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LT3684EDD

36V, 2A, 2.8MHz Step-Down Switching Regulator

Linear Technology 

LT3684

APPLICATIONS INFORMATION

Table 2. Capacitor Vendors

VENDOR

PHONE

Panasonic

(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

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 0.05Ω 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 diode with a reverse

voltage rating greater than the input voltage. 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.

B120

20

B130

30

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

1

500

1

500

2

500

2

500

2

500

1

420

470

2

470

Frequency Compensation

The LT3684 uses current mode control to regulate the

output. This simplifies loop compensation. In particular, the

LT3684 does not require the ESR of the output capacitor

for stability, so you are free to use ceramic capacitors to

achieve low output ripple and small circuit size. Frequency

compensation is provided by the components tied to the

VC pin, as shown in Figure 2. Generally a capacitor (CC)

and a resistor (RC) in series to ground are used. In addi-

tion, there may be lower value capacitor in parallel. This

capacitor (CF) is not part of the loop compensation but

is used to filter noise at the switching frequency, and is

required only if a phase-lead capacitor is used or if the

output capacitor has high ESR.

3684f

13

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