LP3913 datasheet(57/70 Pages) TI1

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Isat > Ioutmax + Iripple

Vin

Vout

Vin - Vout

where

Iripple =

LP3913

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SNVS489I – NOVEMBER 2006 – REVISED SEPTEMBER 2013

APPLICATIONS INFORMATION

Component Selection

Inductors for BUCK1, BUCK2 and BUCK3

There are two main considerations when choosing an inductor; the inductor should not saturate and the inductor

current ripple is small enough to achieve the desired output voltage ripple. Care should be taken when reviewing

the different saturation current ratings that are specified by different manufacturers.

Saturation current ratings are typically specified at 25ºC, so ratings at maximum ambient temperature of the

application should be requested from the manufacturer.

There are two methods to choose the inductor saturation current rating:

Method 1:The saturation current is greater than the sum of the maximum load current and the worst case

average to peak inductor current. This can be written as follows:

IRIPPLE:

Average to peak inductor current

IOUTMAX:

Maximum load current

VIN:

Maximum input voltage to the buck

Min inductor value including worse case tolerances (30% drop can be considered for method

Minimum switching frequency (1.6 mHz)

VOUT:

Buck Output voltage

Method 2:A more conservative and recommended approach is to choose an inductor that has saturation current

rating greater than the maximum current limit of TBA.

Inductor

Value

Unit

Description

Notes

LSW1,2

2.2

µH

BUCK1,2 Inductor

D.C.R. 70 m

Ω

LSW3

2.2

µH

BUCK3 Inductor

D.C.R. 70 m

Ω

External Capacitors

The regulators on the LP3913 require external capacitors for regulator stability. These are specifically designed

for portable applications requiring minimum board space and smallest components. These capacitors must be

correctly selected for good performance.

LDO Capacitor Selection

Input Capacitor

An input capacitor is required for stability. It is recommended that a 1.0 µF capacitor be connected between the

LDO input pin and ground (this capacitance value may be increased without limit).

This capacitor must be located a distance of not more than 1 cm from the input pin and returned to a clean

analog ground. Any good quality ceramic, tantalum, or film capacitor may be used at the input.

Important: Tantalum capacitors can suffer catastrophic failures due to surge currents when connected to a low

impedance source of power (like a battery or a very large capacitor). If a tantalum capacitor is used at the input,

it must be specified by the manufacturer to have a surge current rating sufficient for the application.

There are no requirements for the ESR (Equivalent Series Resistance) on the input capacitor, but tolerance and

temperature coefficient must be considered when selecting the capacitor to ensure the capacitance will remain

approximately 1.0 µF over the entire operating temperature range.

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