SPF8201 Datasheet, PDF (13/19 Pages) Sanken electric – Built-in startup circuit

SPF8201 DATA SHEET
Ver.1
7-4. S/OCP terminal
S/OCP terminal detects the drain current of the internal MOSFET. The sense resistor is connected between S/OCP and
GND terminal externally. S/OCP terminal detects the drain current at pulse by pulse, and when the S/OCP terminal
voltage exceeds “OCP threshold voltage (Vocp = 0.5V typ.)”, the over-current function operates and the internal
MOSFET is turned off every switching period.
Since the high frequency current flows through the sense resistor, low inductance and high surge tolerance resistor shall
be used.
7-5. COMP terminal
COMP terminal is the output of an error amplifier. The capacitor Ccomp and the resistor Rcomp are connected between
COMP and GND terminals generally. Appropriate capacitance must be chosen by checking the operation.
7-6. FB terminal
FB terminal is the input of an error amplifier. To control output voltage of a converter, the SPF8201 has current mode
control, which is superior transient response and stability. The device has an error amplifier between FB terminal and
COMP terminal. FB terminal adjusts input voltage to equalize VFB, Feedback voltage,=2.5V.
In case that a user does not use an optocoupler to regulate secondary voltage, please use an auxiliary winding and make
combined voltage in primary side from secondary output and an transformer to control operation, as described in Fig.7-9.
When the auxiliary winding circuit is composed as Fig.7-9, a smoothing capacitor, V3, and secondly output VOUT is set
by turn ratio of N2 and N3. The following is the equation.
VOUT=(N2/N3)xV3
The voltage,V3, is divided by resistors and input to FB terminal. Then, IC controls voltage on FB terminal to be
equalized VFB=2.5V.
Therefore,
VOUT=(R1+R2)/R2x2.5
Thus, secondly output will be controlled as follow.
VOUT=N2/N3x(R1+R2)/R2x2.5
Fig.7-9 FB terminal and COMP terminal peripheral circuit
SANKEN ELECTRIC CO.,LTD.
13 / 19
2014 Feb