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LT3684

3684f

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

Ω 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

PART NUMBER

(V)

IAVE

(A)

VF AT 1A

(mV)

VF AT 2A

(mV)

On Semicnductor

MBRM120E

MBRM140

530

550

595

Diodes Inc.

B120

B130

B220

B230

DFLS240L

500

International Rectiﬁer

10BQ030

20BQ030

420

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.

VENDOR

PHONE

URL

PART SERIES

COMMANDS

Panasonic

(714) 373-7366

www.panasonic.com

Ceramic,

Polymer,

Tantalum

EEF Series

Kemet

(864) 963-6300

www.kemet.com

Ceramic,

Tantalum

T494, T495

Sanyo

(408) 749-9714

www.sanyovideo.com

Ceramic,

Polymer,

Tantalum

POSCAP

Murata

(408) 436-1300

www.murata.com

Ceramic

AVX

www.avxcorp.com

Ceramic,

Tantalum

TPS Series

Taiyo Yuden

(864) 963-6300

www.taiyo-yuden.com

Ceramic

Table 2. Capacitor Vendors

APPLICATIONS INFORMATION

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