4*f*C

r =

r

2

12

1 -

+

¸¸

¹

·

¨¨

©

§

LM3673

SNVS434L – JULY 2006 – REVISED OCTOBER 2015

www.ti.com

9.2.2.3 Input Capacitor Selection

A ceramic input capacitor of 4.7 µF, 6.3 V is sufficient for most applications. Place the input capacitor as close as

X5R types; do not use Y5V. DC bias characteristics of ceramic capacitors must be considered when selecting

case sizes like 0805 and 0603. The minimum input capacitance to ensure good performance is 2.2 µF at 3-V DC

bias; 1.5 µF at 5-V DC bias including tolerances and over ambient temperature range. The input filter capacitor

supplies current to the PFET switch of the LM3673 in the first half of each cycle and reduces voltage ripple

imposed on the input power source. The low ESR of a ceramic capacitor 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:

(8)

Table 3. Suggested Inductors and Their Suppliers

MODEL

VENDOR

DIMENSIONS L × W × H (mm)

DCR (maximum)

COIL

BRL2518T2R2M

Taiyo Yuden

2.5 × 1.8 × 1.2

135 m

Ω

DO3314-222MX

Coilcraft

3.3 × 3.3 × 1.4

200 m

Ω

LPO3310-222MX

Coilcraft

3.3 × 3.3 × 1

150 m

Ω

CDRH2D14-2R2

Sumida

3.2 × 3.2 × 1.55

94 m

Ω

CHIP

KSLI-2520101AG2R2

Hitachi Metals

2.5 × 2 × 1.0

115 m

Ω

LQM31PN2R2M00

Murata

3.2 × 1.6 × 0.95

220 m

Ω

LQM2HPN2R2MJ0

Murata

2.5 × 2 × 1.2

160 m

Ω

9.2.2.4 Output Capacitor Selection

A ceramic output capacitor of 10 µF, 6.3 V is sufficient for most applications. 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 minimum output capacitance to ensure good performance is 5.75 µF at 1.8-V DC bias including tolerances

and over ambient temperature range. The output filter capacitor smoothes 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.

can be calculated as:

Voltage peak-to-peak ripple due to capacitance can be expressed as follows:

(9)

Voltage peak-to-peak ripple due to ESR can be expressed as follows:

Because these two components are out of phase the root mean squared (RMS) value can be used to get an

approximate value of peak-to-peak ripple.

16

Submit Documentation Feedback

Copyright © 2006–2015, Texas Instruments Incorporated

Product Folder Links: LM3673