AN880 datasheet(10/14 Pages) STMICROELECTRONICS

10 / 14 page

Several ways can achieve the protection task.

First it can be done by sensing the resonant cur-

rent through a MOSFET source resistor or a sec-

ondary winding on the choke. The switching is

stopped when a large current reduction is de-

tected by analog means.

A logic circuit can also detect the presence of the

lamp filaments. One end of a filament is always

connected to a fixed voltage. If the other end of

the filament is connected through a high imped-

ance resistor to another voltage, the absence of

the filament can be easily detected by monitoring

the resistor voltage change as shown on figure

22.

CONCLUSION

The foregoing note shows how high voltage driv-

ers, like the L6569, simplify the design of the

lamp ballast. These devices includes all the cir-

cuitry to drive MOSFETs in half bridge inverter.

Since the optimized switching frequency in-

creases above 50 kHz with a low tolerance, the

size of the passive resonant components is re-

duced, and the ballast becomes cheaper.With its

supply and its oscillator the L6569 is versatile,

and its flexibility permits to design any improved

power control.

BIBLIOGRAPHY

[1]: AN 527 "Electronic fluorescent lamp ballast"

A.Vitanza, R.Scollo, SGS-THOMSON

[2]: Smart Power ICs, Chapter 8, High voltage in-

tegrated circuits for off-line power applications.

C.Diazzi, SGS-THOMSON

[3]: AN 512 "Characteristics of power semicon-

ductors" JM Peter, SGS-THOMSON

[4] : "The L6569 half bridge driver: the shutdown

function"

[5] : "Compatibility test of dimming electronic bal-

lasts used in daylighting and environment con-

trols", A.Buddenberg, Rensselaer Polytechnic In-

stitute

[6]: "PSpice High Frequency Dynamic Fluores-

cent Lamp Model", Bryce Hesterman, APEC’96,

p.641

4_GND

3_CF

18V

10.R

100.R

10.R

LAMP

L6569

11.R

Figure 22: Open load detection example.

AN880 APPLICATION NOTE

10/14

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