Datasheet

I
RMS OUTMAX
x
x (1 -
V
OUT
V
IN
V
OUT
V
IN
= I
=
r
)
+
12
2
The worst case is when
OUT
V
V
IN
( )
OUT
V
V
IN
r
-
x
L
f
x
x x
V
IN
=
2 x
OUT
V
OUTMAX
I
where
V
IN
- V
OUT
2 x L
I
RIPPLE
=
V
OUT
V
IN
1
f
¹
·
©
§
¹
·
©
§
¹
·
©
§
LM3674
www.ti.com SNVS405F –DECEMBER 2005–REVISED MAY 2013
(9)
• I
Ripple
: average to peak inductor current
• I
outmax
: maximum load current (600mA)
• V
IN
: maximum input voltage in application
• L: min inductor value including worst case tolerances (30% drop can be considered for method 1)
• f: minimum switching frequency (1.6 MHz)
• V
OUT
: output voltage
Method 2:
A more conservative and recommended approach is to choose an inductor that has saturation current rating
greater than the max current limit of 1200 mA.
A 2.2 µH inductor with a saturation current rating of at least 1200 mA is recommended for most applications. The
inductor’s resistance should be less than around 0.3Ω for good efficiency. Table 2 lists suggested inductors and
suppliers. For low-cost applications, an unshielded bobbin inductor is suggested. For noise critical applications, a
toroidal or shielded-bobbin inductor should be used. A good practice is to lay out the board with overlapping
footprints of both types for design flexibility. This allows substitution of a low-noise toroidal inductor, in the event
that noise from low-cost bobbin models is unacceptable.
Table 2. Suggested Inductors and Their Suppliers
Model Vendor Dimensions LxWxH (mm) D.C.R (max)
DO3314-222MX Coilcraft 3.3 x 3.3 x 1.4 200 mΩ
LPO3310-222MX Coilcraft 3.3 x 3.3 x 1.0 150 mΩ
ELL5GM2R2N Panasonic 5.2 x 5.2 x 1.5 53 mΩ
CDRH2D14NP-2R2NC Sumida 3.2 x 3.2 x 1.55 94 mΩ
INPUT CAPACITOR SELECTION
A ceramic input capacitor of 4.7 µF, 6.3V is sufficient for most applications. Place the input capacitor as close as
possible to the V
IN
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 minimum input capacitance to ensure good performance is 2.2µF at 3V
dc bias; 1.5µF at 5V dc bias including tolerances and over ambient temperature range. The input filter
capacitor supplies current to the PFET switch of the LM3674 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:
(10)
OUTPUT CAPACITOR SELECTION
A ceramic output capacitor of 10 µF, 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 minimum output capacitance to ensure good performance is 5.75µF at 1.8V dc bias including
tolerances and over ambient temperature range. The output filter capacitor smoothes 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.
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