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LM51551QDSSTQ1 Datasheet(PDF) 27 Page - Texas Instruments |
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LM51551QDSSTQ1 Datasheet(HTML) 27 Page - Texas Instruments |
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27 / 47 page  27 LM5155-Q1, LM51551-Q1 www.ti.com SNVSAY4B – AUGUST 2018 – REVISED JULY 2019 Product Folder Links: LM5155-Q1 LM51551-Q1 Submit Documentation Feedback Copyright © 2018–2019, Texas Instruments Incorporated Table 2. List of Materials() (continued) REFERENCE DESIGNATOR QTY. SPECIFICATION MANUFACTURER PART NUMBER RG 1 RES, 0, 5%, 0.1 W, 0603 Yageo America RC0603JR-070RL CF 1 CAP, CERM, 100 pF, 50 V,+/- 1%, C0G/NP0, 0603 Kemet C0603C101F5GACTU RF 1 RES, 100, 1%, 0.1 W, 0603 Yageo America RC0603FR-07100RL RSNB 0 N/A N/A N/A CSNB 0 N/A N/A N/A RBIAS 1 RES, 0, 5%, 0.1 W, AEC-Q200 Grade 0, 0603 Panasonic ERJ-3GEY0R00V CBIAS 1 CAP, CERM, 0.01 uF, 50 V, +/- 10%, X7R, 0603 Samsung Electro- Mechanics CL10B103KB8NCNC CVCC 1 CAP, CERM, 1 uF, 16 V, +/- 20%, X7R, AEC-Q200 Grade 1, 0603 MuRata GCM188R71C105MA64D RPG 1 RES, 24.9 k, 1%, 0.1 W, 0603 Yageo America RC0603FR-0724K9L 10.2.2.3 Inductor Selection (LM) When selecting the inductor, consider three key parameters: inductor current ripple ratio (RR), falling slope of the inductor current, and RHP zero frequency (fRHP). Inductor current ripple ratio is selected to have a balance between core loss and copper loss. The falling slope of the inductor current must be low enough to prevent sub-harmonic oscillation at high duty cycle (additional RSL resistor is required if not). Higher fRHP (= lower inductance) allows a higher crossover frequency and is always preferred when using a small value output capacitor. The inductance value can be selected to set the inductor current ripple between 30% and 70% of the average inductor current as a good compromise between RR, FRHP and inductor falling slope. 10.2.2.4 Output Capacitor (COUT) There are a few ways to select the proper value of output capacitor (COUT). The output capacitor value can be selected based on output voltage ripple, output overshoot or undershoot due to load transient. The ripple current rating of the output capacitors must be enough to handle the output ripple current. By using multiple output capacitors, the ripple current can be split. In practice, ceramic capacitors are placed closer to the diode and the MOSFET than the bulk aluminum capacitors in order to absorb the majority of the ripple current. 10.2.2.5 Input Capacitor The input capacitors decrease the input voltage ripple. The required input capacitor value is a function of the impedance of the source power supply. More input capacitors are required if the impedance of the source power supply is not low enough. 10.2.2.6 MOSFET Selection The MOSFET gate driver of the device is sourced from the VCC. The maximum gate charge is limited by the 35- mA VCC sourcing current limit. A leadless package is preferred for high switching-frequency designs. The MOSFET gate capacitance should be small enough so that the gate voltage is fully discharged during the off-time. 10.2.2.7 Diode Selection A Schottky is the preferred type for D1 diode due to its low forward voltage drop and small reverse recovery charge. Low reverse leakage current is important parameter when selecting the Schottky diode. The diode must be rated to handle the maximum output voltage plus any switching node ringing. Also, it must be able to handle the average output current. |
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