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LM3674MF-ADJ 데이터시트(PDF) 11 Page - National Semiconductor (TI) |
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LM3674MF-ADJ 데이터시트(HTML) 11 Page - National Semiconductor (TI) |
11 / 15 page Operation Description (Continued) will be controlled down to the lowest possible input voltage. When the device operates near 100% duty cycle, the output voltage supply ripple is slightly higher, approximately 25mV. The minimum input voltage needed to support the output voltage is • I LOAD Load current • R DSON,PFET Drain to source resistance of PFET switch in the triode region • R INDUCTOR Inductor resistance Application Information OUTPUT VOLTAGE SELECTION FOR ADJUSTABLE (LM3674-ADJ) The output voltage of the adjustable parts can be pro- grammed through the resistor network connected from V OUT to FB the to GND. V OUT will be adjusted to make FB equal to 0.5V. The resistor from FB to GND (R2) should be 200 k Ω to keep the current drawn through this network small but large enough that it is not susceptible to noise. If R 2 is 200K Ω, and given the V FB is 0.5V, then the current through the resistor feedback network will be 2.5µA. The output voltage formula is: • V OUT = Output Voltage (V) • V FB = Feedback Voltage (0.5V typ) • R 1 = Resistor from VOUT to FB ( Ω) • R 2 = Resistor from FB to GND ( Ω) For any output voltage greater than or equal to 1.0V a frequency zero must be added at 45KHz for stability. The formula is: For output voltages greater than or equal to 2.5V, a pole must also be placed at 45KHz as well. If the pole and zero are at the same frequency the formula for calculation of C2 is: The formula for location of zero and pole frequency created by adding C1,C2 are given below. It can be seen that by adding C1, a zero as well as a higher frequency pole is introduced. See the " LM3674-ADJ Configurations for " Various V OUT" table. TABLE 1. Adjustable LM3674 Configurations for Various V OUT VOUT (V) R1 (K Ω)R2 (KΩ) C1 (pF) C2 (pF) L (µH) CIN (µF) COUT (µF) 1.0 200 200 18 None 2.2 4.7 10 1.1 191 158 18 None 2.2 4.7 10 1.2 280 200 12 None 2.2 4.7 10 1.5 357 178 10 None 2.2 4.7 10 1.6 442 200 8.2 None 2.2 4.7 10 1.7 432 178 8.2 None 2.2 4.7 10 1.8 464 178 8.2 None 2.2 4.7 10 1.875 523 191 6.8 None 2.2 4.7 10 2.5 402 100 8.2 None 2.2 4.7 10 2.8 464 100 8.2 33 2.2 4.7 10 3.3 562 100 6.8 33 2.2 4.7 10 INDUCTOR SELECTION There are two main considerations when choosing an induc- tor; the inductor should not saturate, and the inductor current ripple should be small enough to achieve the desired output voltage ripple. Different saturation current rating specifica- tions are followed by different manufacturers so attention must be given to details. Saturation current ratings are typi- cally specified at 25˚C. However, ratings at the maximum ambient temperature of application should be requested from the manufacturer. The minimum value of inductance to guarantee good performance is 1.76µH at I LIM (typ) dc current over the ambient temperature range. Shielded inductors radiate less noise and should be preferred. There are two methods to choose the inductor saturation current rating. Method 1: The saturation current is greater than the sum of the maxi- mum load current and the worst case average to peak inductor current. This can be written as: www.national.com 11 |
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