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NCP1381 Datasheet(PDF) 8 Page - ON Semiconductor |
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NCP1381 Datasheet(HTML) 8 Page - ON Semiconductor |
8 / 26 page NCP1381 http://onsemi.com 8 Figure 5. A Typical Startup Sequence Followed by a Faulty Condition Startup Resistor Calculation For the sake of the example, we will go through the calculation of the startup element. Suppose that we have the following information: VCCON = 15 V. VCCOFF = 10 V. ICC2 = 4 mA, given by the selected MOSFET Qg. Startup duration below 2 s at minimum input voltage. Input voltage from 85 VAC to 265 VAC. Standby power below 500 mW. 1. From a startup DV of 15 − 10 = 5 V and a 4 mA total consumption, we can obtain the necessary VCC capacitor to keep enough voltage, assuming the feedback loop is closed within 10ms: CVCC = 4 m x 10 m / 5 = 8 mF or 22 mF for the normalized value if we account for the natural dispersion. 2. If we want a startup below 2 s, then the charging current flowing inside the VCC capacitor must be above: Icharge > 15 x 22 u / 2 > 165 mA. If we add the 15 mA of ICC1, the total startup current shall be above 180 mA. 3. The minimum input voltage is 85 x 1.414 = 120 V. Then, Rstartup should be below (120 − 15) / 180 m < 580 k W. 4. From this value, we can calculate the dissipated power at high line: Pstartup = (265 x 1.414)2 / 580 k = 242 mW. In latched mode, an internal zener diode is activated and clamps VCC to around 6 V. When VCC goes below 4 V, this zener is relaxed and the circuit can startup again. Please note that in fault mode the VCC comparator has the priority and stops the pulses anytime VCC falls below its minimum operating level VCCOFF. |
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