AN2870 datasheet(18/41 Pages) STMICROELECTRONICS

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Device blocks description

AN2870

18/41

Lamp to ground

(Figure 11): the resistive divider senses the voltage across the lamp. As

the L6585DE doesn’t have a negative rail, it is necessary to shift the external signal; this can

be done (for example) using two Zener diodes connected back-to-back between the EOL

pin and the centre of the resistive divider.

The Zener voltages should differ by an amount as close as possible to the double of the

internal reference to have a symmetrical detection, in fact:

Let be VUP and VDOWN the maximum allowed values of the DC component of the Lamp

Voltage divided by the divider factor KD, W the window amplitude, VZ the Zener voltage of a

Zener and VF the forward voltage of the Zener (@ 5.5 μA)

●

VUP = VLAMP,MAX/KD = VREF + W/2 + VZ1 + VF

●

VDOWN = VLAMP,MIN/KD = VREF – W/2 – VZ2 – VF

It must be:

●

VUP = - VDOWN

therefore:

●

2 VREF = VZ2 − VZ1

The biasing current available at pin EOL is equal to 5.5

μA then the V

Z1 Voltage should be

greater than 8 V in order to have a more precise EOL threshold.

In this case the window comparator can be referenced to the 2.5 V internal reference as

external disturbances don’t influence the lamp voltage mean value (Fixed reference

configurations).

In the Figure 11 is shown the case of asymmetric rectification with positive shifting.

To avoid an immediate intervention of the EOL protection, a filtering is introduced: as soon

as the voltage at pin EOL goes outside the window of the comparator a Tch cycle is started.

The IC is stopped if, at the end of the Tch cycle, the EOL voltage is again outside the limits.

AN2870 Datasheet(PDF) 18 Page - STMicroelectronics