LP3907 datasheet(41/58 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

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SNVS511S – JUNE 2007 – REVISED APRIL 2016

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Typical Application (continued)

9.2.2 Detailed Design Procedure

9.2.2.1 Component Selection

9.2.2.1.1

Inductors for SW1 And SW2

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

current ripple is small enough to achieve the desired output voltage ripple. Care must 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 must be requested

from the manufacturer.

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

9.2.2.1.1.1

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:

where

•

IRIPPLE = Maximum load current

•

IOUTMAX = Average to peak inductor current

•

VIN = Maximum input voltage to the buck

•

L = Min inductor value including worse case tolerances (30% drop can be considered for method 1)

•

f = Minimum switching frequency (1.6 MHz)

•

VOUT = Buck output voltage

(6)

9.2.2.1.1.2

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 1250 mA for Buck1 and 1750 mA for Buck2.

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

(7)

space

Table 8. Suggested Inductor Values

INDUCTOR

VALUE (µH)

DESCRIPTION

NOTES

LSW1,2

2.2

SW1,2 inductor

DCR: 70 m

Ω

9.2.2.1.2

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.

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