L4990 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L4990' PDF : 24 Pages View PDF
L4990 - L4990A
Tss
≅
3
⋅
Rsense ⋅
ISSC
IQpk
⋅
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.
Figure 23. Regulation characteristic and re-
lated quantities
VOUT
D.C.M.
A
C.C.M.
TON
D97IN495
B
D
ISHORT IOUT(max)
IQpk
1-2 ·IQpk
IQpk(max)
C
TON(min)
IOUT
Figure 24. Hiccup mode operation.
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
⋅
1
ISSC
+
1
ISSD
⋅
Css
(7)
Since the system tries restarting each hiccup cy-
cle, there is not any latchoff risk.
IOUT
ISEN
FAULT
SS
5V
0.5V
SHORT
7V
time
Thic
D97IN496
11/24
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