NCP1237_11 Datasheet, PDF (27/41 Pages) ON Semiconductor – Fixed Frequency Current Mode Controller for Flyback Converters

V _HV
BO threshold
NCP1237
Peak
detector
t wd
t wd
t wd
time
Reset
IOPC
Sample
Sample
Sample
time
Reset
t wd
time
Figure 56. Overpower Compensation Current with the HV pin connected to a dc voltage
Feedback with Slope Compensation
The ratio from the FB voltage to the current sense setpoint
is typically 5. This means that the FB voltage corresponding
to V_ILIM is 3.5 V. There is a pullup resistor of 20 kW
(typical) from FB pin to the internal reference V_FB(ref).
VDD
20 kW
FB
K _FB
slope
comp.
Oscillator
+
PWM
−
blanking
CS
t LEB
Figure 57. FB Circuitry
In order to allow the NCP1237 to operate in CCM with a
duty cycle above 50%, a fixed slope compensation is
internally applied to the current−mode control. The slope
appearing on the internal voltage setpoint for the PWM
comparator is −32.5 mV/ms typical for the 65 kHz version
(and respectively −50 mV/ms and −67 mV/ms for the
100 kHz and 133 kHz versions).
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