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LT3481EMSE Datasheet(PDF) 10 Page - Linear Technology |
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LT3481EMSE Datasheet(HTML) 10 Page - Linear Technology |
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10 / 24 page  LT3481 10 3481fb frequency will be necessary to achieve safe operation at high input voltages. If the output is in regulation and no short-circuit or start-up events are expected, then input voltage transients of up to 36V are acceptable regardless of the switching frequency. In this mode, the LT3481 may enter pulse skipping opera- tion where some switching pulses are skipped to maintain output regulation. In this mode the output voltage ripple and inductor current ripple will be higher than in normal operation. The minimum input voltage is determined by either the LT3481’s minimum operating voltage of ~3.6V or by its maximum duty cycle (see equation in previous section). The minimum input voltage due to duty cycle is: V VV ft VV IN MIN OUT D SW OFF MIN DSW () () = + + 1– – where VIN(MIN) is the minimum input voltage, and tOFF(MIN) is the minimum switch off time (150ns). Note that higher switching frequency will increase the minimum input voltage. If a lower dropout voltage is desired, a lower switching frequency should be used. Inductor Selection For a given input and output voltage, the inductor value and switching frequency will determine the ripple current. The ripple current ΔIL increases with higher VIN or VOUT and decreases with higher inductance and faster switch- ing frequency. A reasonable starting point for selecting the ripple current is: ΔIL = 0.4((IOUT(MAX)) where IOUT(MAX) is the maximum output load current. To guarantee sufficient output current, peak inductor current must be lower than the LT3481’s switch current limit (ILIM). The peak inductor current is: IL(PEAK) = IOUT(MAX) + ΔIL/2 where IL(PEAK) is the peak inductor current, IOUT(MAX) is the maximum output load current, and ΔIL is the inductor ripple current. The LT3481’s switch current limit (ILIM) is at least 3.5A at low duty cycles and decreases linearly to 2.5A at DC = 0.8. The maximum output current is a func- tion of the inductor ripple current: IOUT(MAX) = ILIM – ΔIL/2 Be sure to pick an inductor ripple current that provides sufficient maximum output current (IOUT(MAX)). The largest inductor ripple current occurs at the highest VIN. To guarantee that the ripple current stays below the specified maximum, the inductor value should be chosen according to the following equation: L VV fI VV V OUT D L OUT D IN MAX = + ⎛ ⎝⎜ ⎞ ⎠⎟ + ⎛ ⎝ ⎜ ⎜ ⎞ ⎠ ⎟ () Δ 1– ⎟⎟ where VD is the voltage drop of the catch diode (~0.4V), VIN(MAX) is the maximum input voltage, VOUT is the output voltage, fSW is the switching frequency (set by RT), and L is in the inductor value. The inductor’s RMS current rating must be greater than the maximum load current and its saturation current should be about 30% higher. For robust operation in fault conditions (start-up or short circuit) and high input voltage (>30V), the saturation current should be above 3.5A. To keep the efficiency high, the series resistance (DCR) should be less than 0.1 Ω, and the core material should be intended for high frequency applications. Table 1 lists several vendors and suitable types. Table 1. Inductor Vendors VENDOR URL PART SERIES TYPE Murata www.murata.com LQH55D Open TDK www.componenttdk.com SLF7045 SLF10145 Shielded Shielded Toko www.toko.com D62CB D63CB D75C D75F Shielded Shielded Shielded Open Sumida www.sumida.com CR54 CDRH74 CDRH6D38 CR75 Open Shielded Shielded Open APPLICATIONS INFORMATION |
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