L99LD21 datasheet(19/73 Pages) STMICROELECTRONICS

19 / 73 page

DS11130 Rev 5

19/73

L99LD21

Buck converters

Figure 7. Peak current control principle

At the beginning of a switching period the MOSFET M1 is turned on, and the inductor

current IL1 increases. The mosfet is activated for a minimum on-time TON_MIN in order to

avoid that the on-phase is ended up by spurious noise, which is caused by the switch-on.

During mosfet activation, the inductor current, IL1, increases until reaching a maximum

value, IL1_PEAK, which is set through a dedicated control register (see bits <23:18> and bits

<17:12> on Table 15: CR#2: Control Register 2). When IL1 reaches its peak value, the

switching mosfet is turned off. The mosfet remains off for a time TOFF, which is derived from

the configured constant VLED1xTOFF1 (see bits <11:8> and bits <7:4> on Table 15: CR#2:

Control Register 2), where VLED1 is the forward voltage of the LED string, which is

connected at the output of the buck converter 1.

During TOFF, the inductor current decreases by:

ΔI

L1_PP

LED 1

F_D1

–

()

--------------------------------------------- T

OFF1

LED 1 TOFF1

⋅

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

∼

⋅

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