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LM3678 Datasheet(PDF) 11 Page - Texas Instruments

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Part # LM3678
Description  High-Performance Miniature 1.5-A Step-Down DC-DC Converter for Handheld Applications
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Manufacturer  TI [Texas Instruments]
Direct Link  http://www.ti.com
Logo TI - Texas Instruments

LM3678 Datasheet(HTML) 11 Page - Texas Instruments

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V
PP-RMS
=
V
PP-C
2 + V
PP-ESR
2
VPP-C =
4*f*C
IRIPPLE
IRMS = IOUTMAX
The worst case is when VIN = 2 VOUT
(VIN - VOUT) VOUT
L f IOUTMAX VIN
r =
VOUT
VIN
r
2
12
1 -
+
VOUT
VIN
¸¸
¹
·
¨¨
©
§
LM3678
www.ti.com
SNVS464C – APRIL 2008 – REVISED MAY 2013
Input Capacitor Selection
A ceramic input capacitor of 10uF, 6.3V is sufficient for most applications. Place the input capacitor as close as
possible to the VIN pin of the device. A larger value may be used for improved input voltage filtering. Use X7R or
X5R types; do not use Y5V. DC bias characteristics of ceramic capacitors must be considered when selecting
case sizes like 0805 and 0603. The input filter capacitor supplies current to the PFET switch of the LM3678 in
the first half of each cycle and reduces voltage ripple imposed on the input power source. A ceramic capacitor’s
low ESR provides the best noise filtering of the input voltage spikes due to this rapidly changing current. Select a
capacitor with sufficient ripple current rating. The input current ripple can be calculated as:
(2)
Table 2. Suggest Inductors and Their Suppliers
Model
Vendor
Dimensions
D.C.R (max)
ISAT
LxWxH (mm)
NR4012T1R0N
Taiyo Yuden
4 x 4 x 1.2
60m
2.5A
LPS4012-102L
Coilcraft
3.9 x 3.9 x 1.2
100m
2.5A
LPS4012-102L
Coilcraft
3.9 x 3.9 x 1.8
40m
3.4A
Output Capacitor Selection
A ceramic output capacitor of 22µF, 6.3V is sufficient for most applications. Use X7R or X5R types; do not use
Y5V. DC bias characteristics of ceramic capacitors must be considered when selecting case sizes like 0805 and
0603. DC bias characteristics vary from manufacturer to manufacturer and dc bias curves should be requested
from them as part of the capacitor selection process.
The output filter capacitor smooths out current flow from the inductor to the load, helps maintain a steady output
voltage during transient load changes and reduces output voltage ripple. These capacitors must be selected with
sufficient capacitance and sufficiently low ESR to perform these functions.
The output voltage ripple is caused by the charging and discharging of the output capacitor and by the RESR and
can be calculated as:
Voltage peak-to-peak ripple due to capacitance can be expressed as follow:
(3)
Voltage peak-to-peak ripple due to ESR can be expressed as follow:
VPP-ESR = (2 * IRIPPLE) * RESR
Because these two components are out of phase the RMS (root mean squared) value can be used to get an
approximate value of peak-to-peak ripple.
The peak-to-peak ripple voltage RMS value can be expressed as follow:
(4)
Note that the output voltage ripple is dependent on the inductor current ripple and the equivalent series
resistance of the output capacitor (RESR).
The RESR is frequency dependent (as well as temperature dependent); make sure the value used for calculations
is at the switching frequency of the part.
Copyright © 2008–2013, Texas Instruments Incorporated
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