LM3668_15 Datasheet, PDF (21/30 Pages) Texas Instruments – 1-A, High-Efficiency Dual-Mode Single-Inductor Buck-Boost DC-DC Converter

www.ti.com
LM3668
SNVS449O – JUNE 2007 – REVISED APRIL 2015
Typical Application (continued)
A 2.2-µH inductor with a saturation current rating of at least 2.05 A 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.
MODEL
LPS4012-222L
LPS4018-222L
1098AS-2R0M (2 µH)
Table 2. Suggest Inductors and Suppliers
VENDOR
Coilcraft
Coilcraft
TOKO
DIMENSIONS
LxWxH (mm)
4 x 4 x 1.2
4 x 4 x 1.8
3 x 2.8 x 1.2
D.C.R (mΩ)(MAX)
100
70
67
I_SAT (A)
2.1
2.5
1.8 (lower current
applications)
9.2.2.2 Input Capacitor Selection
A ceramic input capacitor of at least 10 µF, 6.3 V is sufficient for most applications. Place the input capacitor as
close as possible to the PVIN 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 or 0603. The input filter capacitor supplies current to the PFET switch of the
LM3668 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. For applications where input voltage is 4 V or higher, it is best to use a higher voltage rating capacitor to
eliminate the DC bias affect over capacitance.
9.2.2.3 Output Capacitor Selection
A ceramic output capacitor of 22 µF, 6.3 V (use 10 V or higher rating for 4.5 V-5 V output option) is sufficient for
most applications. Multilayer ceramic capacitors such as X7R or X5R with low ESR is a good choice for this as
well. These capacitors provide an ideal balance between small size, cost, reliability and performance. Do not use
Y5V ceramic capacitors as they have temperature limitation and poor dielectric performance over temperature
and poor voltage characteristic for a given value. In other words, ensure the minimum C_OUT value does not
exceed −40% of the above-suggested value over the entire range of operating temperature and bias conditions.
Extra attention is required if a smaller case size capacitor is used in the application. Smaller case size capacitors
typically have less capacitance for a given bias voltage as compared to a larger case size capacitor with the
same bias voltage. Please contact the capacitor manufacturer for detailed information regarding capacitance
verses case size. Table 3 lists several capacitor suppliers.
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.
Note that the output voltage ripple is dependent on the inductor current ripple and the equivalent series
resistance of the output capacitor (R_ESR).
The R_ESR is frequency dependent (as well as temperature dependent); make sure the value used for calculations
is at the switching frequency of the part.
Table 3. Suggested Capacitors and Suppliers
MODEL
TYPE
VENDOR
VOLTAGE RATING (V)
10 µF FOR C_IN (FOR 4.5/5 V OPTION, USE 10 V OR HIGHER RATING CAPACITOR)
GRM21BR60J106K
Ceramic, X5R
Murata
6.3
JMK212BJ106K
Ceramic, X5R
Taiyo-Yuden
6.3
C2012X5R0J106K
Ceramic, X5R
TDK
6.3
LMK212 BJ106MG (±20%)
Ceramic, X5R
Taiyon-Yuden
10
CASE SIZE
INCH (mm)
0805 (2012)
0805 (2012)
0805 (2012)
0806(2012)
Copyright © 2007–2015, Texas Instruments Incorporated
Product Folder Links: LM3668
Submit Documentation Feedback
21