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LT3990IMS Datasheet(PDF) 10 Page - Linear Technology |
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LT3990IMS Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  LT3990 10 3990p APPLICATIONS INFORMATION The highest allowed VIN during normal operation (VIN(OP-MAX)) is limited by minimum duty cycle and can be calculated by the following equation: V VV ft VV IN OP MAX OUT D SW ON MIN DSW () () • – - = + + where tON(MIN) is the minimum switch on-time (~150ns). However, the circuit will tolerate inputs up to the absolute maximum ratings of the VIN and BOOST pins, regardless of chosen switching frequency. During such transients where VIN is higher than VIN(OP-MAX), the switching frequency will be reduced below the programmed frequency to prevent damage to the part. The output voltage ripple and inductor current ripple may also be higher than in typical operation, however the output will still be in regulation. Inductor Selection For a given input and output voltage, the inductor value and switching frequency will determine the ripple current. The ripple current increases with higher VIN or VOUT and decreases with higher inductance and faster switching frequency. A good starting point for selecting the induc- tor value is: L VV f OUT D SW = + 3 where VD is the voltage drop of the catch diode (~0.7V), L is in μH and fSW is in MHz. 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 500mA. To keep the efficiency high, the series resistance (DCR) should be less than 0.1Ω, and the core material should be intended for high frequency applications. Table 2 lists several vendors and suitable types. This simple design guide will not always result in the optimum inductor selection for a given application. As a general rule, lower output voltages and higher switching frequency will require smaller inductor values. If the ap- plication requires less than 350mA load current, then a lesser inductor value may be acceptable. This allows use of a physically smaller inductor, or one with a lower DCR resulting in higher efficiency. There are several graphs in the Typical Performance Characteristics section of this data sheet that show the maximum load current as a function of input voltage for several popular output voltages. Low inductance may result in discontinuous mode operation, which is acceptable but reduces maximum load current. For details of maximum output current and discontinu- ous mode operation, see Linear Technology Application Note 44. Finally, for duty cycles greater than 50% (VOUT/VIN > 0.5), there is a minimum inductance required to avoid subharmonic oscillations. See Application Note 19. Input Capacitor Bypass the input of the LT3990 circuit with a ceramic capacitor of X7R or X5R type. Y5V types have poor performance over temperature and applied voltage, and should not be used. A 1μF to 4.7μF ceramic capacitor is adequate to bypass the LT3990 and will easily handle Table 2. Inductor Vendors VENDOR URL Coilcraft www.coilcraft.com Sumida www.sumida.com Toko www.tokoam.com Würth Elektronik www.we-online.com Coiltronics www.cooperet.com Murata www.murata.com |
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