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MP2235 数据表(PDF) 14 Page - Monolithic Power Systems |
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MP2235 数据表(HTML) 14 Page - Monolithic Power Systems |
14 / 21 page MP2235 – SYNCHRONOUS STEP-DOWN CONVERTER crossover frequency below one-tenth of the switching frequency. To optimize the compensation components, the following procedure can be used. 1. Choose high-side feedback resistor R1 and calculate the value of low-side resistor R2 according to desired output voltage. Suggest choosing R1 around 40kΩ for >1.2V output condition. 2. Choose the T-Type resistor RT to set the desired crossover frequency. Determine the RT value by the following equation: FB Z 1 2 T OUT i C OUT 1 2 V R R R R V R 2 f C R R RZ is the internal compensation resistor, which equals to 300kΩ. f C is the desired crossover frequency which is typically one tenth of the switching frequency. Ri is the current sense resistance, 0.22Ω. 3. Choose feed forward capacitor CF to achieve sufficient phase margin especially for large output inductor condition. In theory there is no need to add type III zero for peak current mode control, but in real circuit there are some parasitic capacitors or filters internal which induces poles into the control loop. Fortunately, those poles are locating at high frequency range which won’t affect the step 2 bandwidth calculation while it affects the phase margin. For applications with typical inductor values (<4.7µH), setting the compensation zero, fZ3 (formed by R1 and CF) around 1.5 times of crossover frequency fC. Then the CF value can be calculated by following equation: F 1C 1 C 3 R f If electrolytic capacitor is used or the output capacitor has large ESR, the feed forward capacitor CF is not needed any more since there is already one ESR zero in the loop. If large output inductor is used, like 22µH, the phase margin will decrease a lot due to the half switching frequency pole moves towards crossover frequency. In th is condition, it’s suggested increasing feed forward capacitor value of CF to get enough phase margins while it’s better to keep the feed forward zero frequency higher than half of crossover frequency. Figure 7: T-Type Network Table 1 lists the recommended resistors and compensation values for common output voltages (refer to Figure 7). Table 1: Resistor Selection for Common Output Voltages VOUT (V) R1 (kΩ) R2 (kΩ) Rt (kΩ) Cf(pF) L(μH) 1 20.5 84.5 34 33 1 1.2 30.1 61.9 24 33 1 1.8 40.2 32.4 15 33 2.2 2.5 40.2 19.1 6.8 33 2.2 3.3 40.2 13 5.6 33 3.3 5 40.2 7.68 2 33 3.3 For more accurate control loop design, visit MPS website and run the online bode plot simulation by DC/DC designer. Selecting the Inductor Use a 1µH-to-22µH inductor with a DC current rating of at least 25% percent higher than the maximum load current for most applications. For highest efficiency, use an inductor with a DC resistance less than 15mΩ. For most designs, the inductance value can be derived from the following equation. OUT IN OUT 1 IN L OSC V (V V ) L V I f Where ΔI L is the inductor ripple current. |
类似零件编号 - MP2235 |
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类似说明 - MP2235 |
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