¸

¹

·

¨

©

§

x

x

x

f

1

L

2

!

¸

¹

·

¨

©

§

¸

¹

·

¨

©

OUT

LM3686

www.ti.com

SNVS520E – AUGUST 2008 – REVISED MAY 2013

APPLICATION INFORMATION

Application Selection

It is strongly recommended to select the required components of LM3686 as described within the datasheet. If

other components are selected, the device will not perform up to standard and electrical characteristics cannot

be ensured.

Inductor Selection

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

current ripple should be small enough to achieve the desired output voltage ripple. Different saturation current

rating specifications are followed by different manufacturers so attention must be given to details. Saturation

current ratings are typically specified at 25°C. However, ratings at the maximum ambient temperature of

application should be requested from the manufacturer. The minimum value of inductance to ensure good

radiate less noise and should be preferred. There are two methods to choose the inductor saturation current

rating.

METHOD 1

The saturation current should be greater than the sum of the maximum load current and the worst case average

to peak inductor current. This can be written as:

(1)

where

•

•

•

•

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

•

f: minimum switching frequency (2.55 MHz)

(2)

METHOD 2

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

greater than the maximum current limit of 1375 mA.

A 1.0 µH inductor with a saturation current rating of at least 1375 mA is recommended for most applications. The

inductor’s resistance should be less than 0.3

Ω for good efficiency. Table 1 lists suggested inductors and

suppliers. 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.

Table 1. Suggested Inductors and their Suppliers

Model

Vendor

Dimensions LxWxH (mm)

DCR (max)

BRL2518T1R0M

TAIYO YUDEN

2.5 X 1.8 X 1.2

80

MDT2520CR1R0M

TOKO

2.5 X 2.0 X 1.0

80

KSLI252010AG1R0

HITACHI METALS

2.5 X 2.0 X 1.0

75

External Capacitors

As common with most regulators, the LM3686 requires external capacitors to ensure stable operation. The

LM3686 is specifically designed for portable applications requiring minimum board space and the smallest size

components. These capacitors must be correctly selected for good performance.

Copyright © 2008–2013, Texas Instruments Incorporated

Submit Documentation Feedback

17

Product Folder Links: LM3686