AN993 Datasheet, PDF (19/27 Pages) STMicroelectronics – Electronic ballast with PFC using L6574 and L6561

AN993
Description of the demonstration application
Figure 15. Current feedback loop
RS_2
0.68
Q3
STP4NB50
R19 10k
6
C9 8.2nF
5
R16 100K
D4
1N4148
R18 100K
-
7
+
RIGN
4
Figure 16. Cpre waveform (Ch1) and amplifier output (Ch2)
AM01323v1
AM01324v1
4.5
Start-up and supply
The start-up procedure is very important in an application that contains two different
sections.
The ballast section starts before the PFC, avoiding any extra voltage at the output of the
PFC section, and so the L6561 dynamic OVP activation (see AN966). This behavior is
guaranteed under all conditions because the VS turn-on threshold of the L6574 is lower
than that of the L6561/2.
At start-up, the L6574 is powered by the resistor (R₄ + R₅). This resistor must be chosen so
as to ensure the "before start-up current" of both the L6561 and L6574.
When the ballast section is running, the charge pump (C11, R14, R17, D3 and DZ1)
enables supply to both devices. R17 and C10 enable reduction of noise at VCC.
Doc ID 5656 Rev 10
19/27