A5974AD datasheet(32/49 Pages) STMICROELECTRONICS

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Application information

A5974AD

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Doc ID 018762 Rev 1

the inductor and the VFW of the free-wheeling diode) while during TON, the voltage applied

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

So, Equation 29 and the 30 in overcurrent conditions can be simplified to:

Equation 31

considering TON, which has already been reduced to its minimum.

Equation 32

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 18). 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 32)

does not compensate the current increase during TON(Equation 31). Figure 20 shows an

example of a power-up phase with VIN = VIN MAX = 36 V where ΔIL TON > ΔIL TOFF so the

current escalates and the balance between Equation 31 and 32 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 18.

Short-circuit current VIN = 12 V

L TON

DCR

DS(on)

() I

⋅

–

------------------------------------------------------------------ T

ON MIN

()

--------- 250ns

()

≅

L TOFF

out

DCR

⋅

()

–

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

⋅

()

out

DCR

⋅

()

–

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

μs

()

≅

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