FS6X1220RD Fairchild Semiconductor, FS6X1220RD Datasheet - Page 8

• Manufacturer Part Number: FS6X1220RD
• Description: IC SWIT PWM CM OVP UVLO HV D2PAK
• Manufacturer: Fairchild Semiconductor

Specifications of FS6X1220RD

• Output Isolation: Isolated
• Frequency Range: 270 ~ 330kHz
• Voltage - Input: 9 ~ 35 V
• Voltage - Output: 200V
• Power (watts): 45W
• Operating Temperature: -25°C ~ 150°C
• Package / Case: TO-263-6, D²Pak (5 leads + Tab-Variant)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: FS6X1220RDTR; FS6X1220RD_NL; FS6X1220RD_NLTR; FS6X1220RD_NLTR

FS6X1220R
Functional Description
1. Startup : To guarantee stable operation of the control IC,
Vcc has under voltage lockout (UVLO) with 6V hysteresis.
Figure 1 shows the relation between the supply current (Icc)
and the supply voltage (Vcc). Before Vcc reaches 15V, the
start-up current is 60µA, which is usually provided by the
DC link through start-up resistor. When Vcc reaches 15V, the
control IC begins operation and the operating current
increases to 10mA as shown. Once the control IC starts
operation, it continues its normal operation unless Vcc goes
below the stop voltage of 9V.
2. Feedback Control : The FS6X1220R employs current
mode control. The voltage of the feedback pin is compared
with the current sense voltage for pulse width modulation
(PWM). Figure 2 illustrates the simplified PWM block. The
feedback voltage determines the peak drain current of the
SenseFET. Usually opto-coupler along with TL431 are used
to implement feedback network. The collector of the opto-
coupler transistor is connected to feedback pin and the
emitter is connected to the ground pin. When the voltage of
the reference pin of TL431 exceeds the internal reference
voltage of 2.5V, the opto-coupler diode current increases,
pulling down the feedback voltage.
8
Figure 1. Relation between supply current and voltage
60uA
Vo
10mA
Icc
Figure 2. Pulse width modulation (PWM) circuit
431
Power Down
Vfb
Cfb
Vstop=9V
FB
4
5uA
V
Vcc
SD
D1
Vref
Vfb*
0.9mA
Vstart=15V
D2
28R
R
Power Up
OSC
OLP
driver
Gate
Vz
Vcc
3. Protection Circuit : Besides pulse-by-pulse current limit,
the FS6X1220R has 3 self protection functions; over load
protection (OLP), over voltage protection (OVP) and thermal
shutdown (TSD). Because these protection circuits are fully
integrated into the IC without external components, the
reliability can be improved. In the event of these fault
conditions, FS6X1220R enters into auto-restart operation.
Once the fault condition occurs, switching operation is
terminated and MOSFET remains off, which causes Vcc to
be reduced. When Vcc reaches 9V, the protection is reset and
the supply current reduces to 60uA. Then, Vcc begin to
increase with the current provided through the start-up
resistor. When Vcc reaches 15V, FS6X1220R resumes its
normal operation if the fault condition is removed. In this
manner, the auto-restart alternately enables and disables the
switching of the power MOSFET until the fault condition is
eliminated as illustrated in figure 3.
3.1 Pulse-by-pulse current limit : As shown in figure 2, the
drain current of the power MOSFET is limited by the
inverting input of PWM comparator (Vfb*). Assuming that
the 0.9mA current source flows only through the internal
resistor (28R +R= 2.9k), the cathode voltage of diode D2 is
about 2.6V. Since D1 is blocked when the feedback voltage
(Vfb) exceeds 2.6V, the maximum voltage of the cathode of
D2 is 2.6V. Therefore, the maximum value of Vfb* is about
0.1V, which limits the peak value of the power MOSFET
drain current.
10mA
60uA
Vds
Vcc
15V
Icc
9V
Figure 3. Auto restart operation after protection
Power
on
operation
Normal
occurs
Fault
situation
Fault
removed
Fault
operation
Normal
t