LED2001PUR datasheet(29/42 Pages) STMICROELECTRONICS

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DocID024346 Rev 1

29/42

LED2001

Application information

The inductor current ripple during ON and OFF phases can be written as:

•

ON phase

Equation 40

•

OFF phase

Equation 41

where DCRL is the series resistance of the inductor.

The pulse-by-pulse current limitation is effective to implement constant current protection

when:

Equation 42

From Equation 40 and Equation 41 it can be seen that the implementation of the constant

current protection becomes more critical the lower the VOUT and the higher the VIN.

In fact, in short-circuit condition the voltage applied to the inductor during the OFF-time

becomes equal to the voltage drop across parasitic components (typically the DCR of the

inductor and the RDSON of the low-side switch) since VOUT is negligible, while during TON

the voltage applied at the inductor is maximized and is approximately equal to VIN.

In general, the worst case scenario is heavy short-circuit at the output with maximum input

voltage. Equation 40 and Equation 41 in overcurrent conditions can be simplified to:

Equation 43

considering TON which has already been reduced to its minimum.

Equation 44

where TSW=1/fSW and considering the nominal fSW.

At higher input voltage

ΔI

L TON may be higher than

ΔI

L TOFF and so the inductor current can

escalate. As a consequence, the system typically meets Equation 42 at a current level

higher than the nominal value thanks to the increased voltage drop across stray

components. In most of the application conditions the pulse-by-pulse current limitation is

effective to limit the inductor current. Whenever the current escalates, a second level current

protection called “Hiccup mode” is enabled. Hiccup protection offers an additional protection

against heavy short-circuit conditions at very high input voltage even considering the spread

L TON

OUT

–

DCR

DSON HS

() I

⋅

–

----------------------------------------------------------------------------------------------- TON

()

L TON

OUT

DCR

DSON LS

() I

⋅

()

–

---------------------------------------------------------------------------------------- TOFF

()

L TON

L TOFF

L TON

DCR

DSON HS

() I

⋅

–

------------------------------------------------------------------------ TON MIN

()

--------- 90ns

()

≅

L TOFF

DCR

DSON LS

()

–

⋅

-------------------------------------------------------------- TSW 90ns

–

()

DCR

DSON LS

()

–

⋅

-------------------------------------------------------------- 1.18

μs

()

≅

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