SO-20 datasheet(21/33 Pages) STMICROELECTRONICS

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L6585DE

Ballast section

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6.3

Startup sequence with old or damaged lamps

When an old lamp is connected to the ballast the strike voltage is higher than the nominal

voltage and may also be higher than the safety threshold. In this case the lamp can ignite in

a time longer than ignition time or may not ignite. In both cases, during ignition time,

because of the frequency decrease, the voltage at the output of the ballast can easily reach

dangerous values.

The same occurs if the lamp tube is broken: the lamp cannot ignite and the lamp voltage

must be limited.

During ignition time, the L6585DE senses the current flowing into the lamp through a sense

resistor connected to the HBCS pin. If the HBCS pin voltage reaches 1.6 V, a small amount

of current is sunk from the EOI pin causing a small frequency increase. This frequency

modification results, macroscopically, in a frequency regulation and therefore a current

regulation and a lamp voltage limiting.

As soon as the HBCS pin voltage reaches 1.6 V, the TCH pin starts to charge Cd: when the

TCH voltage reaches 4.63 V, the TCH pin is no longer left free (as during preheating), but it

sinks 26 uA, causing a faster discharge of Cd. When the TCH voltage reaches 1.5 V, the pin

is pulled down and HBCS voltage is checked. If it is above 1.05 V the IC is stopped.

If the lamp ignites during this reduced TCH cycle, the EOI pin stops sinking current and if it

reaches 1.9 V, the IC enters run mode and TCH pin is immediately pulled down.

Figure 11.

Startup procedures with old or damaged lamps

It can be noted that the reduced TCH cycle time depends only on the value of Cd. It is

suggested to start from the choice of Cd in order to obtain the protection time, and then can

proceed to the choice of Rd to obtain the desired TPRE.

snk

Tch

source

Tch

reduced

Tch

26974

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