NCP1249 Datasheet, PDF(25/31 Page) ON Semiconductor – High-Voltage Current-Mode PWM Controller Featuring Peak Power Excursion and Extremely Low Stand-by Power Consumption

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NCP1249 Datasheet, PDF (25/31 Pages) ON Semiconductor – High-Voltage Current-Mode PWM Controller Featuring Peak Power Excursion and Extremely Low Stand-by Power Consumption

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NCP1249A, NCP1249B

RoppU

421k

1N4148

100p

OPP

ROPPL

Vlatch

OVP

Vcc

OPP

aux.

winding

Figure 68. Simple resistive divider brings the OPP pin above 3 V in case of a Vcc voltage runaway above 18 V

First, calculate the OPP network with the above equations.

Then, suppose we want to latch off our controller when Vout

exceeds 25 V. On the auxiliary winding, the plateau reflects

the output voltage by the turns ratio between the power and

the auxiliary windings. In case of voltage runaway for our

19 V adapter, the plateau will go up to:

Vaux,OVP + 25

0.18

0.25

(eq. 14)

Since our OVP comparator trips at a 3 V level, across the

1 kW selected OPP pullâdown resistor, it implies a 3 mA

current. From 3 V to go up to 18 V, we need an additional

15 V. Under 3 mA and neglecting the series diode forward

drop, it requires a series resistor of:

ROVP

Vlatch * VVOP

VOVPÅROPPL

18 * 3

3Å1 k

(eq. 15)

In nominal conditions, the plateau establishes to around

14 V. Given the divideâby ratio 6, the OPP pin will swing to

14/6 = 2.3 V during normal conditions, leaving 700 mV for

the noise immunity. A 100 pF capacitor can be added to

improve it and avoids erratic trips in presence of external

surges. Do not increase this capacitor too much otherwise

the OPP signal will be affected by the integrating time

constant.

A second solution for the OVP detection alone, is to use

a Zener diode wired as recommended by Figure 69.

15V