AN1320 Datasheet, PDF (1/20 Pages) STMicroelectronics – IN BALLAST APPLICATIONS

AN1320
APPLICATION NOTE
L6574 & MICROCONTROLLER
IN BALLAST APPLICATIONS
by Francesca Sandrini and Luca Rodeschini
There is an increasing demand for flexibility in ballast applications. This means a request for having
ballast that can be used for different tubes without changing the soldered components. The aim is to
save money by using less parts (resistors, capacitors and so on) and less ballast models to be stored
and managed.
A way that is going to be investigated is the use of microcontroller which can “supervise” the application
in such a way that the key parameters of the application can be modified according to the tube charac-
teristics just by changing the micro-code.
In this application note we will exploit a way to interface a microcontroller with our integrated ballast
controller: the L6574.
1.0 INTRODUCTION
Roughly speaking a HF-TL ballast converts the 50-60Hz input to a high frequency output, usually in the range
of 25-125KHz.
A rectifier block and a DC to high frequency inverter usually make up a ballast. The half bridge of the inverter
can be driven in different ways with different ICs.
We will focus on a specific driver: the L6574. We will see first how the L6574 can drive and control a ballast,
then how it can communicate and be supervised by a µC.
The aim of this paper is to examine if there are advantages in having a µC work with L6574, the feasibility of
this “cooperation”, and a practical example.
2.0 L6574
L6574 is a BCD off line 16 pin IC specifically designed for ballast applications [ref.1](see fig.1). It has both driver
functions and controller functions on board.
The most useful characteristics to control the lamp are:
s Preheat and frequency shifting timing
s Cmos shut down input
s Sense op-amp for closed loop control or protection failures
The parameters of the application are set by external components (resistors and capacitors) connected to the
IC. L6574 allows the user to set all the parameters according to the lamp characteristics, and the ballast will be
a high performance one. There is a specific application note on this IC (ref. [2]): here you find the description of
58W TL ballast with PFC section. Please refer to this application note and to the L6574 datasheet for the IC
details. In the following paragraphs we will focus our attention on a way to interface the L6574 with the micro-
controller rather than on “L6574 - stand alone” performances. The aim of this “supervision” is to control the three
points mentioned above.
December 2000
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