FSCM0765RD Fairchild Semiconductor, FSCM0765RD Datasheet - Page 10

• Manufacturer Part Number: FSCM0765RD
• Manufacturer: Fairchild Semiconductor

Specifications of FSCM0765RD

• Power Switch Family: FSCM0765R
• Input Voltage: -0.3 to 20V
• Power Switch On Resistance: 1.4Ohm
• Output Current: 2.64A
• Number Of Outputs: Single
• Supply Current: 2.5mA
• Operating Temperature (min): -25C
• Operating Temperature (max): 85C
• Operating Temperature Classification: Commercial
• Power Dissipation: 83W
• Lead Free Status / Rohs Status: Compliant

Functional Description
1. Startup: Figure 16 shows the typical startup circuit and
transformer auxiliary winding for the FSCM0765R
application. Before the FSCM0765R begins switching, it
consumes only startup current (typically 25uA) and the
current supplied from the DC link supply current consumed
by the FPS (Icc), and charges the external capacitor (C
is connected to the Vcc pin. When Vcc reaches start voltage
of 12V (V
current consumed by FSCM0765R increases to 3mA. Then,
the FSCM0765R 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 17 shows the relation between the
current consumed by the FPS (Icc) and the supply voltage
(Vcc).
The minimum current supplied through the startup resistor is
given by
Figure 17. Relation Between O
25uA
(V
3mA
I
CC
line
min
AC line
- V
Power Down
START
line
max
), the FSCM0765R begins switching, and the
)
Figure 16. Startup Circuit
FSCM 0765R
Vstop=8V
STOP
and Vcc Voltage
). To guarantee the stable operation
Vstart=12V
perating Supply C
V
I
SUP
I
C C
C C
Power Up
C
Ca
Rstr
DC
Da
Vz
urrent
a
V
) that
CC
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: The FSCM0765R employs current
mode control, as shown in Figure 18. An opto-coupler (such
as the H11A817A) and a 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 SenseFET
is determined by the inverting input of the PWM comparator
(Vfb*) as shown in Figure 18. 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 3A, respectively.
The pulse-by-pulse current limit can be adjusted using a
resistor to GND on the current limit pin (#5). The current
limit level using an external resistor (R
Vo
Figure 18. 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
(
6
4
I
=
delay
V
2 V
Vcc
SD
str
------------------------------------
2.8k
⋅
0.3k
D1
R
is the startup resistor. The startup
line
LIM
Vref
Ω
V
I
min
FB
D2
+
+
fb
-
⋅
*
0.9mA
3A
R
–
2.5R
R
LIM
sup
V
start
OSC
min
LIM
FB
OLP
)
) of 0.9mA flows
is larger than the
) is given by
⋅
----------- -
R str
1
driver
Gate
FSCM0765R
start
R
SenseFET
sense
is the
:
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