L99LD21 datasheet(21/73 Pages) STMICROELECTRONICS

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DS11130 Rev 5

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L99LD21

Buck converters

3.5

Buck converter’s start-up

While the device and the system are protected against short circuit conditions of the buck’s

output to GND, the device inhibits the detection of the short circuit during the startup phase

TSTARTUP.

A startup phase is applied in the following conditions:

•

If one of the buck converters is activated for the first time after a power on reset (POR),

including buck activation after device wake-up;

•

If one of the buck converters has been deactivated for more than tDELAY;

•

If one of the buck converters has been latched off prior to a Read and Clear command;

•

If one of the buck converters is re-activated after a VS under voltage event.

After these events, it is possible that the output capacitors of the buck converters are

completely discharged. The charging of the buck output capacitors might lead switching

cycles with short on-time (shorter than TON_MIN), which could potentially lead to a wrong

detection of a shorted buck output. The introduction of this start-up phase avoids this wrong

diagnostic.

3.6

Switching frequency

For a given buck converter, the switching frequency depends on the buck input voltage

(output of the boost controller) and the forward voltage of the LED string, which is connected

to its output.

In continuous conduction mode, TOFF is given by:

Where D is the buck converter’s duty cycle, T and FSW are respectively the switching period

and frequency.

Neglecting the drop voltage across the mosfet, the inductor’s DC resistance and the diode’s

forward voltage, compared to VBUCKIN and VLED, we have:

For a given application (given inductance and VLED), it is possible to set ILEDx_AVG by

selecting different combinations of ILx_PEAK and VLEDxTOFF in order to avoid critical

frequency ranges such as the AM radio band.

To avoid buck operation at not allowed TON and/or TOFF times, frequency range has to be

kept inside FSWmin and FSWmax, where:

FSWmin = 1/(TON_MAX_BUCK + TOFF_MAX_BUCK)

FSWmax = 1/(TON_MIN_BUCK + TOFF_MIN_BUCK)

OFF

–

() T

⋅

–

-------------

LED

BUCKIN

-------------------------

LED

BU CKIN

-------------------------

–

OFF

-----------------------------------

LED

BUCKIN

-------------------------

–







⋅

LED TOFF

⋅

------------------------------------------------------------

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