R5972D datasheet(24/35 Pages) STMICROELECTRONICS

24 / 35 page

Application information

R5972D

24/35

DocID026873 Rev 1

So Equation 28 and Equation 29 in overcurrent conditions can be simplified to:

Equation 30

considering TON that has been already reduced to its minimum.

Equation 31

considering that fSW has been already reduced to one third of the nominal.

In case a short-circuit at the output is applied and VIN = 12 V the inductor current is

controlled in most of the applications (see Figure 13). When the application must sustain the

short-circuit condition for an extended period, the external components (mainly the inductor

and diode) must be selected based on this value.

In case the VIN is very high, it could occur that the ripple current during TOFF (Equation 31)

does not compensate the current increase during TON (Equation 30). Figure 15 shows an

example of a power-up phase with VIN = VIN MAX = 36 V whereIL TON > IL TOFF, so the

current escalates and the balance between Equation 30 and Equation 31 occurs at a current

slightly higher than the current limit. This must be taken into account in particular to avoid

the risk of an abrupt inductor saturation.

Figure 13. Short-circuit current VIN = 12 V

IL TON



VIN DCRL RDSON

 I



–

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



VIN

--------- 250ns





IL TOFF



VD Vout DCRL I





–

--------------------------------------------------------------- 3T





VD Vout DCRL I





–

--------------------------------------------------------------- 12

s





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