AN993 datasheet(21/27 Pages) STMICROELECTRONICS

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AN993

Design tips

Doc ID 5656 Rev 10

21/27

Design tips

5.1

Inductance and capacitor evaluation

To design an application with the L6574, a preliminary evaluation of the components can be

done by fixing the lamp type and its electrical characteristics only.

This evaluation is an "iterative" process because some assumptions have to be made that

need to be checked at the end of the process. Figure 19 shows a simplified schematic of the

load.

In Figure 19 r is the lamp filament resistance, rr is the operating lamp equivalent resistance

when the lamp is off or during preheating. rr is an open circuit.

First of all, one has to evaluate a proper inductance value.

L has to provide the right current value to the lamp when it is already ignited and is working

("choke" inductance). Therefore, L depends on the current required by the lamp, that is to

say on the lamp’s operating wattage and voltage, on the operating frequency fmin, but also

on the voltage across L and the lamp (Vb). The greater the Vb variation, the greater the

inductance required to give a constant current to the lamp. Vb variations are due to the high-

voltage bus variations and to the ripple on the half battery capacitor (Cb). The ripple

depends on the size of Cb and a proper hypothesis has to be done on it to estimate the

variation of Vb. Taking into account all this data and hypothesis, one can assume that during

an operating condition most of the current flows into the lamp, not into C, and all the power

delivered to the system is delivered to the lamp. Vlamp is the operating voltage across the

lamp and Plamp is the operating lamp wattage, so a good approximation to conduct the

choke inductance is:

Equation 16

The second step is the evaluation of the capacitance across the lamp (C).

Figure 19.

Simplified schematic of the lamp

AM01327v1

lamp

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