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參數(shù)資料
| 型號： | L4990D |
| 廠商： | STMICROELECTRONICS |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 1.5 A SWITCHING REGULATOR, 1000 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封裝： | SO-16 |
| 文件頁數(shù)： | 3/24頁 |
| 文件大小： | 322K |
| 代理商： | L4990D |
Tss
3
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-
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.
VOUT
TON
D.C.M.
C.C.M.
D
A
B
C
IQpk
TON(min)
1-2 IQpk
IQpk(max)
IOUT
ISHORT IOUT(max)
D97IN495
Figure 23. Regulation characteristic and re-
lated quantities
7V
Thic
time
SHORT
IOUT
ISEN
FAULT
SS
5V
0.5V
D97IN496
Figure 24. Hiccup mode operation.
L4990 - L4990A
11/24
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