LT3467AIDDB-TRPBF(RevD) Ver la hoja de datos (PDF) - Linear Technology

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Fabricante

LT3467AIDDB-TRPBF
(Rev.:RevD)

1.1A Step-Up DC/DC Converter with Integrated Soft-Start

Linear Technology 

LT3467/LT3467A

APPLICATIONS INFORMATION

Capacitor Selection

Low ESR (equivalent series resistance) capacitors should

be used at the output to minimize the output ripple voltage.

Multi-layer ceramic capacitors are an excellent choice,

as they have extremely low ESR and are available in very

small packages. X5R dielectrics are preferred, followed

by X7R, as these materials retain the capacitance over

wide voltage and temperature ranges. A 4.7μF to 15μF

output capacitor is sufficient for most applications, but

systems with very low output currents may need only a

1μF or 2.2μF output capacitor. Solid tantalum or OS-CON

capacitors can be used, but they will occupy more board

area than a ceramic and will have a higher ESR. Always

use a capacitor with a sufficient voltage rating.

Ceramic capacitors also make a good choice for the input

decoupling capacitor, which should be placed as close as

possible to the LT3467. A 1μF to 4.7μF input capacitor

is sufficient for most applications. Table 2 shows a list

of several ceramic capacitor manufacturers. Consult the

manufacturers for detailed information on their entire

selection of ceramic parts.

Table 2. Ceramic Capacitor Manufacturers

Taiyo Yuden

(408) 573-4150

www.t-yuden.com

AVX

(803) 448-9411

www.avxcorp.com

Murata

(714) 852-2001

www.murata.com

The decision to use either low ESR (ceramic) capacitors

or the higher ESR (tantalum or OS-CON) capacitors can

affect the stability of the overall system. The ESR of any

capacitor, along with the capacitance itself, contributes

a zero to the system. For the tantalum and OS-CON ca-

pacitors, this zero is located at a lower frequency due to

the higher value of the ESR, while the zero of a ceramic

capacitor is at a much higher frequency and can generally

be ignored.

A phase lead zero can be intentionally introduced by placing

a capacitor (C4) in parallel with the resistor (R1) between

VOUT and VFB as shown in Figure 2. The frequency of the

zero is determined by the following equation.

ƒZ

=

2π

1

• R1• C4

By choosing the appropriate values for the resistor and

capacitor, the zero frequency can be designed to improve

the phase margin of the overall converter. The typical

target value for the zero frequency is between 35kHz

to 55kHz. Figure 3 shows the transient response of the

step-up converter from Figure 8 without the phase lead

capacitor C4. Although adequate for many applications,

phase margin is not ideal as evidenced by 2-3 “bumps”

in both the output voltage and inductor current. A 22pF

capacitor for C4 results in ideal phase margin, which is

revealed in Figure 4 as a more damped response and less

overshoot.

Diode Selection

A Schottky diode is recommended for use with the LT3467

and the LT3467A. The Philips PMEG 2005 is a very good

choice. Where the switch voltage exceeds 20V, use the

PMEG 3005 (a 30V diode). Where the switch voltage

exceeds 30V, use the PMEG 4005 (a 40V diode). These

diodes are rated to handle an average forward current of

0.5A. In applications where the average forward current

of the diode exceeds 0.5A, a Philips PMEG 2010 rated at

1A is recommended. For higher efficiency, use a diode

with better thermal characteristics such as the On Semi-

conductor MBRM120 (a 20V diode) or the MBRM140 (a

40V diode).

3467afd

8

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