AN1320 Datasheet, PDF (6/20 Pages) STMicroelectronics – IN BALLAST APPLICATIONS

AN1320 APPLICATION NOTE
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
fixing the value of the C_F capacitor.
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.
We can not use the PWM to act directly on C_F pin, because the rising edge on C_F is the low side mosfet “on
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
pins that set the current that charges C_F. A push pull output that gives us just 0V or 5V can not be used to in-
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-
alog T_PRE” will be so small to be “invisible” to the lamp (i.e. less than 2ms). During these 2 ms, the oscillating
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.
Only C_F and R_IGN (pin 3 and 4) set it.
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.
6/20