EVAL6569 Datasheet, PDF (13/14 Pages) Panasonic Semiconductor – THE L6569 A NEW HIGH VOLTAGE IC DRIVER FOR ELECTRONIC LAMP BALLAST

APPENDIX B: Rating of the capacitive supply
with the L6569 driver
The supply is made with the snubber and a start
up resistor RS.
A snubber circuit is used to minimize the MOS-
FETs dissipation. It also achieves a non dissipa-
tive supply as shown on figure 10.
The MOSFETs gate charge, the driver consump-
tion, the oscillator, and the shunt regulator, define
the circuit consumption. We can estimate this
current is IS AV:
ISAV > 2 ⋅ IG + IQS + IOSC + IREG =
=
2
⋅
QG
⋅
fsw
+
IQS
+
VS
R_F ⋅ 2
+
IREG
Where QG the MOSFET gate charge
IQS the driver supply current
VS the supply voltage
RF the oscillator resistor and VS the driver
supply voltage
IREG the shunt regulator current.
When V_S is lower than the UVLO threshold U_UVLO, the
driver is only consuming. Its current must be mini-
mal to reduce the dissipation of the resistor RS.
The L6569 has a 150 µA start up current, and the
maximum resistance is 2MΩ for a 230Vac line ap-
plication.
We can also reduce the resistor value to get a
faster start up time TS.
T_S
=
R_S
⋅
C_S ⋅ U_UVLO
V_DC
Where CS is the supply capacitor, and VDC the
line voltage.
When the timer oscillates, the capacitor C sup-
AN880 APPLICATION NOTE
plies the lamp current during the lower MOS turn
off. To avoid any cross conduction its capacitance
is limited by the driver dead time TD (see figure
26). Hence the capacitive supply current IC is also
limited.
C
<
T_D ⋅ I_L
V_DC
ICAV = C ⋅ VDC ⋅ FSW < IL ⋅ TD ⋅ FSW
Where IL is the peak lamp current, and FSW the
switching frequency.
For a ballast such as a CFL one this circuit sup-
plies easily the required current. For instance with
a CF18DT lamp ( IL > 230 mA) the capacitor is
1nF on 120Vac line, 470 pF on 230 Vac line. At
50 kHz the average capacitive current is 6 mA in
both cases.
Figure 26: Cross conduction of the snubber
capacitor with the upper MOSFET:
capacitor current and voltage
waveforms.
TD
V + V HVG
OUT
GND
IC
GND
GND
RF
200 ns/dv ; 50 V/dv ; 0.1 A/dv
13/14