled oscillator; integrated bootstrap diode; clamping on VS; SO16, DIP16 package Figure 2. Internal Block diagram of the L6574 H.V.Bus VS OP AMP OPOUT 12 16 Vboot 5 + - 6 OPIN- UV 15 HVG BOOTSTRAP HVG

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Application Note Lighting Control L6574 EVAL6574B AN993.pdf Datasheet