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SS8051T12TB Folha de dados(PDF) 7 Page - Silicon Standard Corp. |
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SS8051T12TB Folha de dados(HTML) 7 Page - Silicon Standard Corp. |
7 / 9 page www.SiliconStandard.com 3 of 4 SS8051 Rev.2.01 6/06/2003 Here, VD = 0.4V (Schottky diode voltage), ILIM = 350mA and tOFF = 500ns. A larger value can be used to slightly increase the available output current, but limit it to about twice the calculated value. When too large an inductor is used, the output voltage ripple will increase without providing much additional output current. In conditions of varying VIN, such as battery power applications, use the minimum VIN value in the above equation. A smaller value can be used to give smaller physical size, but overshoot of the inductor current will occur (see Current Limit Overshoot section). Inductor Selection – SEPIC Regulator For a SEPIC regulator using the SS8051, the approximate inductance value can be calculated using the formula below. As for the boost inductor selection, a larger or smaller value can be used. Current Limit Overshoot The SS8051 uses a constant off-time control scheme; the MOSFET is turned off after the 350mA current limit is reached. When the current limit is reached and the MOSFET actually turns off, there is a 100ns delay time. During this time, the inductor current exceeds the current limit by a small amount. The formula below can calculate the peak inductor current. VIN(MAX) – VSAT IPEAK = ILIM + L x 100ns Here, VSAT = 0.25V (switch saturation voltage). For systems with high input voltages and smaller inductance values, the current overshoot will be most apparent. This overshoot can be useful as it helps increase the amount of available output current. By using a small inductance value, the current limit overshoot can be quite high. Even though it is internally current limited to 350mA, the internal MOSFET of the SS8051 can handle larger currents without any problem, but the total efficiency will suffer. For best performance, the IPEAK is best kept below 500mA. Capacitor Selection Low ESR (Equivalent Series Resistance) capacitors should be used at the output to minimize the output ripple voltage and the peak-to-peak transient voltage. Multilayer ceramic capacitors (MLCC) are the best choice, as they have a very low ESR and are available in very small packages. Their small size makes them a good match with the SS8051’s SOT-23 package. If solid tantalum capacitors (like the AVX TPS, Sprague 593D families) or OS-CON capacitors are used, they will occupy more volume than ceramic ones and the higher ESR increases the output ripple voltage. It is important to use a capacitor with a sufficient voltage rating. A low ESR surface-mount ceramic capacitor also makes a good selection for the input bypass capacitor, which should be placed as close as possible to the SS8051. A 4.7µF input capacitor is sufficient for most applications. Diode Selection For most SS8051 applications, the high switching frequency requires high-speed Schottky diodes, such as the Motorola MBR0530 (0.5A, 30V) with their low forward voltage drop and fast switching speed. Many different manufacturers make equivalent parts, but make sure that the component is rated for at least 0.35A. To achieve high efficiency, the average current rating of the Schottky diodes should be greater than the peak switching current. Choose a reverse breakdown voltage greater than the output voltage. Lowering Output Ripple Voltage The SS8051 supplies energy to the load in bursts by ramping up the inductor current, then delivering that current to the load. Using low ESR capacitors will help VOUT + VD L = 2 ILIM x tOFF |
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