R5975D datasheet(33/47 Pages) STMICROELECTRONICS

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R5975D

Application information

This can be understood considering the inductor current ripple during the ON and OFF

phases:



ON phase

Equation 33



OFF phase

Equation 34

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 free-wheeling diode), while during TON, the voltage applied

to the inductor is maximized instead as approximately equal to VIN.

So, Equation 33 and 34 in overcurrent conditions can be simplified to:

Equation 35

considering TON which has been already reduced to its minimum.

Equation 36

considering that fSW has already been 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 19). 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 may happen that the ripple current during TOFF (Equation 36)

does not compensate the current increase during TON (Equation 35). Figure 21 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 35 and 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.

IL TON



VIN Vout

–

DCRL RDS(on)

 I



–

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



IL TOFF



VD Vout DCRL I





–

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



IL TON



VIN DCRL RDS(on)

 I



–

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



VIN

--------- 250ns





IL TOFF



VD Vout DCRL I





–

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





VD Vout DCRL I





–

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

s





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