AN993 Datasheet, PDF (8/27 Pages) STMicroelectronics – Electronic ballast with PFC using L6574 and L6561

Device block description
3
Device block description
AN993
The preheating control section and the bootstrap section are tightly linked to the
application’s design. This chapter describes their workings and usage.
3.1
Preheating and ignition section
The L6574’s turn-on sequence is divided into three phases: the preheating phase, the
ignition phase and the normal operation phase (Figure 4). The preheating phase is
characterized by the highest oscillation frequency (f_max) for a period T_pre. During the ignition
phase, the frequency shifts from f_max to f_min (which is the normal operating frequency) in a
period T_sh.
Figure 4. Startup timing diagram and EN2 function
Power-O.K.
VSupply
V(Cpre)
Osc.
freq.
preheating
fMAX
steady state operation
ignition
fMIN
IL
TPRE
Time
AM01312v1
● ^T_pre^{= K ⋅ C}_pre
● ^T_IGN^{= 0.1 ⋅ T}_pre
All the above-mentioned parameters are set by carefully selecting a few external
components. T_pre and T_sh are set by means of the capacitor C_pre that is connected to pin 1.
During the preheating phase (T_pre) the capacitor C_pre is charged by means of a constant
current I_pre, which is generated internally and does not depend on any external components.
The voltage across C_pre increases linearly up to the "preheating threshold" at which the
preheating phase terminates.
Equation 1
8/27
Doc ID 5656 Rev 10