AND9057/D Adjusting the Over Temperature Protection Trip Point in NCP1250-Based Adapters http://onsemi.com APPLICATION NOTE resistor on the OPP pin in relationship with this NTC value. This technique can lead to rather high pull−down resistors on the OPP pin to ground. If this resistance is too high and the converter noisy, the converter immunity can be reduced with possible erratic operations. The solution to fix this kind of problem is to reduce the pull−down resistance. Values below 3 kW give good operating results. There are cases where the noise in the adapter will require low values such as 1 kW. To still meet the OTP trip point, you will need to install a resistor in parallel with the existing NTC device. The solution appears in Figure 1. The NCP1250 features a multi−function pin in which the user can implement Over Power Protection (OPP) and Over Voltage Protection (OVP). If you add a Negative Temperature Coefficient (NTC) resistor in parallel with the OVP Zener diode, you have a means to protect the adapter against thermal runaway. The operating principle is simple: the latch detection is made by observing the OPP pin level via a comparator featuring a 3 V reference voltage. When the NTC resistance decreases as the temperature increases, the off−time voltage on the OPP pin goes up. When it touches the 3 V reference four consecutive times, the controller permanently latches off. Customers often re−use an NTC they have already implemented in previous projects and define the pull−down Aux D5 1N4148 Aux D5 1N4148 R2 X1 NTC R2 ZD1 1N965 R3 X1 NTC ZD1 1N965 OPP C10 22p OPP C10 22p R1 R1 Figure 1. This Picture Shows How to Adapt an Existing NTC Value to Adjust the OTP Trip Point As a design example, let us assume that we have the following component values around the NCP1250: − R1 = 1 kW; the NCP1250 pull−down resistor on the OPP pin. − RNTC = 15 kW; the NTC value at the select OTP trip point. − Npa = 0.2; the turns ratio between the primary and auxiliary windings. © Semiconductor Components Industries, LLC, 2011 November, 2011 − Rev. 0 − Nps = 0.09; the turns ratio between the primary and secondary windings. − Vout = 5 V ; the converter output voltage. − Vf = 0.6 V; the diodes forward drops in the auxiliary and secondary windings. At first, we need to know the voltage across the NTC in fault mode, e.g. when the OPP pin level reaches 3 V. This voltage is defined by the plateau voltage on the auxiliary 1 Publication Order Number: AND9057/D AND9057/D winding during the off time. It also depends on the 0.6 V forward drop of diode D1 and the 3 V OVP level: N pa V NTC + ǒV out ) V f Ǔ The next step is to recalculate the OPP resistor (R2 in Figure 1). For instance, if we need a 300 mV decrease from the 0.8 V setpoint at high line, then the auxiliary diode anode during the on−time swings to: * V OVP * V f N ps (eq. 1) V ANODE + −N pa 0.2 * 3 * 0.6 + 8.8 V 0.09 + (5 ) 0.6) I R1 + 3 + + 3 mA 1000 R1 V R2 + 74.7 * 0.3 + 74.4 V R eq + I R1 + 8.8 + 2.9 kW 3m (eq. 2) I R1 + R 3R NTC R2 + (eq. 3) R eqR NTC R NTC * R eq + 2.9k 15k 15k * 2.9k 300m 1k + 300 mA (eq. 8) 74.4 + 248 kW 300m (eq. 9) This application note explains how we can easily adjust the Over Temperature Protection trip point by paralleling a resistor with an available NTC device. In case of noisy layouts, it helps to decrease the OPP pin pull−down resistor to a value close or below one kW, naturally improving the converter robustness to external perturbations. Solving for R3 leads to: R3 + (eq. 7) Conclusion (eq. 4) R NTC ) R 3 (eq. 6) The R2 value is therefore easily derived: Knowing that the equivalent resistor is the NTC paralleled with the added resistance (R3), we have: R eq + Ǹ2Ǔ + −74.7 V The current flowing in the pull−down resistor R1 in this condition will be: From Equations 1 and 2, we can derive the equivalent resistance made of the NTC in parallel with the resistor we are looking for (R3): V NTC ǒ264 We can evaluate the voltage drop around R2 as: Based on 1 kW pull−down OPP resistor, the current in fault mode (VOVP = 3 V) is: V OVP V IN + −0.2 + 3.7 kW (eq. 5) ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice to any products herein. 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