LT3685E データシートの表示（PDF） - Linear Technology

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LT3685E

36V, 2A, 2.4MHz Step-Down Switching Regulator

Linear Technology 

LT3685

APPLICATIONS INFORMATION

Table 2. Capacitor Vendors

VENDOR

Panasonic

PHONE

(714) 373-7366

URL

www.panasonic.com

Kemet

Sanyo

(864) 963-6300

(408) 749-9714

www.kemet.com

www.sanyovideo.com

Murata

AVX

Taiyo Yuden

(408) 436-1300

(864) 963-6300

www.murata.com

www.avxcorp.com

www.taiyo-yuden.com

PART SERIES

Ceramic,

Polymer,

Tantalum

Ceramic,

Tantalum

Ceramic,

Polymer,

Tantalum

Ceramic

Ceramic,

Tantalum

Ceramic

COMMANDS

EEF Series

T494, T495

POSCAP

TPS Series

where fSW is in MHz, and COUT is the recommended output

capacitance in μF. Use X5R or X7R types. This choice will

provide low output ripple and good transient response.

Transient performance can be improved with a higher value

capacitor if the compensation network is also adjusted

to maintain the loop bandwidth. A lower value of output

capacitor can be used to save space and cost but transient

performance will suffer. See the Frequency Compensation

section to choose an appropriate compensation network.

When choosing a capacitor, look carefully through the

data sheet to find out what the actual capacitance is under

operating conditions (applied voltage and temperature).

A physically larger capacitor, or one with a higher voltage

rating, may be required. High performance tantalum or

electrolytic capacitors can be used for the output capacitor.

Low ESR is important, so choose one that is intended for

use in switching regulators. The ESR should be specified by

the supplier, and should be 0.05 or less. Such a capacitor

will be larger than a ceramic capacitor and will have a larger

capacitance, because the capacitor must be large to achieve

low ESR. Table 2 lists several capacitor vendors.

Catch Diode

The catch diode conducts current only during switch off

time. Average forward current in normal operation can

be calculated from:

ID(AVG) = IOUT (VIN – VOUT)/VIN

where IOUT is the output load current. The only reason to

consider a diode with a larger current rating than necessary

for nominal operation is for the worst-case condition of

shorted output. The diode current will then increase to the

typical peak switch current. Peak reverse voltage is equal

to the regulator input voltage. Use a Schottky diode with a

reverse voltage rating greater than the input voltage. The

overvoltage protection feature in the LT3685 will keep the

switch off when VIN > 38V which allows the use of a 40V

rated Schottky even when VIN ranges up to 60V. Table 3

lists several Schottky diodes and their manufacturers.

Table 3. Diode Vendors

VR

PART NUMBER

(V)

On Semicnductor

MBRM120E

20

MBRM140

40

Diodes Inc.

B220

20

B230

30

DFLS240L

40

International Rectifier

10BQ030

30

20BQ030

30

IAVE

VF AT 1A VF AT 2A

(A)

(mV)

(mV)

1

530

595

1

550

2

500

2

500

2

500

1

420

470

2

470

Ceramic Capacitors

A precaution regarding ceramic capacitors concerns the

maximum input voltage rating of the LT3685. A ceramic

input capacitor combined with trace or cable inductance

3685fb

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