LM3668

www.ti.com

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 m

Ω 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 2. Suggest Inductors and Suppliers

MODEL

VENDOR

DIMENSIONS

D.C.R (m

Ω)(MAX)

LxWxH (mm)

LPS4012-222L

Coilcraft

4 x 4 x 1.2

100

2.1

LPS4018-222L

Coilcraft

4 x 4 x 1.8

70

2.5

1098AS-2R0M (2 µH)

TOKO

3 x 2.8 x 1.2

67

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

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

is at the switching frequency of the part.

Table 3. Suggested Capacitors and Suppliers

CASE SIZE

MODEL

TYPE

VENDOR

VOLTAGE RATING (V)

INCH (mm)

GRM21BR60J106K

Ceramic, X5R

Murata

6.3

0805 (2012)

JMK212BJ106K

Ceramic, X5R

Taiyo-Yuden

6.3

0805 (2012)

C2012X5R0J106K

Ceramic, X5R

TDK

6.3

0805 (2012)

LMK212 BJ106MG (±20%)

Ceramic, X5R

Taiyon-Yuden

10

0806(2012)

Copyright © 2007–2015, Texas Instruments Incorporated

Submit Documentation Feedback

21

Product Folder Links: LM3668