A5975AD datasheet(33/50 Pages) STMICROELECTRONICS

33 / 50 page

A5975AD

Application information

Doc ID 018761 Rev 1

33/50

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

phases:

On phase

Equation 29

Off phase

Equation 30

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 instead maximized as approximately equal to VIN.

So, Equation 29 and 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 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 32)

does not compensate the current increase during TON (Equation 31). Figure 21 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 Equation 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.

L TON

out

–

DCR

DS(on)

() I

⋅

–

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

()

L TOFF

out

DCR

⋅

()

–

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

OFF

()

L TON

DCR

DS(on)

() I

⋅

–

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

ON MIN

()

--------- 250ns

()

≅

L TOFF

out

DCR

⋅

()

–

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

⋅

()

out

DCR

⋅

()

–

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

μs

()

≅

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