LT3686 datasheet(13/28 Pages) LINER

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LT3686

3686fa

The MODE pin serves as mode select for the BD active

load circuit. The active load is enabled when MODE is

tied high and disabled when MODE is tied low. See Fixed

Frequency at Light Load section.

Inductor Selection and Maximum Output Current

A good first choice for the inductor value is:

4(VOUT + VD)

where VD is the voltage drop of the catch diode (~0.4V), L

is in μH, frequency is in MHz. With this value there will be

no subharmonic oscillation. The inductor’s RMS current

rating must be greater than the maximum load current

and its saturation current should be about 30% higher. For

robust operation during fault conditions, the saturation

current should be above 2A. To keep efficiency high, the

series resistance (DCR) should be less than 0.1Ω. Table

2 lists several vendors and types that are suitable.

There are several graphs in the Typical Performance

Characteristics section of this data sheet that show the

maximum load current as a function of input voltage and

inductor value for several popular output voltages. Low

inductance may result in discontinuous mode opera-

tion, which is okay, but further reduces maximum load

current. For details of the maximum output current and

discontinuous mode operation, see Linear Technology

Application Note 44.

ApplicAtions inForMAtion

Catch Diode

A low capacitance 1-2A Schottky diode is recommended

for the catch diode, D1. The diode must have a reverse

voltage rating equal to or greater than the maximum input

voltage. The MBRM140 is a good choice; it is rated for

1A continuous forward current and a maximum reverse

voltage of 40V.

Input Capacitor

Bypass the input of the LT3686 circuit with a 2.2μF or

higher value ceramic capacitor of X7R or X5R type. Y5V

types have poor performance over temperature and ap-

plied voltage and should not be used. A 2.2μF ceramic is

adequate to bypass the LT3686 and will easily handle the

ripple current. However, if the input power source has

high impedance, or there is significant inductance due to

long wires or cables, additional bulk capacitance may be

necessary. This can be provided with a low performance

electrolytic capacitor. Step-down regulators draw current

from the input supply in pulses with very fast rise and

fall times. The input capacitor is required to reduce the

resulting voltage ripple at the LT3686 and to force this very

high frequency switching current into a tight local loop,

minimizing EMI. A 2.2μF capacitor is capable of this task,

but only if it is placed close to the LT3686 and the catch

diode (see the PCB Layout section). A second precaution

regarding the ceramic input capacitor concerns the maxi-

mum input voltage rating of the LT3686. A ceramic input

capacitor combined with trace or cable inductance forms

Table 2

VENDOR

URL

PART SERIES

INDUCTANCE RATE (µH)

SIZE (mm)

Sumida

www.sumida.com

CDRH4D28

CDRH5D28

CDRH8D28

1.2 to 4.7

2.5 to 10

2.5 to 33

4.5

× 4.5

5.5

× 5.5

8.3

× 8.3

Toko

www.toko.com

A916CY

D585LC

2 to 12

1.1 to 39

6.3

× 6.2

8.1

× 8

Würth Elektronik

www.we-online.com

WE-TPC(M)

WE-PD2(M)

WE-PD(S)

1 to 10

2.2 to 22

1 to 27

4.8

× 4.8

5.2

× 5.8

7.3

× 7.3

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