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LTC3704 数据表(PDF) 24 Page - Linear Technology |
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LTC3704 数据表(HTML) 24 Page - Linear Technology |
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24 / 36 page  LTC1871 24 1871fe APPLICATIONS INFORMATION that the input filter capacitor for the LTC1871 is not shared with other converters. AC input current from another converter could cause substantial input voltage ripple, and this could interfere with the operation of the LTC1871. A few inches of PC trace or wire (L ≈ 100nH) between the CIN of the LTC1871 and the actual source VIN should be sufficient to prevent current sharing problems. SEPIC Converter Applications The LTC1871 is also well suited to SEPIC (single-ended primary inductance converter) converter applications. The SEPIC converter shown in Figure 16 uses two inductors. The advantage of the SEPIC converter is the input voltage may be higher or lower than the output voltage, and the output is short-circuit protected. Figures 16. SEPIC Topology and Current Flow + + • • • • SW L2 COUT RL VOUT VIN C1 D1 L1 16a. SEPIC Topology + + • RL VOUT VIN D1 16c. Current Flow During Switch Off-Time + + • RL VOUT VIN VIN VIN 16b. Current Flow During Switch On-Time The first inductor, L1, together with the main switch, resembles a boost converter. The second inductor, L2, together with the output diode D1, resembles a flyback or buck-boost converter. The two inductors L1 and L2 can be independent but can also be wound on the same core since identical voltages are applied to L1 and L2 throughout the switching cycle. By making L1 = L2 and winding them on the same core the input ripple is reduced along with cost and size. All of the SEPIC applications information that follows assumes L1 = L2 = L. SEPIC Converter: Duty Cycle Considerations For a SEPIC converter operating in a continuous conduction mode (CCM), the duty cycle of the main switch is: D = VO + VD VIN + VO + VD where VD is the forward voltage of the diode. For convert- ers where the input voltage is close to the output voltage the duty cycle is near 50%. The maximum output voltage for a SEPIC converter is: VO(MAX) = VIN + VD () DMAX 1– DMAX –VD 1 1– DMAX The maximum duty cycle of the LTC1871 is typically 92%. SEPIC Converter: The Peak and Average Input Currents The control circuit in the LTC1871 is measuring the input current (either using the RDS(ON) of the power MOSFET or by means of a sense resistor in the MOSFET source), so the output current needs to be reflected back to the input in order to dimension the power MOSFET properly. Based on the fact that, ideally, the output power is equal to the input power, the maximum input current for a SEPIC converter is: IIN(MAX) =IO(MAX) • DMAX 1– DMAX The peak input current is: IIN(PEAK) = 1+ 2 •IO(MAX) • DMAX 1– DMAX The maximum duty cycle, DMAX, should be calculated at minimum VIN. The constant ‘χ’ represents the fraction of ripple current in the inductor relative to its maximum value. For example, if 30% ripple current is chosen, then χ = 0.30 and the peak current is 15% greater than the average. |
类似零件编号 - LTC3704 |
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类似说明 - LTC3704 |
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