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LP8866S-Q1 Datasheet(PDF) 58 Page - Texas Instruments |
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LP8866S-Q1 Datasheet(HTML) 58 Page - Texas Instruments |
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58 / 82 page  • VIN(min): Minimum input voltage • ƒSW: Minimum switching frequency of the converter • L: Inductance • VOUT: Output voltage • η: Efficiency of boost converter 8.2.1.2.2 Output Capacitor Selection Recommended voltage rating for output capacitors is 50% higher than maximum output voltage level. Capacitance value determines voltage ripple and boost stability. The DC-bias effect can reduce the effective capacitance significantly, by up to 80%, a consideration for capacitance value selection. The conservative target effective capacitance is 50 µF to achieve good phase and gain margin levels. A design table in product webpage could be refered for the target effective capacitance in a certain application. TI recommends using 33-µF Al- polymer electrolytic capacitor together with 10-µF ceramic capacitors in parallel to reduce ripple, increase stability, and reduce ESR effect. 8.2.1.2.3 Input Capacitor Selection Recommended input capacitance is the same as output capacitance although input capacitors are not as critical to boost operation. Input capacitance can be reduced but must ensure enough filtering for input power. 8.2.1.2.4 Charge Pump Output Capacitor TI recommends a ceramic capacitor with at least 10-V voltage rating for the output capacitor of the charge pump. A 10-μF capacitor can be used for most applications. 8.2.1.2.5 Charge Pump Flying Capacitor TI recommends a ceramic capacitor with at least 10-V voltage rating for the flying capacitor of the charge pump. One 2.2-μF capacitor connecting C1P and C1N pins can be used for most applications. 8.2.1.2.6 Output Diode A Schottky diode must be used for the boost output diode. Current rating must be at least 25% higher than the maximum output current. Schottky diodes with a low forward drop and fast switching speeds are ideal for increasing efficiency. At maximum current, the forward voltage must be as low as possible; less than 0.5 V is recommended. Reverse breakdown voltage of the Schottky diode must be significantly larger than the output voltage, 25% higher voltage rating is recommended. Do not use ordinary rectifier diodes, because slow switching speeds and long recovery times cause efficiency and load regulation to suffer. 8.2.1.2.7 Switching FET Gate-drive voltage for the FET is 5V. Switching FET is a critical component for determining power efficiency of the boost converter. Several aspects need to be considered when selecting switching FET such as voltage and current rating, RDSON, power dissipation, thermal resistance and rise/fall times. An N type MOSFET with at least 25% higher voltage rating than maximum output voltage must be used. Current rating of switching FET should be same or higher than inductor rating. RDSON must be as low as possible, less than 20 mΩ is recommended. Thermal resistance (RθJA) must also be low to dissipate heat from power loss on switching FET. In most cases, a resistance is recommended between GD pin and Switching FET's gate terminal. It could be used to control the rising/falling time of the switching FET. This gate resistance could offer the flexibility of balancing between EMC performance and efficiency. 8.2.1.2.8 Boost Sense Resistor The RSENSE resistor determines the boost overcurrent limit and is sensed every boost switching cycle. A high- power 20-mΩ resistor can be used for sensing the boost SW current and setting maximum current limit at 10 A (typical). RSENSE can be increased to lower this limit and can be calculated with Equation 22. In typical condition, to avoid too much efficiency loss on RSENSE resistor, boost overcurrent limit is recommended to be set above 4A, therefore RSENSE doesn't exceed 50 mΩ. Power rating can be calculated from the inductor current and sense resistor resistance value. LP8866S-Q1 SNVSBD1A – AUGUST 2020 – REVISED FEBRUARY 2021 www.ti.com 58 Submit Document Feedback Copyright © 2021 Texas Instruments Incorporated Product Folder Links: LP8866S-Q1 |
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