STRF6626 Datasheet, PDF (7/16 Pages) Allegro MicroSystems – OFF-LINE QUASI-RESONANT FLYBACK SWITCHING REGULATORS

Series STR-F6600
OFF-LINE
QUASI-RESONANT FLYBACK
SWITCHING REGULATORS
Functional Description and Operation (cont’d)
+ OUTPUT
4
DRIVE
REG.
UVLO
R
3
OVER-VOLT.
PROTECT
FAULT
LATCH
2
REF.
S
Q
+
TSD
OSC
+
–
+
1.45 V
–
1
+
0.73 V
5
+
VOLTAGE
SENSE
– OUTPUT
Figure 6 – Series STR-F6600 Typical Application
Dwg. EK-003-5A
WARNING — These devices are designed to be operated at lethal voltages and energy levels. Circuit
designs that embody these components must conform with applicable safety requirements. Precau-
tions must be taken to prevent accidental contact with power-line potentials. Do not connect
grounded test equipment.
The use of an isolation transformer is recommended during circuit development and breadboarding.
Soft Start with Voltage Feedback (refer to figure 7)
Output voltage control is achieved by sensing the opto-
coupled feedback current (proportional to the output
voltage error signal) across resistor R4 and summing this
with the drain current ramp on R5. The signal on pin 1 is
therefore the opposite of the output voltage error signal
and the drain current ramp. The dc bias signal across R4 is
thus a function of the load. Consequently at light load, the
bias signal on R4 is closer to the threshold voltage of the
comparator.
To eliminate the possibility of false shutdown at
MOSFET turn on (when there is a current spike due to the
discharge of primary capacitance), a constant-current sink
of 1.35 mA is turned on, effectively lowering the input
impedance on pin 1, and momentarily increasing the
shutdown threshold.
7