LP3910 datasheet(65/72 Pages) TI1

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VPP-RMS =

VPP-C

2 + V

PP-ESR

VPP-C =

IRIPPLE

IRMS = IOUTMAX

1 -

VIN

VOUT

L * f * IOUTMAX * VIN

where

r =

(VIN ± VOUT) * VOUT)

LP3910

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SNVS481M – NOVEMBER 2006 – REVISED DECEMBER 2015

9.2.2.2.2

Buck1, Buck2 and Buck-Boost Capacitor Selection

9.2.2.2.2.1

Input Capacitor Selection for Buck1, Buck2 and Buck-Boost

A ceramic input capacitor of 10

μF, 6.3 V is sufficient for the magnetic DC-DC converters. Place the input

capacitor as close as possible to the input of the device. A large value may be used for improved input voltage

filtering. The recommended capacitor types are X7R or X5R. Y5V-type capacitors must not be used. DC bias

characteristics of ceramic capacitors must be considered when selecting case sizes like 0805 and 0603. The

input filter capacitor supplies current to the PFET switch of the DC-DC converter in the first half of each cycle

and reduces voltage ripple imposed on the input power source. Low ESR in a ceramic capacitor provides the

best noise filtering of the input voltage spikes due to fast current transients. A capacitor with sufficient ripple

current rating must be selected. The Input current ripple can be calculated as:

(6)

The worse case is when VIN = 2 × VOUT.

9.2.2.2.2.2

Output Capacitor Selection for Buck1, Buck2 and Buck-Boost

A 10-

μF, 6.3-V ceramic capacitor must be used on the output of the Buck1 and Buck2 magnetic DC-DC

converters. The buck-boost needs a 22-

μF capacitor. The output capacitor must be mounted as close as

possible to the output of the device. A large value may be used for improved input voltage filtering. The

recommended capacitor types are X7R or X5R. Y5V-type capacitors should not be used. 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 and analyzed as

part of the capacitor selection process.

The output filter capacitor of the magnetic DC-DC converter 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 ESD to perform these

functions.

The output voltage ripple is caused by the charging and the discharging of the output capacitor and also due to

its ESR and can be calculated using Equation 7:

(7)

Voltage peak-to-peak ripple due to ESR can be expressed by Equation 8:

VPP-ESR = 2 × IRIPPLE × RESR

(8)

Because the VPP-C and VPP-ESR are out of phase, the RMS value can be used to get an approximate value of the

peak-to-peak ripple:

(9)

The output voltage ripple is dependent on the inductor current ripple and the ESR of the output capacitor (RESR).

The RESR is frequency dependent as well as temperature dependent. The RESR must be calculated with the

applicable switching frequency and ambient temperature.

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