LED2000DR datasheet(29/40 Pages) STMICROELECTRONICS

29 / 40 page

DocID023432 Rev 4

29/40

LED2000

Application information

In overcurrent condition, the duty cycle is strongly reduced and, in most applications, this is

enough to limit the switch current to the current threshold.

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

I

L TON may be higher than IL TOFF and so the inductor current can

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

IL TON



VIN VOUT

–

DCRL RDSON HS

 I



–

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



IL TON



VOUT

DCRL RDSON LS

 I





–

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



IL TON



IL TOFF



IL TON



VIN DCRL RDSON HS

 I



–

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



VIN

--------- 90ns





IL TOFF



DCRL RDSON LS



–



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

–



DCRL RDSON LS



–



-------------------------------------------------------------- 1,18

s





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