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поискавой системы для электроныых деталей |
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поискавой системы для электроныых деталей |
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SP7656EN2-L/TR датащи(PDF) 6 Page - Exar Corporation |
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SP7656EN2-L/TR датащи(HTML) 6 Page - Exar Corporation |
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6 / 10 page  11/07/08 SP7656 PowerBlox Copyright 2008 EXAR Corporation 6 the capacitor discharges linearly as it supplies IOUT-Iin. The contribution to Input voltage ripple by each term can be calculated from: 2 ) ( , Vin Cin fs Vout Vin Vout Iout Cin V Irip Iout ESR ESR V 5 . 0 , Trise Irip Iout ESL ESL V 5 . 0 , Where Trise is the rise time of current through Capacitor. The total input voltage ripple is sum of the above: Output Capacitor Selection Select the output capacitor for voltage rating, capacitance and Equivalent Series Resistance (ESR). Nominally the voltage rating is selected to be twice as large as the output voltage. Select the capacitance to satisfy the specification for output voltage overshoot/undershoot caused by current step load. A steady-state output current IOUT corresponds to inductor stored energy of ½ L IOUT2. A sudden decrease in IOUT forces the energy surplus in L to be absorbed by COUT. This causes an overshoot in output voltage that is corrected by the power switch reducing in duty cycle. Use the following equation to calculate COUT: 2 2 2 1 2 2 Vout Vos I I L Cout Where: L is the output inductance I2 is the step load high current I1 is the step load low current Vos is output voltage including overshoot VOUT is steady state output voltage Output voltage undershoot calculation is more complicated. Test results for SP7656 buck circuits show that undershoot is approximately equal to overshoot. Therefore above equation provides a satisfactory method for calculating COUT. Select ESR such that output voltage ripple (VRIP) specification is met. There are two components to the output ripple voltage: The first component arises from charge transferred to and from COUT during each cycle. The second component is due to inductor ripple current flowing through the output capacitor’s ESR. It can be calculated from: 2 2 8 1 fs Cout ESR Irip Vrip Where: IRIP is inductor ripple current f s is switching frequency COUT is output capacitor calculated above Note that a smaller inductor results in a higher inductor ripple current, therefore requiring a larger COUT and/or lower ESR in order to meet the output voltage ripple requirement. Schottky Rectifier Selection Select the Schottky based on the voltage rating VR, Forward voltage Vf, and thermal resistance Rthja. For a low duty cycle application the Schottky is conducting most of the time and its conduction losses are the largest component of losses in the converter. Conduction losses can be estimated from: Vin Vout Iout Vf Pc 1 where: Vf is diode forward voltage at IOUT The AC losses from the switching capacitance of a Schottky are negligible and can be ignored. |
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