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LT3505

3505fc

Table 2. Inductor Vendors

VENDOR

URL

PART SERIES

INDUCTANCE RANGE (µH)

Size (mm)

Sumida

www.sumida.com

CDRH4D28

CDRH5D28

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

× 8.0

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

Inductor Selection and Maximum Output Current

A good ﬁrst choice for the inductor value is:

L = 1.2 (VOUT + VD)/fSW

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

L is in µH and fSW is in MHz. With this value there will

be no subharmonic oscillation for applications with 50%

or greater duty cycle. The inductor’s RMS current rating

must be greater than your maximum load current and

its saturation current should be about 30% higher. For

robust operation in fault conditions, the saturation current

should be above 2.2A. To keep efﬁciency high, the series

resistance (DCR) should be less than 0.1 . Table 2 lists

several vendors and types that are suitable.

Of course, such a simple design guide will not always

result in the optimum inductor for your application. A

larger value provides a higher maximum load current and

reduces output voltage ripple at the expense of slower

transient response. If your load is lower than 1.2A, then

you can decrease the value of the inductor and operate

with higher ripple current. This allows you to use a physi-

cally smaller inductor, or one with a lower DCR resulting

in higher efﬁciency. There are several graphs in the Typical

PerformanceCharacteristicssectionofthisdatasheetthat

show the maximum load current as a function of input

voltage and inductor value for several popular output volt-

ages. Low inductance may result in discontinuous mode

operation, which is okay, but further reduces maximum

load current. For details on maximum output current and

discontinuous mode operation, see Linear Technology

Application Note 44.

Catch Diode

Depending on load current, a 1A to 2A Schottky diode is

recommended for the catch diode, D1. The diode must

have a reverse voltage rating equal to or greater than the

maximuminputvoltage.TheONSemiconductorMBRM140

is a good choice; it is rated for 1A continuous forward

current and a maximum reverse voltage of 40V.

Input Capacitor

The input of the LT3505 circuit must be bypassed with a

X7R or X5R type ceramic capacitor. Y5V types have poor

performance over temperature and applied voltage and

should not be used. For switching frequencies higher than

750kHz,bypasstheinputwitha1µForhighervalueceramic

capacitor.Forswitchingfrequenciesbelow750kHz,bypass

the input with a 2.2

µF or higher value ceramic capacitor.

If the input power source has high impedance, or there is

signiﬁcantinductanceduetolongwiresorcables,additional

bulk capacitance may be necessary. This can be provided

with a low performance electrolytic capacitor.

Step-down regulators draw current from the input sup-

ply in pulses with very fast rise and fall times. The input

capacitor is required to reduce the resulting voltage

ripple at the LT3505 and to force this very high frequency

switching current into a tight local loop, minimizing EMI.

To accomplish this task, the input bypass capacitor must

be placed close to the LT3505 and the catch diode; see

the PCB Layout section. A second precaution regarding

the ceramic input capacitor concerns the maximum input

voltage rating of the LT3505. A ceramic input capacitor

combined with trace or cable inductance forms a high

quality (underdamped) tank circuit. If the LT3505 circuit

is plugged into a live supply, the input voltage can ring to

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