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LM3678 数据表(PDF) 10 Page - Texas Instruments |
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LM3678 数据表(HTML) 10 Page - Texas Instruments |
10 / 18 page ¸ ¹ · ¨ © § f 1 L 2 ! ISAT IOUTMAX + IRIPPLE where IRIPPLE = VOUT VIN ¸ ¹ · ¨ © § ¸ ¹ · ¨ © §V IN - VOUT LM3678 SNVS464B – APRIL 2008 – REVISED JANUARY 2009 www.ti.com INTERNAL SYNCHRONOUS RECTIFICATION The LM3678 uses an internal NFET as a synchronous rectifier to reduce rectifier forward voltage drop and associated power loss. Synchronous rectification provides a significant improvement in efficiency whenever the output voltage is relatively low compared to the voltage drop across an ordinary rectifier diode. CURRENT LIMITING A current limit feature allows the LM3678 to protect itself and external components during overload conditions by implementing current limiting with an internal comparator that trips at 2.15A (typ). If the output is shorted to ground the device enters a timed current limit mode where the NFET is turned on for a longer duration until the inductor current falls below a low threshold. This allows the inductor current more time to decay, thereby preventing runaway. SHUTDOWN MODE Setting the EN input pin low (<0.4V) places the LM3678 in shutdown mode. During shutdown the PFET switch, NFET switch, reference, control and bias circuitry of the LM3678 are turned off. Setting EN high (>1.0V) enables normal operation. It is recommended to set EN pin low to turn off the LM3678 during system power up and undervoltage conditions when the supply is less than 2.5V. Do not leave the EN pin floating. SOFT START The LM3678 has a soft-start circuit that limits in-rush current during start-up. During start-up the switch current limit is increased in steps. Soft start is activated only if EN goes from logic low to logic high after VIN reaches 2.5V. Soft start is implemented by increasing switch current limit in steps of 250mA, 500mA, 1A and 2A (typical switch current limit). The start-up time thereby depends on the output capacitor and load current demanded at start-up. Inductor Selection There are two main considerations when choosing an inductor; 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 specifications are followed by different manufacturers so attention must be given to details. Saturation current ratings are typically specified at 25°C. However, ratings at the maximum ambient temperature of application should be requested from the manufacturer. 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 should be greater than the sum of the maximum load current and the worst case average to peak inductor current. This can be written as (1) • IRIPPLE: average to peak inductor current • IOUTMAX: maximum load current (1.5A) • VIN: maximum input voltage in application • L : minimum inductor value including worst case tolerances (30% drop can be considered for method 1) • f : minimum switching frequency (2.7Mhz) • VOUT: output voltage For a more conservative approach, a 1µH inductor with a saturation current rating of at least 2.5A is recommended for most applications. 10 Submit Documentation Feedback Copyright © 2008–2009, Texas Instruments Incorporated Product Folder Links: LM3678 |
类似零件编号 - LM3678_14 |
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类似说明 - LM3678_14 |
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