LP3907 datasheet(41/57 Pages) TI1

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L t

VIN - VOUT

IPP

VOUT

VIN ¹

where

Iripple =

VIN - VOUT

VOUT

VIN ¹

Isat > Ioutmax + Iripple

LP3907

www.ti.com

SNVS511N – JUNE 2007 – REVISED AUGUST 2011

COMPONENT SELECTION

Inductors for SW1 and SW2

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:

(5)

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 1)

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 1250mA for Buck1 and 1750mA for Buck2.

Given a peak-to-peak current ripple (IPP) the inductor needs to be at least

(6)

Inductor

Value

Unit

Description

Notes

LSW1,2

2.2

µH

SW1,2 inductor

D.C.R. 70m

Ω

External Capacitors

The regulators on the LP3907 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 1cm 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.

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