LM3676 datasheet(16/25 Pages) TI

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PP-RMS

PP-C

2 + V

PP-ESR

VPP-C =

4*f*C

IRIPPLE

IRMS = IOUTMAX

The worst case is when VIN = 2 VOUT

(VIN - VOUT) VOUT

L f IOUTMAX VIN

r =

VOUT

VIN

1 -

VOUT

VIN

¸¸

¨¨

LM3676

SNVS426C – NOVEMBER 2006 – REVISED MAY 2013

www.ti.com

(7)

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 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 5.75µF at 1.8V 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.

The output voltage ripple is caused by the charging and discharging of the output capacitor and by the RESR and

can be calculated as:

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

(8)

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

VPP-ESR = (2 * IRIPPLE) * RESR

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.

The peak-to-peak ripple voltage, rms value can be expressed as follow:

(9)

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

resistance of the output capacitor (RESR).

The RESR 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 Their Suppliers

Case Size

Model

Type

Vendor

Voltage Rating

Inch (mm)

4.7 µF for CIN

C2012X5R0J475K

Ceramic, X5R

TDK

6.3V

0805 (2012)

JMK212BJ475K

Ceramic, X5R

Taiyo-Yuden

6.3V

0805 (2012)

GRM21BR60J475K

Ceramic, X5R

Murata

6.3V

0805 (2012)

C1608X5R0J475K

Ceramic, X5R

TDK

6.3V

0603 (1608)

10 µF for COUT

GRM21BR60J106K

Ceramic, X5R

Murata

6.3V

0805 (2012)

JMK212BJ106K

Ceramic, X5R

Taiyo-Yuden

6.3V

0805 (2012)

C2012X5R0J106K

Ceramic, X5R

TDK

6.3V

0805 (2012)

C1608X5R0J106K

Ceramic, X5R

TDK

6.3V

0603 (1608)

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