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LM3678 数据表(PDF) 10 Page - Texas Instruments

部件名 LM3678
功能描述  High-Performance Miniature 1.5-A Step-Down DC-DC Converter for Handheld Applications
Download  18 Pages
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制造商  TI1 [Texas Instruments]
网页  http://www.ti.com
标志 TI1 - Texas Instruments

LM3678 数据表(HTML) 10 Page - Texas Instruments

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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
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Product Folder Links: LM3678


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