AN880 datasheet(13/14 Pages) STMICROELECTRONICS

13 / 14 page

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 +

⋅ 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 VS is lower than the UVLO threshold UUVLO, 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 2M

Ω for a 230Vac line ap-

plication.

We can also reduce the resistor value to get a

faster start up time TS.

⋅ CS ⋅ UUVLO

VDC

Where CS is the supply capacitor, and VDC the

line voltage.

When the timer oscillates, the capacitor C sup-

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.

⋅ IL

VDC

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.

VHVG + VOUT

GND

200 ns/dv ; 50 V/dv ; 0.1 A/dv

Figure 26: Cross conduction of the snubber

capacitor with the upper MOSFET:

capacitor current and voltage

waveforms.

AN880 APPLICATION NOTE

13/14

AN880 Datasheet(PDF) 13 Page - STMicroelectronics

AN880 Datasheet(HTML) 13 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Datasheet Download

Link URL