FSDM07652R Fairchild Semiconductor, FSDM07652R Datasheet - Page 12

• Manufacturer Part Number: FSDM07652R
• Description: SMPS Power Switch For LCD Monitors Flyback, (Green)The FSDM07652R is an Integrated Pulse Width Modulator (PWM) And Sense Fet Specifically Designed For High Performance Offline Switch Mode Power Supplies (SMPS) With Minimal External Components. This D
• Manufacturer: Fairchild Semiconductor

FSDM07652R
3.1 Over Load Protection (OLP) : Overload is defined as
the load current exceeding a pre-set level due to an unex-
pected event. In this situation, the protection circuit should
be activated in order to protect the SMPS. However, even
when the SMPS is in the normal operation, the over load pro-
tection circuit can be activated during the load transition. In
order to avoid this undesired operation, the over load protec-
tion circuit is designed to be activated after a specified time
to determine whether it is a transient situation or an overload
situation. Because of the pulse-by-pulse current limit capa-
bility, the maximum peak current through the Sense FET is
limited, and therefore the maximum input power is restricted
with a given input voltage. If the output consumes beyond
this maximum power, the output voltage (Vo) decreases
below the set voltage. This reduces the current through the
opto-coupler LED, which also reduces the opto-coupler tran-
sistor current, thus increasing the feedback voltage (Vfb). If
Vfb exceeds 2.5V, D1 is blocked and the 3.5uA current
source starts to charge C
tion, Vfb continues increasing until it reaches 6V, when the
switching operation is terminated as shown in figure 7. The
delay time for shutdown is the time required to charge C
from 2.5V to 6.0V with 3.5uA. In general, a 10 ~ 50 ms
delay time is typical for most applications.
12
Vds
Vcc
12V
8V
Power
on
Figure 6. Auto restart operation
operation
Normal
occurs
B
Fault
slowly up to Vcc. In this condi-
situation
Fault
removed
Fault
operation
Normal
B
t
3.2 Abnormal Over Current Protection (AOCP) : Even
though the FPS
rent mode PWM feedback, these are not enough to protect
the FPS
former pin short occurs. The FPS
(Abnormal Over Current Protection) circuit as shown in fig-
ure 8. When the gate turn-on signal is applied to the power
Sense FET, the AOCP block is enabled and monitors the cur-
rent through the sensing resistor. The voltage across the
resistor is then compared with a preset AOCP level. If the
sensing resistor voltage is greater than the AOCP level for
longer than 300ns, the reset signal is applied to the latch,
resulting in the shutdown of SMPS.
3.3 Over voltage Protection (OVP) : If the secondary side
feedback circuit were to malfunction or a solder defect
caused an open in the feedback path, 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 more
energy than required is provided to the output, the output
6.0V
2.5V
V
FB
2.5R
R
TM
when a secondary side diode short or a trans-
AOCP
Figure 7. Over load protection
OSC
TM
T
1
PWM
has OLP (Over Load Protection) and cur-
Figure 8. AOCP block
LEB
T
Over load protection
S
R
12
= Cfb*(6.0-2.5)/I
Q
Q
+
-
TM
has an internal AOCP
Vaocp
driver
Gate
delay
R
sense
T
2
2
GND
t