L99LD21 datasheet(20/73 Pages) STMICROELECTRONICS

20 / 73 page

Buck converters

L99LD21

20/73

DS11130 Rev 5

where

ΔIL1_PP is the inductor peak to peak current and VF_D1 is the forward voltage of the

diode D1. As D1 is a Schottky diode with a low forward voltage, VF_D1 can be in general

neglected, compared to VLED1.

Note:

Once the VLEDxTOFF constant for a given buck converter is selected by SPI, the peak-to-

peak inductor current ripple is constant. In particular, it depends neither on the boost voltage

nor on the LED forward voltage.

The ripple current through the LED strings is reduced by means of an external capacitor in

parallel with the LEDs.

Figure 8. Inductor and mosfet current waveforms

Referring to the Figure 7 and Figure 8 the average LED current - valid for both Buck 1 and

Buck 2 - is therefore:

where IL1_PEAK* results from IL1_PEAK (see Table 40) corrected with loop delay (tloop_delay)

In order to achieve the best accuracy versus input voltage variation during current sensing

process, a defined buck input voltage window must be selected, by means of a dedicated

control register (see bits <5:4> and bits <3:2> on Table 16: CR#3: Control Register 3).

3.4

Buck converter’s blank time

The buck converters have a minimum on-time TBLANK_BUCK. Although the inductor’s target

peak current ILx_PEAK is reached before this time has elapsed, the switch is kept on. This

delay is used as a leading-edge blank time, in order to avoid a premature end of the

switching cycle, which might be caused by the noise, which results from the commutation of

the buck’s mosfet.

LED1_AVG

L1_AVG

L1_PEAK*

ΔI

L1_PP

()

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

–

L1_PEAK*

LED TOFF1

⋅

()

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

–

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