AN1320 APPLICATION NOTE

6/20

4.0 L6574: HOW TO SET FREQUENCIES, TIMING, FAULT SIGNALS IN A “MICROCONTROLLED

APPLICATION”

We have seen that the frequencies in L6574 are set by fixing the current that flows out from pin 2 and 4 and

A microcontroller output pin can give us a high logic level (5V) a low logic one (0V) or a PWM output at fixed

frequency and variable duty cycle.

time” and the falling edge is the high side mosfet one. The half bridge would oscillate in asymmetrical way at

fixed frequency instead of at 50% duty cycle and variable frequency. So we have to interface the

µC with the

terface pin 2 and 4 because they have a maximum voltage level up to 2V. We have to use the integrated value

of a PWM signal to set a voltage level between 0V and 2V.

We can use a PWM output also to give the op amp the voltage reference to change the load current (and so the

lamp power to perform dimming)

Acting on L6574 pin 4 and (or) on pin 7 (opamp+) we can control the inverter working frequency.

If we want to control the preheat timing and frequency we have to act on pins 2 and 1. First we have to avoid

that the L6574 fixes the preheat time by itself. If we connect to pin 1 a very small cap (e.g. 1nF), the L6574 “an-

frequency has to be high enough to avoid lamp filaments preheat (> 150KHz). The resistor connected to pin 2

has to be sized properly.

After these 2 ms L6574 is in “working mode”: it means that pin 4 is no more involved in fixing the frequency.

Now the effective preheat time can be decided by the

µC just acting on the PWM that gives the voltage reference

to pin 4. For example, it can have a certain duty cycle (appropriate for the preheat freq.) for a fixed time, than it

can change the duty cycle (i.e. the voltage reference) to set the ignition profile and the final working frequency.

Now we are able to change all the frequencies and the timing involved in lamp turning on and dimming with two

connections between L6574 and the

µC.

The fault management can also be done by the

µC: all the fault signals will be brought to it, and then it will react

according to the code. A connection that can be useful is the one to pin 8 (shut down pin) that can be direct

because the ICs levels are compatible. In this way the

µC can react to a signal either by stopping the inverter

or by changing the frequency (i.e. repeating a preheat sequence if there is the no-ignition alarm, or bringing the

frequency to a very high value…).

Just with these 3 connection between the L6574 and the

µC we can set nearly all the parameters of the appli-

cation by software.

The number of

µC inputs we need for fault signals depends only on what we want to control.

We need another input pin to give the

µC the information about the dimming level: this is the interface between

the ballast and the “final user”. We can use either switches or an AD input. The first solution is more expensive

in terms of number of pin, the AD input requires some attention for the code part but allows a much larger num-

ber of levels.

5.0 HOW TO APPLY THIS INTERFACE TO A BALLAST

We started from AN993 demo board to build a

µC application with the same performances and some additional

degrees of freedom.

We will now apply all the concepts already discussed and put them into a working board.