Datasheet

I
SAT
>
+ I
RIPPLE
I
SAT
> I
OUTMAX
+ I
RIPPLE
For Buck
(V
OUT
- V
IN
)
(V
OUT
)
Where D =
& '¶= (1-D)
For Boost
I
OUTMAX
'¶
V
OUT
Where I
RIPPLE
=
(V
IN
- V
OUT
)
(2 x L x f)
x
V
IN
(2 x L x f)
V
OUT
x
V
IN
(V
OUT
- V
IN
)
Where I
RIPPLE
=
LM3668
www.ti.com SNVS449M –JUNE 2007–REVISED MAY 2013
where
• I
RIPPLE
: Peak inductor current
• I
OUTMAX
: Maximum load current
• V
IN
: Maximum input voltage in application
• L : Min inductor value including worst case tolerances (30% drop can be considered)
• f : Minimum switching frequency
• V
OUT
: Output voltage
• D: Duty Cycle for CCM Operation
• V
OUT
: Output Voltage
• V
IN
: Input Voltage
Example using above equations:
• V
IN
= 2.8V to 4V
• V
OUT
= 3.3V
• I
OUT
= 500 mA
• L = 2.2 µH
• F = 2 MHz
• Buck: I
SAT
= 567 mA
• Boost: I
SAT
= 638 mA (1)
As a result, the inductor should be selected according to the highest of the two I
SAT
values.
A more conservative and recommended approach is to choose an inductor that has a saturation current rating
greater than the maximum current limit of 2.05A.
A 2.2 µH inductor with a saturation current rating of at least 2.05A is recommended for most applications. The
inductor’s resistance should be less than 100 mΩ for good efficiency. For low-cost applications, an unshielded
bobbin inductor could be considered. 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 shielded inductor, in the event that noise from low-cost bobbin model is
unacceptable.
Table 3. Suggest Inductors and Suppliers
Model Vendor Dimensions D.C.R (max) I
SAT
LxWxH (mm)
LPS4012-222L Coilcraft 4 x 4 x 1.2 100 mΩ 2.1A
LPS4018-222L Coilcraft 4 x 4 x 1.8 70 mΩ 2.5A
1098AS-2R0M (2 µF) TOKO 3 x 2.8x 1.2 67 mΩ 1.8A ( lower current
applications)
Copyright © 2007–2013, Texas Instruments Incorporated Submit Documentation Feedback 19
Product Folder Links: LM3668