AN993 Datasheet, PDF (11/27 Pages) STMicroelectronics – Electronic ballast with PFC using L6574 and L6561

AN993
Equation 10
Equation 11
Device block description
These equations fit well with the measured values, especially in the frequency range of 30 to
100 kHz. Figure 8 shows a comparison between the measured and computed data of f_min
(at C_f = 470 pF).
Figure 7. C_pre voltage and frequency Figure 8. Operating frequency at C_f =
shifting
470 pF
V(Cpre)
120
measured values
computed values
100
80
time
freq
60
preheating
ignition
40
To summarize:
time
AM01315v1
Table 2.
Parameter dependencies
Parameter
T_pre
Tsh
Δf = f_max - f_max
f_min
20
40
60
80
100
120
R_ign (kΩ)
AM01316v1
Set by choosing...
C_pre
C_pre
R_pre
R_ign and C_f
If R_pre is not connected at pin 2, there is no Δf, so f_max and f_min have the same value.
Furthermore, there is no I_max, that is to say there is no Ifs current. In this case, C_pre is
charged only once by I_pre = 2.3 µA up to 3.5 V (Figure 7 shows the first voltage rise only).
When C_pre is discharged, no Ifs current is present to recharge it. If R_pre is connected after
the preheating and ignition phases, pin 1(C_pre) is at 4.8 to 5 V with a current capability of a
few µA (1/6 IR_pre). If R_pre is not connected, C_pre is at GND level in the normal operation
phase.
Doc ID 5656 Rev 10
11/27