ST1CC40 datasheet(27/37 Pages) STMICROELECTRONICS

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ST1CC40

Application information

The pulse-by-pulse current limitation is effective in implementing constant current protection

when:

Equation 38

From Equation 36 and Equation 37 we can gather that the implementation of the constant

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

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 RDS(on) of the low-side switch) since VOUT is negligible, while during TON

the voltage applied at 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. Equation 36 and Equation 37 in overcurrent conditions can be simplified to:

Equation 39

considering TON that has already been reduced to its minimum.

Equation 40

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

may escalate. As a consequence, the system typically meets Equation 38 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 condition at very high input voltage even considering the spread

of the minimum conduction time of the power element. If the hiccup current level (6.2 A typ.)

is triggered, the switching activity is prevented for 12 cycles.

Figure 15 shows the operation of the constant current protection when a short-circuit is

applied at the output at the maximum input voltage.

IL TON



IL TOFF



DCR

IL TON



VIN DCRL RDS(on) HS

 I



–

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



VIN

--------- 90ns





IL TOFF



DCRL RDS(on) LS



–



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

–



DCRL RDS(on) LS



–



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

s





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