LM3674_15 Datasheet, PDF (13/26 Pages) Texas Instruments – LM3674 2-MHz, 600-mA Step-Down DC-DC Converter in SOT-23

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LM3674
SNVS405G – DECEMBER 2005 – REVISED APRIL 2015
8.2.1.2 Detailed Design Procedure
8.2.1.2.1 Inductor Selection
There are two main considerations when choosing an inductor:
• The inductor should not saturate.
• 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 the application should be requested from the manufacturer. The minimum value of
inductance to ensure good performance is 1.76 µH at I_LIM (typical) 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 maximum load current and the worst case average to peak
inductor current. This can be written as:
I_SAT > I_OUTMAX + I_RIPPLE
(4)
where
I_RIPPLE
=
§ ^{VIN - VOUT}· § ^VOUT·§
© 2 x L ¹ © V_IN ¹©
1
f
·
¹
and
• I_RIPPLE is average-to-peak inductor current
• I_OUTMAX is maximum load current (600 mA)
• V_IN is maximum input voltage in application
• L is minimum inductor value including worst case tolerances (30% drop can be considered for method 1
• f is minimum switching frequency (1.6 MHz)
• V_OUT is output voltage
(5)
Method 2:
A more conservative and recommended approach is to choose an inductor that has saturation current rating
greater than the maximum 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
resistance of the inductor should be less than 0.3 Ω for good efficiency. Table 1 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.
MODEL
DO3314-222MX
LPO3310-222MX
ELL5GM2R2N
CDRH2D14NP-2R2NC
Table 1. Suggested Inductors and Their Suppliers
VENDOR
Coilcraft
Coilcraft
Panasonic
Sumida
DIMENSIONS L×W×H (mm)
3.3 x 3.3 x 1.4
3.3 x 3.3 x 1.0
5.2 x 5.2 x 1.5
3.2 x 3.2 x 1.55
D.C.R (maximum) (mΩ)
200
150
53
94
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