AN2870 datasheet(14/41 Pages) STMICROELECTRONICS

14 / 41 page

Device blocks description

AN2870

14/41

3.3.2

Overcurrent control and protections

Limiting the current flowing into the half bridge:

●

The lamp voltage during the ignition phase is limited

●

The power of the lamp during run mode is limited

Ignition phase: (see Figure 8) if the VHBCS high threshold (HBCSH = 1.6 V) is crossed

(because the lamp doesn’t ignite), the following actions are taken by L6585DE:

A current, whose amplitude is proportional to the time the VHBCS is above threshold, is

sunk from EOI and consequently from RF pin. This results in a frequency increase that

reduces the resonant network current and therefore the lamp voltage.

A reduced time is calculated by Tch pin:

The 31µA generator charges CD to 4.63 V

Instead of leaving CD to be discharged by RD, a 26 µA current generator discharge

quickly CD to 1.5 V (S4 on)

The pull down switch S3 completes the reduced cycle

The reduced Tch cycle depends only on CD value and is equal to:

Equation 1

At the end of the Tch cycle, during the first subsequent low side on time, the HBCS voltage

is checked: if VHBCS is higher than a threshold (HBCSH,test) the IC is stopped in latched

condition, otherwise EOI pin is released in high impedance status. When EOI voltage

reaches 1.9 V the IC enters the run mode.

The sense resistor value defines the maximum current that can flow during ignition and then

the maximum allowed lamp voltage.

The linear growth of the lamp voltage, thanks to the exponential decrease of the operating

frequency during ignition allows a better control of the voltage thanks to a lower dV/dFsw.

In case of choke saturation the intense current results in very high VHBCS. The 2.75

threshold triggers this event and stops immediately the IC.

Run mode: During this phase, current control similar to the one present during ignition is

available in case of an over-current due to symmetrical ageing of the lamp. It follows the

same rules, but the threshold is equal to 1.05 V instead of 1.6 V.

Also during run mode the saturation protection is active: in case of choke saturation due to

lamp breaking, lamp removal and capacitive mode where VHBCS experiences a spike whose

amplitude is higher than 1.6 V and whose duration is longer than 300 ns. This kind of event

causes the IC turn-off in latched condition.

The lamp ageing causes the shift of peak of the resonance curve towards the run frequency.

This results in hard switching behavior: the half bridge doesn’t work at ZVS and spikes

appears at HBCS pin. These spikes have very high amplitude (up to 8 V) and short duration

(30 ns-50 ns).

snk

Tch

source

Tch

reduced

Tch

269740

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