AN2870 datasheet(13/41 Pages) STMICROELECTRONICS

13 / 41 page

AN2870

Device blocks description

13/41

3.3

Ballast controller section

3.3.1

Oscillator and timer

The half bridge driver oscillation is regulated by a current controlled oscillator (CCO): it

needs a capacitor connected to OSC pin (pin 1) and uses the current flowing outside RF pin

(pin 2) as reference.

The RF pin has a 2 V precise voltage reference that let the designer fix the run mode

frequency simply connecting a resistor between RF pin and GND (Rrun).

The EOI pin (pin 3) is driven by the internal logic in order to set the frequency during the

preheating and to control the lamp current during an over-current event in the half bridge.

Preheating frequency is set by the parallel of Rrun and a resistor (Rpre) placed between RF

and EOI: in fact during the preheating the EOI pin is pulled to GND.

TCH pin is connected to the parallel of a resistor (RD) and a capacitor (CD) and is used in

order to define the preheating time and the protection time; its cycle (Tch cycle) is

composed by the following steps:

A 31

μA current generator charges the C

D causing TCH voltage to rise linearly,

When TCH voltage reaches 4.63 V, the TCH pin is left in high impedance status and CD

is discharged by RD,

When TCH voltage reaches 1.5 V the cycle finishes and an internal resistor pulls down

the TCH pin to GND.

Figure 7.

Oscillator and starting sequence

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