データシートサーチシステム |
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LM2611AMFX データシート(PDF) 9 Page - National Semiconductor (TI) |
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LM2611AMFX データシート(HTML) 9 Page - National Semiconductor (TI) |
9 / 13 page Operation (Continued) A general rule of thumb is to keep f ESR > 80kHz for LM2611 Cuk designs. Low ESR tantalum capacitors will usually be rated for at least 180mA in a voltage rating of 10V or above. However the ESR in a tantalum capacitor (even in a low ESR tantalum capacitor) is much higher than in a ceramic capaci- tor and could place f ESR low enough to cause the LM2611 to run unstable. Improving Transient Response/Compensation The compensator in the LM2611 is internal. However, a zero-pole pair can be added to the open loop frequency response by inserting a feed forward capacitor, C FF, in par- allel to the top feedback resistor (R FB1). Phase margin and bandwidth can be improved with the added zero-pole pair. This inturn will improve the transient response to a step load change (see Figure 6 and Figure 7). The position of the zero-pole pair is a function of the feedback resistors and the capacitor value: (1) (2) The optimal position for this zero-pole pair will vary with circuit parameters such as D, I OUT,COUT, L1, L2, and CCUK. For most cases, placing the zero at 34 krad/s (5.4 kHz) is effective (this corresponds to the values on the front page schematic). Notice how the pole position, ω p, is dependant on the feedback resistors R FB1 and RFB2, and therefore also dependant on the output voltage. As the output voltage becomes closer to −1.26V, the pole moves towards the zero, tending to cancel it out. If the absolute magnitude of the output voltage is less than 3.3V, adding the zero-pole pair will not have much effect on the response. Hysteric Mode As the output current decreases, there will come a point when the energy stored in the Cuk capacitor is more than the energy required by the load. The excess energy is absorbed by the output capacitor, causing the output voltage to in- crease out of regulation. The LM2611 detects when this happens and enters a pulse skipping, or hysteretic mode. In hysteretic mode, the output voltage ripple will increase, as illustrated in Figure 8 and Figure 9. 20018120 ) LEVEL 3 FIGURE 6. 130mA to 400mA Transient Response of the circuit in Figure 10 with C FF= 1nF 20018122 ) LEVEL 3 FIGURE 7. 130mA to 400mA Transient Response of the circuit in Figure 10 with C FF disconnected 20018121 FIGURE 8. The LM2611 in PWM mode has very low ripple www.national.com 9 |
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同様の説明 - LM2611AMFX |
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