Datasheet

¨
¨
©
§
I
RMS
= I
OUTMAX
where r =
L
*
f
*
I
OUTMAX
*
V
IN
(V
IN
- V
OUT
)
*
V
OUT
1 -
¨
¨
©
§
V
OUT
V
IN
*
V
OUT
V
IN
+
r
2
12
*
I
SAT
> I
OUTMAX
+ I
RIPPLE
where I
RIPPLE
=
*
¨
¨
©
§
¨
¨
©
§
V
IN
- V
OUT
2
¨
¨
©
§
¨
¨
©
§
V
OUT
V
IN
¨
¨
©
§
¨
¨
©
§
f
1
*
L
*
LP3972
SNVS468K –SEPTEMBER 2006–REVISED MAY 2013 www.ti.com
where
• I
RIPPLE
: Average to peak inductor current
• I
OUTMAX
: Maximum load current (1500 mA)
• 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 (2)
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 3A.
A 2.2 µH inductor with a saturation current rating of at least 3A is recommended for most applications. The
inductor’s resistance should be less than 0.3Ω for a good efficiency. Table 1 lists suggested inductors and
suppliers. 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 models is unacceptable.
Input Capacitor Selection
A ceramic input capacitor of 10 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 input filter capacitor supplies current to the PFET switch of the converter 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:
(3)
The worst case is when V
IN
= 2 * V
OUT
Table 1. Suggested Inductors and Their Suppliers
Model Vendor Dimensions LxWxH (mm) D.C.R. (Typ)
FDSE0312-2R2M Toko 3.0 x 3.0 x 1.2 160 mΩ
DO1608C-222 Coilcraft 6.6 x 4.5 x 1.8 80 mΩ
Output Capacitor Selection
Use a 10 µF, 6.3V ceramic capacitor. 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.
46 Submit Documentation Feedback Copyright © 2006–2013, Texas Instruments Incorporated
Product Folder Links: LP3972