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參數(shù)資料
| 型號： | LM3101 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Secondary-Side PWM Controller |
| 中文描述： | 二次側(cè)PWM控制器 |
| 文件頁數(shù)： | 11/20頁 |
| 文件大小： | 319K |
| 代理商： | LM3101 |
Functional Description
(Continued)
Soft-Start Section
Soft-Start is accomplished by gradually increasing the refer-
ence voltage during start-up. The gradual increase is imple-
mented by charging the Soft-Start capacitor, C
SFST
, on pin
3 (the SFST pin). The charging current is set according to
the following equation:
I
SFST
e
0.45/R
T
[
A,
X
]
.
TL/H/11436–12
FIGURE 6. Soft-Start Block Diagram
Typically, I
SFST
starts to flow when the supply voltage is
raised above 3V.
As shown inFigure 6, at the beginning of start-up, C
SFST
is
not charged up, and the SFST pin pulls down the reference
voltage from its nominal value to:
V
SFSTO
e
I
SFST
#
R
1
V
SFSTO
is designed to be 85% of the nominal reference
voltage. The reference voltage rises smoothly from V
SFSTO
[
V, A,
X
]
.
to its nominal value as C
SFST
charges up. When C
SFST
charges up completely, the reference voltage is at its nomi-
nal value, start-up is over and steady-state operation be-
gins.
The discharge time of C
SFST
is set by the RC network as:
t
DS
e
5
#
C
SFST
#
R
2
.
This is the time delay required to prepare the Soft-Start cy-
cle after the LM3101 and the entire regulator has been
turned off. R
2
must be large enough so that the final value
of V
SFST
is greater than the reference voltage:
V
SFST
e
I
SFST
#
(R
1
a
R
2
)
t
1.24V
[
V, A,
X
]
.
Power Supply Monitor Functions
The LM3101 provides two monitor functions, a power-on
reset flag with programmable delay, and a crowbar driver
output for overvoltage conditions.
POWER-ON RESET
The power-on reset (POR) flag monitors the output voltage
via the feedback pin (FB - pin 13). The POR flag will go low
after the output voltage reaches 95% of its nominal value,
and the subsequent programmed delay has passed. The
POR flag pin (pin 11) is an open-collector pin which needs
an external resistor to pull it up. This pin is valid with supply
voltages as low as 1V while sinking 1.6 mA.
To program the reset delay, connect an external capacitor
to the C
RD
pin (pin 12). The practical range of delay is from
10
m
s to 5 ms, and follows the equation:
T
RD
e
C
RD
#
60
#
10
3
For a power-on reset delay of 120
m
s, the reset delay ca-
pacitor must be 0.002
m
F.
[
s, F
]
.
CROWBAR DRIVER OUTPUT
The second monitor function is a crowbar driver output
(CD–pin 10). If the output voltage gets higher than 120% of
its nominal value, the CD pin can supply more than 200 mA
to an external SCR trigger input. The SCR will fire, shorting
the regulator output and saving the load circuitry from ex-
cessive supply voltage.
11
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