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参数资料
| 型号： | FSDL312 |
| 厂商： | FAIRCHILD SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 4 A SWITCHING REGULATOR, PDIP8 |
| 封装： | DIP-8 |
| 文件页数： | 3/16页 |
| 文件大小： | 607K |
| 代理商： | FSDL312 |
FSDL312
11
Figure 6. Over load protection
4.2 Thermal Shutdown (TSD) : The Sense FET and the con-
trol IC are integrated, making it easier for the control IC to
detect the temperature of the Sense FET. When the tempera-
ture exceeds approximately 140
°C, thermal shutdown is acti-
vated.
4.3 Abnormal Over Current Protection (AOCP) :
Figure 7. AOCP Function & Block
Even though the FPSTM has OLP (Over Load Protection)
and current mode PWM feedback, these are not enough to
protect the FPSTM when a secondary side diode short or a
transformer pin short occurs. In addition to start-up, soft-
start is also activated at each restart attempt during auto-
restart and when restarting after latch mode is activated. The
FPSTM has an internal AOCP (Abnormal Over Current Pro-
tection) circuit as shown in figure 7. When the gate turn-on
signal is applied to the power Sense FET, the AOCP block is
enabled and monitors the current through the sensing resis-
tor. The voltage across the resistor is then compared with a
preset AOCP level. If the sensing resistor voltage is greater
than the AOCP level, pulse by pulse AOCP is triggered
regardless of uncontrollable LEB time. Here, pulse by pulse
AOCP stops Sense FET within 350nS after it is activated.
4.4 Over Voltage Protection (OVP) : In case of malfunc-
tion in the secondary side feedback circuit, or feedback loop
open caused by a defect of solder, the current through the
opto-coupler transistor becomes almost zero. Then, Vfb
climbs up in a similar manner to the over load situation, forc-
ing the preset maximum current to be supplied to the SMPS
until the over load protection is activated. Because excess
energy is provided to the output, the output voltage may
exceed the rated voltage before the over load protection is
activated, resulting in the breakdown of the devices in the
secondary side. In order to prevent this situation, an over
voltage protection (OVP) circuit is employed. In general,
Vcc is proportional to the output voltage and the FPSTM uses
Vcc instead of directly monitoring the output voltage. If
VCC exceeds 19V, OVP circuit is activated resulting in ter-
mination of the switching operation. In order to avoid undes-
ired activation of OVP during normal operation, Vcc should
be properly designed to be below 19V.
5. Soft Start : The FPSTM has an internal soft start circuit that
increases the feedback voltage together with the Sense FET
current slowly after it starts up. The typical soft start time is
15msec, as shown in figure 8, where progressive increments
of Sense FET current are allowed during the start-up phase.
The pulse width to the power switching device is progres-
sively increased to establish the correct working conditions
for transformers, inductors, and capacitors. The voltage on
the output capacitors is progressively increased with the
intention of smoothly establishing the required output volt-
age. It also helps to prevent transformer saturation and
reduce the stress on the secondary diode.
1
t
2
t
3
t
4
t
3V
6V
Vcc
8V
Delay current (5uA) charges the Cfb
FPS switching
OLP
Following Vcc
2
.
_
,
8
.
2
,
3
)
1
(
);
)
1
(
1
(
1
fig
fb
C
K
R
V
t
V
R
t
V
In
RC
t
=
=
=
V
t
V
t
V
uA
I
t
V
t
V
C
t
delay
fb
3
)
1
(
)
2
1
(
,
5
;
))
1
(
)
2
1
(
2
=
+
=
+
=
R
SQ
Vsense
Vfb
Out Driver
Rsense
CLK
Drain
V
AOCP
PWM
COMPARATOR
AOCP
COMPARATOR
LEB
1mS
15steps
Current limit
0.98A
2.15A
t
Drain current
[A]
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