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SC2308A Datasheet(PDF) 10 Page - Semtech Corporation |
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SC2308A Datasheet(HTML) 10 Page - Semtech Corporation |
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10 / 17 page  0 Applications Information (Continued) SC2308A losses in both the switch and the diode, an expression for the maximum available output current of a boost converter can be derived using Equation (5): IN CESAT D D OUT IN LIM ) MAX ( OUT V V V D V 45 D 1 V V I I (5) Since switching losses are excluded in the derivation, the actual output current is over-estimated in Equation (5). Nevertheless, this calculation still provides a useful initial approximation. Inductor Selection The inductor must be able to handle the peak current I LIM. First, the inductor should not saturate at I LIM. Second, the inductor needs to have low core loss at the switching fre- quency. Inductors with ferrite cores are preferrable. More- over, the inductor should have low DCR for low copper loss. The inductance can be selected such that the inductor ripple current is between 20% to 40% of its average current for improved efficiency. The inductance can be calculated using Equation (6): SW L CESAT IN f I V V D L (6) The Coilcraft DO336P series and the Sumida CDRH8D38NP series inductors perform well in boost converters. The inductors selected must be suitable for a 750kHz switch- ing frequency. Input Capacitor Selection The input current in a boost converter is the inductor cur- rent, which is continuous with low RMS current ripples. A 2.2mF~4.7mF ceramic capacitor is adequate for most applications. Use X5R or better ceramic capacitors, since they have stable temperature and voltage coefficients. The voltage rating for the input capacitor should exceed the maximum input voltage by 0% to 25%. Murata and TDK are two ceramic capacitor suppliers. Output Capacitor Selection Ceramic and low equivalent series resistance (ESR) tantalum or polymer capacitors can be used for output filtering. In a buck converter, the inductor ripple current flows in the output capacitor, whereas in a boost converter, the output capacitor current is the difference between the rectifying diode current and the output current (Figure 4). This cur- rent is discontinuous with high current amplitudes. For this reason, the output ripple voltage of a boost converter is always higher than that of a buck converter with the same inductor current and the same output capacitor. If tantalum or polymer capacitors are used at the converter output, then the converter output ripple voltage will be primarily determined by the capacitor ESR, due to the rel- atively high ESR of these capacitors. The output voltage ripple is the product of the peak inductor current and the output capacitor ESR. For example, if two Sanyo 6TPG00M (00mF, ESR=70mW) polymer capacitors are used for output filtering, then the output peak-to-peak ripple voltage will be 70mV, assuming a 2A peak inductor current. Tantalum capacitor voltage derating is generally 50%. Multi-layer ceramic capacitors, due to their extremely low ESR (<5mΩ), are particularly well suited for output filtering. It is worth noting that the output ripple voltage resulting from charging and discharging of a 0μF or a 22mF ceramic capacitor is higher than the ripple voltage resulting from the capacitor ESR. The output ripple voltage due to charging and discharging effects is calculated using the following equation: OUT SW OUT OUT C f DI V (7) X5R and X7R ceramic capacitors are the preferred types. Rectifying Diode For high efficiency, Schottky barrier diodes should be used as rectifying diodes for the SC2308A. These diodes should have an average forward current rating at least equal to the output current. The reverse blocking voltage of the Schottky diode should be derated by 0%-20% for reliability. The Schottky diode used in a 2V output step- up converter should have a reverse voltage rating of at least 5V (20% derating). |
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