A5974D datasheet(31/46 Pages) STMICROELECTRONICS

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A5974D

Application information

8.5

Short-circuit protection

In overcurrent protection mode, when the peak current reaches the current limit, the device

reduces the TON down to its minimum value (approximately 250 nsec) and the switching

frequency to approximately one third of its nominal value even when synchronized to an

external signal (see Section 5.4: Current protection on page 11). In these conditions, the

duty cycle is strongly reduced and, in most applications, this is enough to limit the current to

ILIM. In any event, in case of heavy short-circuit at the output (VO = 0 V) and depending on

the application conditions (VCC value and parasitic effect of external components) the

current peak could reach values higher than ILIM. 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 freewheeling diode), while during TON the voltage applied

the inductor is instead maximized as approximately equal to VIN.

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

Equation 35

considering TON that has been already reduced to its minimum.

Equation 36

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 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 could occur that the ripple current during TOFF (Equation 36)

does not compensate the current increase during TON(Equation 35). The 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 Equation 36 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 RDSON

 I



–

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



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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