FSCM0565R Fairchild Semiconductor, FSCM0565R Datasheet - Page 10

• Manufacturer Part Number: FSCM0565R
• Description: Green Mode Fairchild Power Switch Fps
• Manufacturer: Fairchild Semiconductor

Functional Description
. . . . Startup : Figure 4 shows the typical startup circuit and
transformer auxiliary winding for FSCM0565R application.
Before
consumes only startup current (typically 25uA) and the
current supplied from the DC link supply ccurrent consumed
by FPS (Icc) and charges the external capacitor (C
connected to the Vcc pin. When Vcc reaches start voltage of
12V (V
current consumed by FSCM0565R increases to 3mA. Then,
FSCM0565R continues its normal switching operation and
the power required for this device is supplied from the
transformer auxiliary winding, unless Vcc drops below the
stop voltage of 8V (V
of the control IC, Vcc has under voltage lockout (UVLO)
with 4V hysteresis. Figure 5 shows the relation between the
current consumed by FPS (Icc) and the supply voltage (Vcc).
Figure 5. Relation between operating supply current and
25uA
(V
3mA
Icc
line
AC line
min
START
FSCM0565R
- V
Power Down
line
), FSCM0565R begins switching, and the
max
)
Figure 4. Startup circuit
FSCM0565R
Vstop=8V
STOP
Vcc voltage
begins
). To guarantee the stable operation
Vstart=12V
switching,
Vcc
I
sup
I
cc
Power Up
C
Ca
Rstr
DC
FSCM0565R
Da
Vz
a
) that is
Vcc
The minimum current supplied through the startup resistor is
given by
where V
start voltage (12V) and R
resistor should be chosen so that I
maximum startup current (40uA). If not, Vcc can not be
charged to the start voltage and FPS will fail to start up.
2. Feedback Control : FSCM0565R employs current mode
control, as shown in Figure 6. An opto-coupler (such as the
H11A817A) and shunt regulator (such as the KA431) are
typically used to implement the feedback network.
Comparing the feedback voltage with the voltage across the
Rsense resistor makes it possible to control the switching
duty cycle. When the reference pin voltage of the KA431
exceeds the internal reference voltage of 2.5V, the
H11A817A LED current increases, thus pulling down the
feedback voltage and reducing the duty cycle. This event
typically happens when the input voltage is increased or the
output load is decreased.
2.1 Pulse-by-pulse current limit: Because current mode
control is employed, the peak current through the Sense FET
is determined by the inverting input of PWM comparator
(Vfb*) as shown in Figure 6. When the current through the
opto transistor is zero and the current limit pin (#5) is left
floating, the feedback current source (I
only through the internal resistor (R+2.5R=2.8k). In this
case, the cathode voltage of diode D2 and the peak drain
current have maximum values of 2.5V and 2.5A, respec-
tively. The pulse-by-pulse current limit can be adjusted using
a resistor to GND on current limit pin (#5). The current limit
level using an external resistor (R
Vo
Figure 6. Pulse width modulation (PWM) circuit
H11A817A
line
KA431
I sup
min
Vfb
min
R
is the minimum input voltage, V
LI M
I
LIM
=
C
B
5
4
I
=
delay
V
2 V
Vcc
SD
str
----------------------------------- -
2.8k
R
0.3k
D1
is the startup resistor. The startup
line
LIM
Vref
I
V
min
FB
D2
+
fb
-
+
*
2.5A
0.9mA
LIM
R
–
2.5R
R
LIM
sup
V
) is given by
start
OSC
min
FB
OLP
) of 0.9mA flows
is larger than the
----------- -
R str
1
driver
Gate
FSCM0565R
start
R
SenseFET
sense
is the
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