AN993 Datasheet, PDF (13/27 Pages) STMicroelectronics – Electronic ballast with PFC using L6574 and L6561

AN993
Figure 10. Startup timing diagram and EN2 function
Device block description
VSupply
V(Cpre)
preheating
fMAX
EN2
ignition _preheating
fMIN
Foult-ignition
ignition
IL
TPRE
TIGN (TSH)
Time
AM01317v1
3.3
3.3.1
Bootstrap section
Bootstrap circuitry is needed to supply the high-voltage section. This function is normally
accomplished by a high-voltage fast-recovery diode (Figure 11). In the L6574, a patented
integrated structure replaces the external diode. It is realized by means of a high-voltage
DMOS, driven synchronously with the low-side driver (LVG), with a diode connected in
series, as shown in Figure 12. An internal charge pump (Figure 12) provides the DMOS
driving voltage.
The diode connected in series to the DMOS has been added to avoid it being unintentionally
turned on.
C_boot selection and charging
To choose the proper C_boot value, the external MOS can be seen as an equivalent capacitor.
This capacitor C_ext is related to the total gate charge of the MOS.
Equation 12
The ratio between the capacitors C_ext and C_boot is proportional to the cyclical voltage loss. It
has to be:
Equation 13
For example, if Q_gate is 30 nC and V_gate is 10 V, C_ext is 3 nF. With C_boot = 100 nF, the drop
would be 300 mV.
Doc ID 5656 Rev 10
13/27