AN2708 datasheet(16/42 Pages) STMICROELECTRONICS

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High-frequency ballast

AN2708

16/42

possible to modulate the output power in order to allow dimming by varying the switching

frequency.

The ballast operating frequencies determine the L6574 biasing circuitry as explained in

AN993 "Electronic Ballast With PFC Using L6574 and L6561" and as summarized below.

2.2.1

Lamp dimming

In this system the lamps are dimmed down to 5% by interfacing the ST7FDALI

microcontroller with the analog driver L6574.

A PWM output of the ST7FDALI microcontroller is used to generate a 0-5 V PWM at 4 kHz.

Its integrated value gives the op amp voltage reference. The dimming level is set by varying

the PWM duty cycle from 70% (100% dimming) to 14% (5% dimming). This modification

allows changing the L6574 op-amp positive reference voltage from 120 mV to 20 mV which

increases the switching frequency and reduces the current in the load.

On the slave unit the duty cycle values have been calculated according the DALI protocol

brightness values, listed in Figure 7.

Table 8.

L6574 biasing circuitry design equations for operating conditions

Pin 1 (CPRE)

Pin 2 (RPRE)

Pin 3 (CF)

Pin 4 (RIGN)

Pin 5 (OPOUT)

A capacitor is connected

between this pin and OPIN-

for the current feedback loop

compensation. It set also the

turn on delay in a dimming

application.

1.5

PRE

⋅

PRE

-----------------

PRE

⋅

fph

frun

–

1.41

PRE

------------------------

frun

1.41

RignCf

--------------------

frun

1.41

RignCf

--------------------

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