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LT3474EFE-1-TRPBF Datasheet(PDF) 11 Page - Linear Technology |
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LT3474EFE-1-TRPBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  LT3474/LT3474-1 11 3474fd APPLICATIONS INFORMATION The optimum inductor for a given application may differ from the one indicated by this simple design guide. A larger value inductor provides a higher maximum load current, and reduces the output voltage ripple. If your load is lower than the maximum load current, then you can relax the value of the inductor and operate with higher ripple current. This allows you to use a physically smaller inductor, or one with a lower DCR resulting in higher efficiency. Be aware that if the inductance differs from the simple rule above, then the maximum load current will depend on input voltage. In addition, low inductance may result in discontinuous mode operation, which further reduces maximum load current. For details of maximum output current and discontinuous mode operation, see Linear Technology’s Application Note 44. Finally, for duty cycles greater than 50% (VOUT/VIN > 0.5), a minimum inductance is required to avoid sub-harmonic oscillations. See Application Note 19. The current in the inductor is a triangle wave with an average value equal to the load current. The peak switch current is equal to the output current plus half the peak-to-peak inductor ripple current. The LT3474 limits its switch cur- rent in order to protect itself and the system from overload faults. Therefore, the maximum output current that the LT3474 will deliver depends on the switch current limit, the inductor value, and the input and output voltages. When the switch is off, the potential across the inductor is the output voltage plus the catch diode drop. This gives the peak-to-peak ripple current in the inductor ΔIL = 1– DC () VOUT + VF () L•f () where f is the switching frequency of the LT3474 and L is the value of the inductor. The peak inductor and switch current is I SW PK () = I LPK () = I OUT + ΔIL 2 Inductor Selection and Maximum Output Current A good first choice for the inductor value is LV V kHz f OUT F =+ • () 900 where VF is the voltage drop of the catch diode (~0.4V), f is the switching frequency and L is in μH. With this value the maximum load current will be 1.1A, independent of input voltage. The inductor’s RMS current rating must be greater than the maximum load current and its saturation current should be at least 30% higher. For highest efficiency, the series resistance (DCR) should be less than 0.2Ω. Table 2 lists several vendors and types that are suitable. For robust operation at full load and high input voltages (VIN > 30V), use an inductor with a saturation current higher than 2.5A. Table 2. Inductors PART NUMBER VALUE (μH) IRMS (A) DCR (Ω) HEIGHT (mm) Sumida CR43-3R3 3.3 1.44 0.086 3.5 CR43-4R7 4.7 1.15 0.109 3.5 CDRH4D16-3R3 3.3 1.1 0.063 1.8 CDRH4D28-3R3 3.3 1.57 0.049 3 CDRH4D28-4R7 4.7 1.32 0.072 3 CDRH5D28-100 10 1.3 0.048 3 CDRH5D28-150 15 1.1 0.076 3 CDRH73-100 10 1.68 0.072 3.4 CDRH73-150 15 1.33 0.13 3.4 Coilcraft DO1606T-332 3.3 1.3 0.1 2 DO1606T-472 4.7 1.1 0.12 2 DO1608C-332 3.3 2 0.08 2.9 DO1608C-472 4.7 1.5 0.09 2.9 MOS6020-332 3.3 1.8 0.046 2 MOS6020-472 10 1.5 0.05 2 |
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