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LM2611 Datasheet(PDF) 7 Page - National Semiconductor (TI) |
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LM2611 Datasheet(HTML) 7 Page - National Semiconductor (TI) |
7 / 15 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). 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 ≤ V OUT ≤ −3.3V, and 2.7V ≤ V IN ≤ 30V (V IN 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: 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 5 and Figure 6). The position of the zero-pole pair is a function of the feedback resistors and the capacitor value: 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. www.national.com 7 |
Similar Part No. - LM2611_05 |
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Similar Description - LM2611_05 |
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