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AN1320

APPLICATION NOTE

December 2000

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.

by Francesca Sandrini and Luca Rodeschini

L6574 & MICROCONTROLLER

IN BALLAST APPLICATIONS

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.