FSL206MRBN Fairchild Semiconductor, FSL206MRBN Datasheet - Page 9

• Manufacturer Part Number: FSL206MRBN
• Description: The FSL206MRBN integrated Pulse-Width Modulator (PWM) and SenseFET is specifically designed for high-performance offline Switched-Mode Power Supplies (SMPS) with minimal external components
• Manufacturer: Fairchild Semiconductor

© 2011 Fairchild Semiconductor Corporation
FSL206MRBN • Rev. 1.0.1
Functional Description
Startup
At startup, an internal high-voltage current source
supplies the internal bias and charges the external
capacitor (C
Figure 36. An internal high-voltage regulator (HV REG)
located between the V
V
FSL206MRBN needs no auxiliary bias winding.
Oscillator Block
The oscillator frequency is set internally and the FPS™
has a random frequency fluctuation function.
Fluctuation of the switching frequency can reduce EMI
by spreading the energy over a wider frequency range
than the bandwidth measured by the EMI test
equipment. The amount of EMI reduction is directly
related to the range of the frequency variation. The
range of frequency variation is fixed internally; however,
its selection is randomly chosen by the combination of
an external feedback voltage and internal free-running
oscillator. This randomly chosen switching frequency
effectively spreads the EMI noise near switching
frequency and allows the use of a cost-effective inductor
instead of an AC input line filter to satisfy world-wide
EMI requirements.
Figure 37.
CC
to 7.8V and supplies operating current. Therefore,
C
A
V
A
CC
) connected to the V
Figure 36. Startup Block
Frequency Fluctuation Waveform
3
7.8V
V
STR
REF
I
I
CH
START
V
and V
DC,link
UVLO
HV/REG
CC
CC
pin, as illustrated in
pins regulates the
V
STR
2
Feedback Control
FSL206MRBN employs Current-Mode control, as shown
in Figure 38. An opto-coupler (such as the FOD817A)
and shunt regulator (such as the KA431) are typically
used to implement the feedback network. Comparing the
feedback voltage with the voltage across the R
resistor makes it possible to control the switching duty
cycle. When the shunt regulator reference pin voltage
exceeds the internal reference voltage of 2.5V; the opto-
coupler LED current increases, feedback voltage V
pulled down, and the duty cycle is reduced. This
typically occurs when input voltage is increased or
output load is decreased.
Figure 38. Pulse-Width-Modulation (PWM) Circuit
Leading-Edge Blanking (LEB)
At the instant the internal SenseFET is turned on, the
primary-side capacitance and secondary-side rectifier
diode reverse recovery typically cause a high-current
spike through the SenseFET. Excessive voltage across
the R
in the Current-Mode PWM control. To counter this
effect, the FPS employs a leading-edge blanking (LEB)
circuit (see Figure 38). This circuit inhibits the PWM
comparator for a short time (t
turned on.
Protection Circuits
The protective functions include Overload Protection
(OLP), Over-Voltage Protection (OVP), Under-Voltage
Lockout (UVLO), Line Under-Voltage Protection (LUVP),
Abnormal Over-Current Protection (AOCP), and thermal
shutdown (TSD). Because these protection circuits are
fully
components, reliability is improved without increasing
cost. Once a fault condition occurs, switching is
terminated and the SenseFET remains off. This causes
V
V
voltage current source charges the V
V
V
manner, auto-restart can alternately enable and disable
the switching of the power SenseFET until the fault
condition is eliminated.
CC
STOP
STR
START
to fall. When V
SENSE
pin. When V
(7V), the protection is reset and the internal high-
integrated
(8V), the FPS resumes normal operation. In this
resistor leads to incorrect feedback operation
inside
CC
CC
reaches the UVLO start voltage
reaches the UVLO stop voltage
the
LEB
) after the SenseFET is
IC
CC
without
capacitor via the
www.fairchildsemi.com
external
SENSE
FB
is