NCP1336A Datasheet, PDF (24/26 Pages) ON Semiconductor – Quasi-Resonant Current Mode Controller for High Power Universal Off-Line Supplies

NCP1336A/B
V_HV
Figure 29. Peak Current Setpoint Variation vs. OPP Pin Voltage
By adding a zener diode in series with the resistor divider,
the user has the choice to adjust the level at which the OPP
is applied to the power chip.
Design Example
Let us assume we need a current setpoint reduction of 25%
at 370 Vdc, which corresponds to a sense voltage of 600 mV.
We thus need to apply 600 mV − 800 mV = −200 mV on
OPP pin to perform the expected compensation.
Knowing that the voltage that appears on the auxiliary
winding during the on−time is −N_p,aux V_IN, with N_p,aux the
auxiliary to primary turn ratio of the transformer (N_p,aux =
N_aux/N_p), we can simply calculate the ratio of the resistor
divider:
R_upper
R_lower
+
*
N_p,auxV_IN *
V_OPP
V_OPP
Assuming the turn ratio of the transformer is N_p,aux = 0.25,
we obtain:
R_upper
R_lower
+
*
0.25
370 *
−0.2
(−0.2)
+
461.5
With R_upper = 470 kW and R_lower = 1 kW for instance, the
OPP function is performed with negligible power wasted in
the resistor divider.
Brownout
The NCP1336 features a brownout pin to protect the
power supply against low input voltage condition. This pin
permanently monitors a fraction of the bulk voltage through
a voltage divider. When this image of bulk voltage is below
the VBO threshold, the controller stops switching. When the
bulk voltage comes back within safe limits, the circuit goes
through a new startup sequence including soft−start and
restarts switching (Figure 30). The hysteresis on brownout
pin is implemented with a low side current source sinking
10 mA when the brownout comparator is low (V_bulk <
V_bulkON).
Figure 30. Brownout Operating Chronograms
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