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AN1320 APPLICATION NOTE

The hardware and software development can not be separated: it is a very interactive process. Starting from

the basic point there are different way to act on the L6574, depending on what we want to be software depen-

dent and what we want to be hardware dependent in the final application.

We will describe the two sections separately just for ease of understanding.

5.1 HARDWARE CHOICES

First we have to choose the

µC. We have chosen the ST62E62C, a 16 pin µC belonging to the ST6 family.

This device is able to perform all the functions we need with pins left over.

In this microcontroller we have just an auto reload timer, this means that we have just one PWM available. We

can call this PWM the HARDWARE PWM in the following pages, just to differentiate it from the second one we

need and that is obtained by software from a standard output pin. We will call this second PWM the “SOFT-

WARE PWM”

We have discussed in the previous chapters how to interface the

µC and the L6574; the remaining problem is

how to supply the ICs.

We have chosen a cheap way to supply the

µC: we use a L7805, which is a linear regulator (5V output). It has

been connected to the L6574 supply. This choice (mainly based on economical reasons) has many conse-

quences:

s

As L6574 supply is given by a charge pump connected to the middle point of the half bridge,

the L6574 has to work (to oscillate) to give the supply to the micro

s

The ICs are not supplied at the same time, but as a sequence: the micro (the supervisor)

has to be told when everything is OK to start the preheat sequence

s

If the L6574 is disabled (after a fault condition for example) the micro no longer has a voltage

supply. When the circuit starts again the micro is reset and the program starts from the very

beginning

It can happen that the micro must always be supplied, even when L6574 is off. In this case another solution has

to be used, e.g. a regulator connected to the rectified mains.

The board start up sequence management takes into account the above mentioned point 1 and 2. Using the

start up resistors we let the L6574 start, so its oscillation allows the charge pump work. Then the L6561 is sup-

plied and the L7805 feeds the

µC. This sequence is an inheritance of L6561+L6574 demo board. The co-ordi-

nation between the L6574 and the L6561 is done by choosing the start up resistors and the charge pump

components. The co-ordination between the micro and the analog part is a mix of hardware and software. By

software we let the micro wait for a “power OK” signal from the L6574, that in the meanwhile keeps on oscillating

at a very high frequency, so as to let the filament not to be preheated. The L6574 can provide this “power OK”

signal for free. The internal sequence of the L6574 charges the preheat cap connected to pin 1 to 5V after the

preheat and the ignition sweep has ended. We can deliver this 5V signal to a

µC input, and hereafter the µC will

start to set all the application parameters: in this way we are sure that everything is correctly supplied. Moreover

the overall timing: it is hundreds of time shorter than a typical preheat time.

To be sure that the third condition does not happen and that there are no out of control situations in which we

do not know which part of the code the micro is running we have added additional safety circuitry. After the alarm

for open load or disconnection, the micro sets an output pin to the high logic level in order to activate an SCR

to disconnect the VCC from all the ICs, PFC included. In this way the

µC can supervise not only the L6574 but

also the L6561.