FSD200
8
Figure 4. Protection block
4.1 Over Load Protection (OLP) :
Overload is a load
current that exceeds a pre-set level due to an abnormal
situation. If this occurs, the protection circuit should be
triggered to protect the SMPS. It is possible that a short
term load transient can occur under normal operation. If
this occurs the system should not shut down. In order
to avoid false shut-downs, the over load protection
circuit is designed to trigger after a delay. Therefore the
device can discriminate between transient overloads
and true faul conditions. The device is pulse-by-pulse
current limited and therefore, for a given input voltage,
the maximum input power is limited. If the load tries to
draw more than this, the output voltage will drop below
its set value. This reduces the opto-coupler LED
current which in turn will reduce the photo-transistor
current. Therefore, the 250uA current source will
charge the feedback pin capacitor, Cfb, and the
feedback voltage, Vfb, will increase. The input to the
feedback comparator is clamped at around 3V.
Therefore, once Vfb reaches 3V, the device is
switching at maximum power. At this point the 250uA
current source is blocked and the 5uA source continues
to charge Cfb. Once Vfb reaches 4V, switching stops.
Therefore the shutdown delay time is set by the time
required to charge Cfb from 3V to 4V with 5uA as
shown in Fig. 5.
Figure 5. Over load protection delay
4.2 Thermal Shutdown (TSD) :
The SenseFET and
the control IC are assembled in one package. This
makes it easy for the control IC to detect the
temperature of the SenseFET. When the temperature
exceeds approximately 150
°
C, thermal shutdown is
activated. Thermal shutdown has a Hysteresis of 50
°
C
and so the temperature must drop to 100
°
C before the
device attempts to restart.
5. Soft Start
: FSD200/210 has an internal soft start
circuit that increases the feedback voltage together
with the MOSFET current slowly at start up. The soft
start time is 3msec in FSD200/210.
OSC
4
Vfb
S
R
Q
GATE
DRIVER
FSD2xx
OLP, TSD
Protection Block
5uA
250uA
RESET
Vth 4V
OLP
+
-
TSD
His 50
℃
S
R
Q
/8
Cfb
3V
R
Vfb
t
3V
OLP
4V
t1
t3
10V
t1<<t2, t3
t1 = -1/RC
Χ
ln( 1-v(t1)/R )
v(t1)=3V
t2 = C
Χ
[v(t1+t2)-v(t1)]
Χ
Idelay
[v(t1+t2)-v(t1)]=1V, Idelay=5uA
t2
FPS Switching Area
Idelay (5uA) charges Cfb
TSD - 50℃
Temperature
t
TSD(
℃)
FPS Switching Area
TSD Hysteresis
