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LM2636M bảng dữ liệu(PDF) 9 Page - National Semiconductor (TI) |
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LM2636M bảng dữ liệu(HTML) 9 Page - National Semiconductor (TI) |
9 / 14 page Applications Information (Continued) The control-to-output transfer function is The ESR zero frequency is: The power stage double pole frequency is: The corresponding Bode plots are shown in Figure 5. Since the ESR zero frequency is so low, it effectively cancels the phase shift from one of the power stage poles. This limits the total phase shift to 90%. Although this regulator design is stable (phase shift is <90˚ when gain = 0dB), it needs compensation to improve the DC gain and cut off frequency (0dB frequency). Otherwise, the low DC gain may cause a poor line regulation, and the low cutoff frequency will hurt transient response performance. The transfer function for the 2-pole-1-zero compensation network shown in Figure 4 is: where One of the poles is located at origin to help achieve the high- est DC gain. So there are three parameters to determine, the position of the zero, the position of the second pole, and the constant A. To determine the cutoff frequency and phase margin, the loop bode plots need to be generated. The loop transfer function is: TF = −TF1 x TF2 By choosing the zero close to the double pole position and the second pole to half of the switching frequency, the closed loop transfer function turns out to be very good. That is, if f Z = 1.32 kHz, fP = 153 kHz, and A = 4.8 x 10 −6 ΩF, then the cutoff frequency will be 50 kHz, the phase margin will be 72˚, and the DC gain will be that of the error amplifier. See Figure 6 below. The compensation network component values can be deter- mined by the following equations: Notice there are three equations but four variables. So one of the variables can be chosen arbitrarily. Since the current driving capability of the error amplifier is limited to around 3 mA, it is a good idea to have a high impedance path from the output of the error amplifier to the output of the converter. From the above equations it can be told that a larger R 2 will DS100834-13 FIGURE 5. Control-to-Output Bode Plots DS100834-17 FIGURE 6. Loop Bode Plots www.national.com 9 |
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Mô tả tương tự - LM2636M |
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