LP3913 datasheet(60/70 Pages) TI1

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+ Vpp-esr

Vpp-rms

Vpp-c

4 * f * C

Vpp-c =

Iripple

LP3913

SNVS489I – NOVEMBER 2006 – REVISED SEPTEMBER 2013

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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 as follows:

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

VPP-ESR = 2 * IRIPPLE * RESR

(6)

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:

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.

The RESR should be calculated with the applicable switching frequency and ambient temperature.

Capacitor

Min Value

Unit

Description

Recommended Type

CVDD

4.7

μF

Charger Input Capacitor

Ceramic, 6.3V, X5R

CCHG_DET

4.7

μF

Charger Input Capacitor

Ceramic, 6.3V, X5R

CUSB

4.7

μF

USB Power (VBUS)

Ceramic, 6.3V, X5R

Capacitor

CBATT

4.7

μF

Li-ion Battery Capacitor

Ceramic, 6.3V, X5R

CLDO1

1.0

μF

LDO Output Capacitor

Ceramic, 6.3V, X5R

CLDO2

1.0

μF

LDO Output Capacitor

Ceramic, 6.3V, X5R

CVREFH

0.1

μF

Bypass Capacitor for

Ceramic, PolyPropylene and

Internal Voltage

Polycarbonate Film

Reference

CVIN2,3

μF

Buck1, Buck2 Input

Ceramic, 6.3V, X5R

Capacitor

CVBUCK1,2

μF

BUCK1,2 Output

Ceramic, 6.3V, X5R

Capacitor

CSW3

μF

BUCK3 Output Capacitor

Ceramic, 6.3V, X5R

CVIN1

μF

LDO Bypass Capacitor

Ceramic, 6.3V, X5R

CVIN4

μF

Buck Bypass Capacitor

Ceramic, 6.3V, X5R

Schottky Diode on Charger Input CHG_IN

A Schottky diode is required in the external adapter path to block the reverse current from either the USB or the

battery source. The most critical parameter in the selection of the right Schottky diode is the leakage current,

which needs to be below 10 µA over the temperature range in order to prevent false detection of the presence of

an external adapter. In addition the Schottky diode should have a maximum voltage rating of 10V or higher. The

current rating depends on the current limit of the adapter. The forward voltage should be limited to 500 mV at its

maximum current. The recommended Schottky diode is MBRA210ET3 from ON Semiconductor which has a

reverse leakage current under 1 µA at room temperature and a forward voltage drop of 500 mV at their max

rated current IF = 2A.

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