IRMS = IOUTMAX

The worst case is when VIN = 2 VOUT

(VIN - VOUT) VOUT

L f IOUTMAX VIN

r =

VOUT

VIN

r

2

12

1 -

+

VOUT

VIN

¸¸

¹

·

¨¨

©

§

¸

¹

·

¨

©

§

f

1

L

2

!

ISAT

IOUTMAX + IRIPPLE

where IRIPPLE =

VOUT

VIN

¸

¹

·

¨

©

§

¸

¹

·

¨

©

§V

IN - VOUT

LP3905

SNVS374D – JUNE 2006 – REVISED MAY 2013

www.ti.com

Buck Inductor Selection

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. Different saturation current rating

specs are followed by different manufacturers so attention must be given to details. Saturation current ratings are

typically specified at 25°C so ratings at max ambient temperature of application should be requested from

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:

(5)

•

IRIPPLE: average to peak inductor current

•

IOUTMAX: maximum load current (600mA)

•

VIN: maximum input voltage in application

•

L : min inductor value including worst case tolerances (30% drop can be considered for method 1)

•

f : minimum switching frequency (1.6Mhz)

•

VOUT: output voltage

Method 2:

A more conservative and recommended approach is to choose an inductor that has saturation current rating

greater than the max current limit of 1220mA.

A 2.2µH inductor with a saturation current rating of at least 1250mA is recommended for most applications.The

inductor’s resistance should be less than 0.3

Ω for good efficiency. For low-cost applications, an unshielded

bobbin inductor could be considered. For noise critical applications, a toroidal or shielded-bobbin inductor should

be used. A good practice is to lay out the board with overlapping footprints of both types for design flexibility.

This allows substitution of a low-noise shielded inductor, in the event that noise from low-cost bobbin models is

unacceptable.

Buck DC/DC Convertor Input Capacitor Selection

A ceramic input capacitor of 10µF, 6.3V is sufficient for most applications. Place the input capacitor as close as

possible to the VIN pin of the device. A larger value may be used for improved input voltage filtering. Use X7R or

X5R types, do not use Y5V. 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 LP3905 in the

first half of each cycle and reduces voltage ripple imposed on the input power source. A ceramic capacitor’s low

ESR provides the best noise filtering of the input voltage spikes due to this rapidly changing current. Select a

capacitor with sufficient ripple current rating. The input current ripple can be calculated as:

(6)

14

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