V

PP-RMS

=

V

PP-C

PP-ESR

2

4*f*C

r =

r

2

12

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+

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¹

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LM3679

SNVS510A – SEPTEMBER 2007 – REVISED FEBRUARY 2008

www.ti.com

INPUT CAPACITOR SELECTION

A ceramic input capacitor of 4.7 µF, 6.3V 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 0603 and 0805. The minimum input capacitance to guarantee good performance is 2.2µF at

3V dc bias; 1.5µF at 5V dc bias including tolerances and over ambient temperature range. The input filter

capacitor supplies current to the PFET switch of the LM3679 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:

(2)

Table 2. Suggested Inductors and Their Suppliers

Model

Vendor

Dimensions LxWxH(mm)

D.C.R (max)

LQM21PN1R0M *

Murata

2.0 x1.25 x 0.5

190m

Ω

MIPSA2520D 1R0

FDK

2.5 x 2.0 x 1.2

100 m

Ω

LQM2HP 1R0

Murata

2.5 x 2.0 x 0.95

100 m

Ω

BRL2518T1R0M

Taiyo Yuden

2.5x 1.8 x 1.2

80 m

Ω

Note : *For Low Profile Solution

OUTPUT CAPACITOR SELECTION

A ceramic output capacitor of 10 µF, 6.3V 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 0603 and

0805. 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 guarantee good performance is 5.75µF at 2.5V 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

(3)

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

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

approximate value of peak-to-peak ripple.

Voltage peak-to-peak ripple, rms can be expressed as follow:

(4)

Note that the output voltage ripple is dependent on the inductor current ripple and the equivalent series

12

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