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LM2698 Arkusz danych(PDF) 9 Page - National Semiconductor (TI) |
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LM2698 Arkusz danych(HTML) 9 Page - National Semiconductor (TI) |
9 / 17 page Operation (Continued) If V L(t) is constant, diL / dt must be constant, thus the current in the inductor changes at a constant rate. This is the case in DC/DC converters since the voltages at the input and output can be approximated as a constant. The current through the inductor of the LM2698 boost converter is shown in Figure 2(a). The important quantities in determining a proper induc- tance value are I L(AVG) (the average inductor current) and ∆iL (the inductor current ripple). If ∆i L is larger than IL(AVG), the inductor current will drop to zero for a portion of the cycle and the converter will operate in discontinuous conduction mode. If ∆i L is smaller than IL(AVG), the inductor current will stay above zero and the converter will operate in continuous conduction mode (CCM). All the analysis in this datasheet assumes operation in continuous conduction mode. To op- erate in CCM: I L(AVG) > ∆iL Choose the minimum I OUT to determine the minimum L for CCM operation. A common choice is to set ∆i L to 30% of I L(AVG). The inductance value will also affect the stability of the converter. Because the LM2698 utilizes current mode con- trol, the inductor value must be carefully chosen. See the COMPENSATION section for recommended inductance val- ues. Choosing an appropriate core size for the inductor involves calculating the average and peak currents expected through the inductor. In a boost converter, and I L(Peak) =IL(AVG) + ∆iL, where A core size with ratings higher than these values should be chosen. If the core is not properly rated, saturation will dramatically reduce overall efficiency. Current Limit The current limit in the LM2698 is referenced to the peak switch current. The peak currents in the switch of a boost converter will always be higher than the average current supplied to the load. To determine the maximum average output current that the LM2698 can supply, use: I OUT(MAX) =(ICL − ∆iL)*(1−D) = (ICL − ∆iL)*VIN/VOUT Where I CL is the switch current limit (see Electrical Chara- teristics table and Typical Performance Curves). Hence, as V IN increases, the maximum current that can be supplied to the load increases, as shown in Figure 3. Diode The diode in a boost converter such as the LM2698 acts as a switch to the output. During the first cycle, when the transistor is closed, the diode is reverse biased and current is blocked; the load current is supplied by the output capaci- tor. In the second cycle, the transistor is open and the diode is forward biased; the load current is supplied by the induc- tor. Observation of the boost converter circuit shows that the average current through the diode is the average load cur- rent, and the peak current through the diode is the peak current through the inductor. The diode should be rated to handle more than its peak current. To improve efficiency, a low forward drop Schottky diode is recommended. Input Capacitor Due to the presence of an inductor at the input of a boost converter, the input current waveform is continuous and triangular. The inductor ensures that the input capacitor sees fairly low ripple currents. However, as the inductor gets smaller, the input ripple increases. 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 boost applications, a 10 µF 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 6) of the IC. This capacitor must 20012673 FIGURE 3. Maximum Output Current vs Input Voltage www.national.com 9 |
Podobny numer części - LM2698 |
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Podobny opis - LM2698 |
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