AN2870 datasheet(16/41 Pages) STMICROELECTRONICS

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Device blocks description

AN2870

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3.3.3

End of life detection

When the lamp becomes older and approaches its end of life, its equivalent resistance

increases symmetrically or asymmetrically.

In symmetrical ageing a modification of the frequency response of the resonance network

can be seen and, consequently, an increasing of lamp current and the appearance of hard

switching events: in fact the resonance frequency is now closer to operating frequency.

In asymmetrical ageing the current flowing in one direction is greater than the current

flowing in the other; this means that lamp voltage and current waveforms have no longer

zero mean value.

A window comparator measures the variation of the DC component of the

lamp voltage that can be either positive or negative. The reference and the amplitude of this

comparator can be set choosing the value of a resistor connected between EOLP pin and

GND accordingly with the following table.

This comparator can be used in both the two most used ballast configurations: Blocking

capacitor to ground and lamp to ground.

Block capacitor to ground

(Figure 10): During normal operation the DC mean value

across Cblock is equal to the half of the output voltage of the PFC. A resistive divider is

placed across the block capacitor to sense its DC voltage: the asymmetric effect appears as

a shifting of this DC value.

Any voltage ripple or disturbance across the output voltage is present also on Blocking

Capacitor and may alter the correct detection of a lamp at the end of its life.

In order to reject all this disturbances, the reference of the window comparator is connected

to CTR pin (Tracking reference configurations): in fact this pin is connected directly to the

output voltage and experiences the same ripple voltage. The rejection of the PFC output

voltage low frequency ripple allows using a smaller bulk capacitance.

Table 1.

Comparator amplitude

EOLP resistor

Symbol

Reference

Half–window amplitude

REOLP > 620 k

RFH

Fixed 2.5 V

± 720 mV

220 k

Ω < REOLP < 270 kΩ

RTL

Tracking with CTR

- 240 mV / + 250 mV

75 k

Ω < REOLP < 91 kΩ

RFL

Fixed 2.5 V

± 240 mV

22 k

Ω < REOLP < 27 kΩ

RTL

Tracking with CTR

- 150 mV / + 160 mV

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