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LM2611AMFX 데이터시트(PDF) 8 Page - National Semiconductor (TI) |
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LM2611AMFX 데이터시트(HTML) 8 Page - National Semiconductor (TI) |
8 / 13 page Operation (Continued) The following equations define values given in Figure 2 and Figure 3: I L2 =IOUT Use these equations to choose correct core sizes for the inductors. The design of the LM2611’s internal compensa- tion assumes L1 and L2 are equal to 10 - 22 µH, thus it is recommended to stay within this range. Switch Current Limit The LM2611 incorporates a separate current limit compara- tor, making current limit independent of any other variables. The current limit comparator measures the switch current versus a reference that represents current limit. If at any time the switch current surpasses the current limit, the switch opens until the next switching period. To determine the maxi- mum load for a given set of conditions, both the input and output inductor currents must be considered. The switch current is equal to i L1 +iL2, and is drawn in Figure 4.In summary: i SW(PEAK) must be less than the current limit (1.2A typical), but will also be limited by the thermal resistivity of the LM2611’s SOT23-5 package ( θ JA = 265˚C/W). Figure 5 shows the maximum output current vs. input voltage that can be expected from a typical layout using 1oz. copper (no heatsink or fan), it is limited by thermal shutdown rather than current limit. Input Capacitor The input current waveform to a Cuk converter is continuous and triangular, as shown in Figure 2. The input inductor insures that the input capacitor sees fairly low ripple cur- rents. However, as the input inductor gets smaller, the input ripple goes up. The RMS current in the input capacitor is given by: The input capacitor should be capable of handling the RMS current. Although the input capacitor is not so critical in a Cuk converter, a 10µF or higher value good quality capacitor prevents any impedance interactions with the input supply. A 0.1µF or 1µF ceramic bypass capacitor is also recom- mended on the V IN pin (pin 5) of the IC. This capacitor must be connected very close to pin 5 (within 0.2 inches). Output Capacitor Like the input current, the output current is also continuous, triangular, and has low ripple (see I L2 in Figure 3). The output capacitor must be rated to handle its RMS current: For example, I COUT(RMS) can range from 30mA to 180mA with 10µH ≤ L 1,2 ≤ 22µH, −10V ≤ VOUT ≤ −3.3V, and 2.7V ≤ V IN ≤ 30V (VIN may be 30V if using separate power and analog supplies, see Split Supply Operation in the APPLI- CATIONS section). The worst case conditions are with L 1,2, V OUT(MAX), and VIN(MAX). Many capacitor technologies will provide this level of RMS current, but ceramic capacitors are ideally suited for the LM2611. Ceramic capacitors provide a good combination of capacitance and equivalent series re- sistance (ESR) to keep the zero formed by the capacitance and ESR at high frequencies. The ESR zero is calculated as: 20018102 FIGURE 4. Switch Current Waveform in a Cuk Converter. The peak value is equal to the sum of the average currents through L1 and L2 and the average-to-peak current ripples through L1 and L2. 20018126 ) LEVEL 3 FIGURE 5. I OUT(MAX) vs VIN using 1oz. copper layout. See Figure 14 for the test circuit. www.national.com 8 |
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