L6585D datasheet(13/25 Pages) STMICROELECTRONICS

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L6585D

Application information

13/25

5.1.2

Ignition (time interval B Figure 5)

When the voltage at pin TCH drops down to 1.50V (typ.), the pin EOI is driven in high

impedance state and CIGN is exponentially charged according to the time constant τ given

by CIGN*RPRE that defines the ignition time and the frequency shift starts.

The ignition time is the time necessary to EOI voltage to reach 1.9V, so, by means of simple

calculation:

Equation 3

During this phase, the half-bridge current control can limit the maximum voltage applied

to the lamp by forcing small frequency increases whenever the half-bridge sense resistor

voltage exceeds the HBCSH threshold (see the “Half-Bridge current control” paragraph).

Figure 5, centre and right, shows the L6585D behavior as the lamp gets older; if it doesn’t

ignite for a time longer than the pre-heating one (counted by a cycle charge/discharge of the

TCH pin), the IC is stopped, enters low consumption and waits for either a re-lamp or an

UVLO.

IGN

PRE

⋅⋅

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