L99LD01 datasheet(24/69 Pages) STMICROELECTRONICS

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Functional description

L99LD01

24/69

DocID025319 Rev 3

If a LED overvoltage failure occurs afterwards, the failure will be latched and the converter is

stopped until a read and clear of the status register 1.

Note that during tDStart, the converter is stopped to enable the buffer capacitor C9 to charge

at a sufficient voltage to correctly drive the mosfet M2. This delay prevents the converter to

turn on, while M2 stays off, avoiding a LED overvoltage event.

If the application uses a big capacitor(a), it is recommended to keep the PWM_L signal low

after a power on reset or after a VS under voltage, until C9 is totally charged, to avoid a LED

overvoltage. Figure 13 shows the device behavior in case of no LED overvoltage failure,

after tDStart.

If C9, after tDStart time, should be not enough charged to allow correct driving operation, a

possible LED overvoltage will appear when, the converter will be released. Figure 14 shows

what happens in this case.

After tDStart, the converter is released while the C9 capacitor is only partially charged.

Consequently, VLED increases up to LED OV_TH1 and a LED overvoltage event is detected

during the tEnRecov phase. The LED_Ov_Rec bit is not reset at the end of the tEnRecov phase

due to the LED overvoltage event. The discharge path is activated until VLED crosses LED

OV_TH2. Then, the LED_Ov_Rec bit is reset, the converter is released, and the buffer

capacitor C9 is now fully charged, enabling the dimming mosfet M2 to turn on.

Figure 15 shows the case of LED_Ov_Rec bit during a start up with a rising edge on

PWM_L = High after the expiration of tDStart. In this case, the tEnRecov phase starts only

when the PWM_L signal goes High. Figure 16 shows the case of LED overvoltage event,

which could appear during normal functioning.

The LED overvoltage status bit is set (latched) and the discharge path is activated until VLED

crosses LED OV_TH2. The converter is stopped, independently from PWM_L, until a read

and clear command of the status register 1 (LED_Ov_Rec bit is reset).

If a LED overvoltage failure event occurs during VS overvoltage, (battery OV), the discharge

path for the output capacitor is inhibited and the LED overvoltage status bit is not set.

When the VS overvoltage event disappears, (VS crosses VS OV_TH2), the LED overvoltage

status bit is set (latched) and the discharge path is activated until VLED crosses LED

OV_TH2. The converter is stopped, independently from PWM_L, until the LED ov status bit

is cleared (read and clear of the status register 1). Figure 17 shows such a case.

Finally Figure 18 shows how will be managed the LED_Ov_Rec bit in case signal PWM_L

has a low on-time. In this case the LED_Ov_Rec bit is reset when the cumulated running

time of tEnRecov exceeds typ. 5 ms. This feature enables a single recovery of a LED

overvoltage event, due to a too fast regulation loop (set by the resistor and capacitor

connected to RCCOMP pin), even in PWM operation with low on-time. However, a proper

choice of RC network values, avoiding fast transients on the LED string voltage, when the

converter is switched ON, it is carefully recommended

More than 22 nF

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