FSL136MR Datasheet, PDF (9/13 Pages) Fairchild Semiconductor – Green Mode Fairchild Power Switch (FPS™)

Functional Description
Startup
At startup, an internal high-voltage current source
supplies the internal bias and charges the external
capacitor (CA) connected with the VCC pin, as illustrated
in Figure 14. When VCC reaches the start voltage of
12V, the FPS™ begins switching and the internal high-
voltage current source is disabled. The FPS continues
normal switching operation and the power is provided
from the auxiliary transformer winding unless VCC goes
below the stop voltage of 8V.
Feedback Control
FSL136MR employs current-mode control, as shown in
Figure 16. 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 RSENSE
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, the feedback
voltage VFB is pulled down, and the duty cycle is
reduced. This typically occurs when the input voltage is
increased or the output load is decreased.
Figure 14. Startup Circuit
Oscillator Block
The oscillator frequency is set internally and the FPS
has a random frequency fluctuation function.
Fluctuation of the switching frequency of a switched
power supply 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 external feedback voltage
and internal free-running oscillator. This randomly
chosen switching frequency effectively spreads the EMI
noise nearby switching frequency and allows the use of
a cost-effective inductor instead of an AC input line filter
to satisfy the world-wide EMI requirements.
Figure 15. Frequency Fluctuation Waveform
Figure 16. Pulse-Width-Modulation 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 RSENSE resistor leads to incorrect feedback
operation in the current-mode PWM control. To counter
this effect, the FPS employs a leading-edge blanking
(LEB) circuit (see the Figure 16). This circuit inhibits the
PWM comparator for a short time (tLEB) after the
SenseFET is turned on.
Protection Circuits
The FPS has several protective functions, such as
overload protection (OLP), over-voltage protection
(OVP), output-short protection (OSP), under-voltage
lockout (UVLO), abnormal over-current protection
(AOCP), and thermal shutdown (TSD). Because these
various protection circuits are fully integrated in the IC
without external components, the reliability is improved
without increasing cost. Once a fault condition occurs,
switching is terminated and the SenseFET remains off.
This causes VCC to fall. When VCC reaches the UVLO
stop voltage, VSTOP (8V), the protection is reset and the
internal high-voltage current source charges the VCC
capacitor via the VSTR pin. When VCC reaches the UVLO
start voltage, VSTART (12V), the FPS resumes normal
operation. In this manner, the auto-restart can
alternately enable and disable the switching of the
power SenseFET until the fault condition is eliminated.
© 2009 Fairchild Semiconductor Corporation
FSL136MR • Rev. 1.0.7
9
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