The auxiliary supply of the converter

The circuit consumption is defined by the MOS-

FETs gate charge, the I.C. consumption, the os-

cillator, and the shunt regulator. Several circuits

are possible.

In many applications a snubber is used to reduce

the dissipation in the MOSFETs. When this snub-

ber is used in conjunction with a start up resistor

achieved almost for free.

At start up the I.C. is consuming 150

µA, and there-

fore only a small supply resistor is required. During

operation the capacitor provides the supply current.

To avoid cross conduction, the capacitance is lim-

last

this

circuit

easily

supplies

the required

mA) the required capacitance is 470 pF on 230 Vac

line. At 50 kHz the average capacitive current is 6

mA, as described in appendix B.

When the required driver current is higher than 10

mA, a secondary winding on the resonant choke

is an easy supply alternative.

The ballast shutdown

The L6569 allows several ways (see figg. 11, 12

and 13) to shutdown the ballast [4]: by acting on

the Under Voltage Lock Out.

When the shutdown is realized acting on Vs pin,

(see fig. 12) a limiting resistor Rs must be used to

slow down the discharge of the supply filter Cs.

The constant time of the discharge must be

greater than 10 periods of the switching fre-

quency:

≥

10

oscillator, and the lower MOSFET will remain ON.

Therefore

the

bootstrap

capacitor

remains

L6569

I

15V

H.V.+

ON

OFF

Figure 9. L6569 driver protection against voltage spikes.

bootstrap

circuit

L6569

6 mA WHEN 50 kHz SWITCHING

1mA WHEN STARTING

220k

Ω

470 pF

310 V

Rs

Cs

C

Figure 10: Non dissipative auxiliary supply using the transistor snubber.

AN880 APPLICATION NOTE

5/14