R5973D datasheet(27/39 Pages) STMICROELECTRONICS

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R5973D

Application information

Equation 29

where VD is the voltage drop across the diode, DCRL is the series resistance of the inductor.

In short-circuit conditions VOUT is negligible so during TOFF the voltage across the inductor

is very small as equal to the voltage drop across parasitic components (typically the DCR of

the inductor and the VFW of the freewheeling diode) while during TON the voltage applied the

inductor is instead maximized as approximately equal to VIN.

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 15). 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 17 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 15. Short-circuit current VIN = 12 V

IL TOFF



VD Vout DCRL I





–

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



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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