L99LD01 datasheet(28/69 Pages) STMICROELECTRONICS

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Functional description

L99LD01

28/69

DocID025319 Rev 3

voltage string, according to the equation reported below, the LED overvoltage thresholds

are given by the following formulas:

VLED_OV =OV_TH1 / KL; being KL =R6 / (R5+R6);

OV_TH1 is the reference for the OV internal comparator. Typical value for OV_TH1 is 3.5 V.

LED OV event makes the converter and also mosfet M2 immediately switched OFF, in order

to prevent any damage to the LED string or to the driver. Furthermore, following an OV

event, the LED_OV status register is set and an internal load is applied between ISENSE+ pin

and ground in order to fast discharge the voltage across capacitor C4.

In the boost topology application, if a short circuit between the source of external mosfet M2

and GND occurs, an uncontrolled current could flow. In order to avoid this situation, a

maximum LED current protection has been inserted, which continuously monitors the

voltage across the sense resistor RSENSE. If this voltage reaches a value in excess of an

internal fixed threshold of (see Table 24 - LED over current protection threshold parameter),

the status bit LED_OC (led over current) is set and the converter and also mosfet M2 will be

immediately switched OFF.

Following a stop of the converter due to an OV event, the device can not be restarted before

of C4 discharge (VLED is below OV_TH2).

After an OV event, the converter could restart if a read and clear command of the LED_OV

status bit is done. Table 3 summarizes the suggested value of KL resistor ratio, supposing to

have a LED_OV event, when the voltage across LED string, reaches a value in excess of

50 % of its nominal value.

Figure 19. LED chain overvoltage thresholds settings. An example for boost and fly

back converters

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