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LM25149 データシート(PDF) 28 Page - Texas Instruments |
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LM25149 データシート(HTML) 28 Page - Texas Instruments |
28 / 49 page • VMAX is the allowed dBμV noise level for the applicable conducted EMI specification, for example CISPR 25 Class 5 • CIN is the existing input capacitance of the buck regulator • DMAX is the maximum duty cycle • IPEAK is the peak inductor current For filter design purposes, the current at the input can be modeled as a square-wave. Determine the passive EMI filter capacitance CF from Equation 21. 2 Attn 40 F IN SW 1 10 C L 2 F S § · ¨ ¸ ¨ ¸ ˜ ¨ ¸ ¨ ¸ © ¹ (21) Adding an input filter to a switching regulator modifies the control-to-output transfer function. The output impedance of the filter must be sufficiently small so that the input filter does not significantly affect the loop gain of the buck converter. The impedance peaks at the filter resonant frequency. The resonant frequency of the passive filter is given by Equation 22. res IN F 1 f 2 L C S ˜ ˜ (22) The purpose of RD is to reduce the peak output impedance of the filter at the resonant frequency. Capacitor CD blocks the DC component of the input voltage to avoid excessive power dissipation in RD. Capacitor CD should have lower impedance than RD at the resonant frequency with a capacitance value greater than that of the input capacitor CIN. This prevents CIN from interfering with the cutoff frequency of the main filter. Added input damping is needed when the output impedance of the filter is high at the resonant frequency (Q of filter formed by LIN and CIN is too high). An electrolytic capacitor CD can be used for input damping with a value given by Equation 23. D IN C 4 C t ˜ (23) Select the input damping resistor RD using Equation 24. IN D IN L R C (24) 9.1.2 Error Amplifier and Compensation A Type-ll compensator using a transconductance error amplifier (EA) is shown in Figure 9-3. The dominant pole of the EA open-loop gain is set by the EA output resistance, RO-EA, and effective bandwidth-limiting capacitance, CBW, as shown by Equation 25. O-EA EA(openloop) O-EA BW g R G (s) 1 s R C ˜ ˜ ˜ m (25) The EA high-frequency pole is neglected in the above expression. The compensator transfer function from output voltage to COMP node, including the gain contribution from the (internal or external) feedback resistor network, is calculated in Equation 26. LM25149 SNVSBV5 – DECEMBER 2020 www.ti.com 28 Submit Document Feedback Copyright © 2020 Texas Instruments Incorporated Product Folder Links: LM25149 |
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同様の説明 - LM25149 |
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