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NCP1027 Datasheet(PDF) 12 Page - ON Semiconductor |
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NCP1027 Datasheet(HTML) 12 Page - ON Semiconductor |
12 / 30 page NCP1027 http://onsemi.com 12 Figure 25. A typical startup sequence showing the VCC capacitor voltage evolution versus time. Suppose our power supply takes 10 ms (tstartup) to bring the output voltage to its target value. We know that the switcher consumption is around 2.0 mA (ICC1). Therefore, we can calculate the amount of capacitance we need, to hold VCC above 7.5 V at least for 10 ms while delivering 2.0 mA: C w ICC1tstartup DVCC or, by replacing with the above values, C w 2m · 10 m 1 w 20 mF then select a 33 mF for the VCC capacitor. Fault Condition – Short−Circuit on VCC In some fault situations, a short−circuit can purposely occur between VCC and GND. In high line conditions (VHV = 370 VDC) the current delivered by the startup device will seriously increase the junction temperature. For instance, since IC1 equals 3.0 mA (the min corresponds to the highest TJ), the device would dissipate 370 3 m = 1.1 W. To avoid this situation, the controller includes a novel circuitry made of two startup levels, IC1 and IC2. At powerup, as long as VCC is below a 1.3 V level, the source delivers IC1 (around 650 mA typical), then, when VCC reaches 1.3 V, the source smoothly transitions to IC2 and delivers its nominal value. As a result, in case of short−circuit between VCC and GND, the power dissipation will drop to 370 650 m = 240 mW. Figure 26 portrays this particular behavior. Figure 26. The startup source now features a dual−level startup current. The first startup period is calculated by the formula C V = I t, which implies a 33 m 1.3/650 m = 66 ms startup time for the first sequence (t1). The second sequence (t2) is obtained by toggling the source to 4.0 mA with a delta V of VCCON – VCCth = 8.5 – 1.5 = 7.0 V, which finally leads to a second startup time of 7 33 m/6.0 m = 39 ms. The total startup time becomes 66 m + 39 m = 105 ms as a typical value. Please note that this calculation is approximated by the presence of the knee in the vicinity of the transition. |
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