Data Sheet

© 2006 Fairchild Semiconductor Corporation www.fairchildsemi.com
FSQ-Series • Rev. 1.1.3 15
FSCQ-Series — Green Mode Fairchild Power Switch (FPS™)
Vsyn
c
Vds
MOSFET Gate
2V
RO
4.6V
2.6V
3V
1.8V
ONON
Figure 31. Extended QR Operation Waveforms
3. Feedback Control: The FSCQ series employs
current mode control, as shown in Figure 32. An opto-
coupler (such as Fairchild’s H11A817A) and shunt
regulator (such as Fairchild’s KA431) are typically used
to implement the feedback network. Comparing the
feedback voltage with the voltage across the R
sense
resistor, plus an offset voltage, makes it possible to
control the switching duty cycle. When the reference pin
voltage of the shunt regulator exceeds the internal
reference voltage of 2.5 V, the opto-coupler LED current
increases, pulling down the feedback voltage and
reducing the duty cycle. This typically occurs when input
voltage is increased or output load is decreased.
3.1 Pulse-by-Pulse Current Limit: Because current
mode control is employed, the peak current through the
SenseFET is limited by the inverting input of the PWM
comparator (V
fb
*) as shown in Figure 32. The feedback
current (I
FB
) and internal resistors are designed so that
the maximum cathode voltage of diode D
2
is about
2.8 V, which occurs when all IFB flows through the
internal resistors. Since D
1
is blocked when the
feedback voltage (V
fb
) exceeds 2.8 V, the maximum
voltage of the cathode of D
2
is clamped at this voltage,
thus clamping V
fb
*. Therefore, the peak value of the
current through the SenseFET is limited.
3.2 Leading Edge Blanking (LEB): At the instant the
internal SenseFET is turned on, there is usually a high
current spike through the SenseFET, caused by the
external resonant capacitor across the MOSFET and
secondary-side rectifier reverse recovery. Excessive
voltage across the R
sense
resistor can lead to incorrect
feedback operation in the current mode PWM control.
To counter this effect, the FSCQ series employs a
leading edge blanking (LEB) circuit. This circuit inhibits
the PWM comparator for a short time (t
LEB
) after the
Sense FET is turned on.
4
OSC
V
CC
V
ref
I
delay
I
FB
V
SD
R
2.5R
Gate
Driver
OLP
D1 D2
+
V
fb
*
-
V
fb
KA431
C
B
V
O
H11A817A
R
sense
SenseFET
Figure 32. Pulse Width Modulation (PWM) Circuit
4. Protection Circuits: The FSCQ series has several
self-protective functions such as overload protection
(OLP), abnormal over-current protection (AOCP), over-
voltage protection (OVP), and thermal shutdown (TSD).
OLP and OVP are auto-restart mode protections, while
TSD and AOCP are latch mode protections. Because
these protection circuits are fully integrated into the IC
without external components, the reliability can be
improved without increasing cost.
- Auto-Restart Mode Protection: Once the fault
condition is detected, switching is terminated and
the SenseFET remains off. This causes V
CC
to fall.
When V
CC
falls to the under voltage lockout (UVLO)
stop voltage of 9 V, the protection is reset and the
FSCQ series consumes only startup current
(25 μA). Then, the V
CC
capacitor is charged up,
since the current supplied through the startup
resistor is larger than the current that the FPS
consumes. When V
CC
reaches the start voltage of
15 V, the FSCQ series resumes its normal
operation. If the fault condition is not removed, the
SenseFET remains off and VCC drops to stop
voltage again. In this manner, the auto-restart can
alternately enable and disable the switching of the
power SenseFET until the fault condition is
eliminated (see Figure 33).
- Latch Mode Protection: Once this protection is
triggered, switching is terminated and the
SenseFET remains off until the AC power line is
unplugged. Then, V
CC
continues charging and
discharging between 9 V and 15 V. The latch is
reset only when V
CC
is discharged to 6 V by
unplugging the AC power line.