L4990 datasheet(11/24 Pages) STMICROELECTRONICS

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Tss ≅

⋅ Rsense ⋅ IQpk

ISSC

⋅ Css

(6)

where Rsense is the current sense resistor (see pin

13) and IQpk is the switch peak current (flowing

through Rsense), which depends on the output

load. Usually, CSS is selected for a TSS in the or-

der of milliseconds.

As mentioned before, the soft-start intervenes

also in case of severe overload or short circuit on

the output. Referring to fig. 23, pulse-by-pulse

current limitation is somehow effective as long as

the ON-time of the power switch can be reduced

(from A to B). After the minimum ON-time is

reached (from B onwards) the current is out of

control.

To prevent this risk, a comparator trips an over-

current handling procedure, named ’hiccup’ mode

operation, when a voltage above 1.2V (point C) is

detected on current sense input (ISEN, pin 13).

Basically, the IC is turned off and then soft-started

as long as the fault condition is detected. As a re-

sult, the operating point is moved abruptly to D,

creating a foldback effect. Fig. 24 illustrates the

operation.

The oscillation frequency appearing on the soft-

start capacitor in case of permanent fault, referred

to as ’hiccup” period, is approximately given by:

Thic ≅ 4.5 ⋅ 





ISSC

ISSD







⋅ Css (7)

Since the system tries restarting each hiccup cy-

cle, there is not any latchoff risk.

VOUT

TON

D.C.M.

C.C.M.

IQpk

TON(min)

1-2 ·IQpk

IQpk(max)

IOUT

ISHORT IOUT(max)

D97IN495

Figure 23. Regulation characteristic and re-

lated quantities

Thic

time

SHORT

IOUT

ISEN

FAULT

0.5V

D97IN496

Figure 24. Hiccup mode operation.

L4990 - L4990A

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