L6567 datasheet(7/15 Pages) STMICROELECTRONICS

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L6567

Figure 2. Preheating and ignition state.

Ignition mode

At the end of the preheat phase the frequency decreses to the minimum frequency (FMIN), causing an increased

coil current and a high voltage appearing across the lamp. That is because the circuit works near resonance.

This high voltage normally ignites the lamp. There is no protection to avoid high ignition currents through the

MOS transistors when the lamp doesn’t ignite. This only occurs in an end of lamp life situation in which the circuit

may break. Now the lowest frequency is the resonance frequency of L and CL (the capacitor across the lamp).

The ignition phase finishes when the frequency reaches FMIN or (at maximum) when the ignition time has

elapsed. The ignition time is related to TPRE:TIGN = (15/16) · TPRE. The CP capacitor is charged 15 times with

the same current used to charge it during TPRE.

The frequency shifting slope is determined by CI.

During the ignition time the VRS monitoring function changes in the capacitive mode protection.

Steady state operation: feed forward frequency

The lamp starts operating at FMIN, determined by RREF and CF directly after the ignition phase. To prevent too

high lamp power at high mains voltages, a feed forward correction is implemented. At the end of the preheat

phase the RHV pin is connected to an internal resistor to sense the High Voltage Bus. If the current in this resistor

increases and overcomes a value set by RREF , the current that charges the oscillator capacitor CF increases

too. The effect is an increase in frequency limiting the power in the lamp. In order to prevent feed forward of the

ripple of the VHV voltage, the ripple is filtered with capacitor CP on pin 8 and an integrated resistor RAVERAGE.

Figure 3. Burn state

TIME

FREQUENCY

FMAX

FMIN

preheating

state

ignition

state

burning state

FMIN

feed forward mode

FREQUENCY

Irhv

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