A5975D datasheet(33/50 Pages) STMICROELECTRONICS

33 / 50 page

A5975D

Application information

Doc ID 018760 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 maximized instead as approximately equal to VIN.

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

Equation 31

considering TON which has been already 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 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 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 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

()

≅

A5975D датащи(PDF) 33 Page - STMicroelectronics

A5975D датащи(HTML) 33 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

датащи скачать

ссылки URL