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NCP431B Datasheet(PDF) 11 Page - ON Semiconductor |
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NCP431B Datasheet(HTML) 11 Page - ON Semiconductor |
11 / 17 page NCP431A, SC431A, NCP431B, NCP432B Series http://onsemi.com 11 Figure 32. Simplified NCP431/NCP432 Device Model NCP431/NCP432 OPEN−LOOP VOLTAGE GAIN VERSUS FREQUENCY Figure 33. Example 1 Circuit Open Loop Gain Plot Example 2. IC = 7.5 mA, RL = 2.2 kW, CL = 0.01 mF. Cathode tied to reference input pin. An examination of the data sheet stability boundary curve (Figure 17) shows that this value of load capacitance and cathode current is on the boundary. Define the transfer gain. The DC gain is: G + GMRGMGoRL + (2.138)(1.0M)(1.25m)(230) + 6389 + 76 dB The resulting open loop Bode plot is shown in Figure 34. The asymptotic plot may be expressed as the following equation: Av + 615 1 ) jf 500 kHz 1 ) jf 8.0 kHz 1 ) jf 60 kHz 1 ) jf 7.2 kHz Note that the transfer function now has an extra pole formed by the load capacitance and load resistance. Note that the crossover frequency in this case is about 250 kHz, having a phase margin of about −46 °. Therefore, instability of this circuit is likely. NCP431/NCP432 OPEN−LOOP BODE PLOT WITH LOAD CAP Figure 34. Example 2 Circuit Open Loop Gain Plot With three poles, this system is unstable. The only hope for stabilizing this circuit is to add a zero. However, that can only be done by adding a series resistance to the output capacitance, which will reduce its effectiveness as a noise filter. Therefore, practically, in reference voltage applications, the best solution appears to be to use a smaller value of capacitance in low noise applications or a very large value to provide noise filtering and a dominant pole rolloff of the system. The NCP431/NCP432 is often used as a regulator in secondary side of a switch mode power supply (SMPS). The benefit of this reference is high and stable gain under low bias currents. Figure 35 shows dependence of the gain (dynamic impedance) on the bias current. Value of |
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