AN4460 datasheet(11/54 Pages) STMICROELECTRONICS

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AN4460

Design concept

The STEVAL-ILL057V1 demonstration board is based on a modified buck topology

(reversed buck) and the fixed-off-time (FOT) principle is used to regulate the current into

each LED string. The advantage of the FOT principle is that the high frequency current

ripple and, therefore, the average LED current depend only on the off-time and the LED

output voltage. With a fixed-off-time, the output voltage depends on the temperature and the

number of LEDs. To compensate this effect, the STLUX385A implements the adaptive

voltage compensation algorithm, which constantly monitors the LED voltage drop and

adjusts the off-time. The input voltage variations do not affect the average current.The

average LED current is directly controlled by regulating the LED peak current (configurable),

which is sensed during the MOSFET on-time.

While the off-time parameter is fixed, the on-time is a function of the time that the current

takes to reach the selected level: this time depends on the LED current, power voltage and

LED voltage. The firmware implements the following two functions to compute the OFF and

ON-time:

Equation 1

Equation 2

where:



VLED: output voltage of the LED string (equal to VLED_PW -VCOMx signals on the

schematics)



IMAX: maximum (peak) current imposed by user and defined by Table 1 (near

92 mA/step)



IAVR: current imposed by design (IMAX - 10%)



L: inductance imposed by design (470 µH)



VLED_PW: input voltage present at the main power pin

The on-time calculated in Equation 2 has to be considered as an ideal value: the real one is

actually a consequence of the triggering of a comparator monitoring the peak value of the

LED current. For this reason, a percentage of the calculated on-time is added (Tx). This is

empirically imposed by adding 20% to the on-time and doubling the resulting value.

Equation 3

TOFF

2IMAX IAVR

–





 L



VLED

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

TOFF

2IMAX IAVR

–





 L



VLED_PW VLED

–

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

TON MAX



TON TX

 2



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