LED5000 datasheet(30/52 Pages) STMICROELECTRONICS

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Application notes - buck conversion

LED5000

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Doc ID 023951 Rev 1

Equation 45

where DCRL is the series resistance of the inductor and VFWDIODE is the forward voltage

drop across the external rectifying diode.

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

when:

Equation 46

From

Equation 44 and Equation 45 we can gather that the implementation of the constant

current protection becomes more critical the lower is the VOUT and the higher is 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 forward voltage of the diode) since VOUT is negligible, while during TON the

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

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

voltage. The

Equation 44 and Equation 45 in overcurrent conditions can be simplified to:

Equation 47

considering TON that has been already reduced to its minimum.

Equation 48

where TSW=1/fSW considering the nominal fSW.

At high input voltage

ΔI

L TON could be higher than

ΔI

L TOFF and so the inductor current could

escalate. As a consequence, the system typically meets the

Equation 46 at a current level

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

components. In most 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. The hiccup protection offers an additional

protection against heavy short-circuit condition at very high input voltage even considering

the spread of the minimum conduction time of the power element. In case the hiccup current

level (6.2 A typical) is triggered the switching activity is prevented for 16 msec typ. (see

hiccup time in

Table 5: Electrical characteristics).

Figure 19 shows the operation of the constant current protection when a short-circuit is

applied at the output at the maximum input voltage.

L TON

OUT

DCR

FW DIODE

⋅

()

–

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

OFF

()

L TON

L TOFF

L TON

DCR

DSON HS

() I

⋅

–

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

ON MIN

()

--------- 90ns

()

≅

L TOFF

DCR

⋅

FW DIODE

()

–

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

90ns

–

()

DCR

⋅

FW DIODE

()

–

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

μs

()

≅

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