6

LT1613

OPERATIO

Table 1 lists several inductors that will work with the

LT1613, although this is not an exhaustive list. There are

many magnetics vendors whose components are suitable

for use.

Diodes

A Schottky diode is recommended for use with the LT1613.

The Motorola MBR0520 is a very good choice. Where the

input to output voltage differential exceeds 20V, use the

MBR0530 (a 30V diode). If cost is more important than

efficiency, the 1N4148 can be used, but only at low current

loads.

Capacitors

The input bypass capacitor must be placed physically

close to the input pin. ESR is not critical and in most cases

an inexpensive tantalum is appropriate.

The choice of output capacitor is far more important. The

quality of this capacitor is the greatest determinant of the

output voltage ripple. The output capacitor must have

enough capacitance to satisfy the load under transient

conditions and it must shunt the switched component of

current coming through the diode. Output voltage ripple

results when this switched current passes through the

finite output impedance of the output capacitor. The

capacitor should have low impedance at the 1.4MHz

switching frequency of the LT1613. At this frequency, the

impedance is usually dominated by the capacitor’s equiva-

lent series resistance (ESR). Choosing a capacitor with

lower ESR will result in lower output ripple.

Ceramic capacitors can be used with the LT1613 provided

loop stability is considered. A tantalum capacitor has

some ESR and this causes an “ESR zero” in the regulator

loop. This zero is beneficial to loop stability. The internally

compensated LT1613 does not have an accessible com-

pensation node, but other circuit techniques can be em-

ployed to counteract the loss of the ESR zero, as detailed

in the next section.

Some capacitor types appropriate for use with the LT1613

are listed in Table 2.

OPERATION WITH CERAMIC CAPACITORS

Because the LT1613 is internally compensated, loop sta-

bility must be carefully considered when choosing an

output capacitor. Small, low cost tantalum capacitors

have some ESR, which aids stability. However, ceramic

capacitors are becoming more popular, having attractive

characteristics such as near-zero ESR, small size and

reasonable cost. Simply replacing a tantalum output ca-

pacitor with a ceramic unit will decrease the phase margin,

in some cases to unacceptable levels. With the addition of

the LT1613 can be used successfully with ceramic output

capacitors as described in the following figures.

A boost converter, stepping up 2.5V to 5V, is shown in

Figure 5. Tantalum capacitors are used for the input and

output (the input capacitor is not critical and has little

Table 1. Inductor Vendors

VENDOR

PHONE

URL

PART

COMMENT

Sumida

(847) 956-0666

www.sumida.com

CLS62-22022

22

µH Coupled

CD43-220

22

µH

Murata

(404) 436-1300

www.murata.com

LQH3C-220

22

µH, 2mm Height

LQH3C-100

10

µH

LQH3C-4R7

4.7

µH

Coiltronics

(407) 241-7876

www.coiltronics.com

CTX20-1

20

µH Coupled, Low DCR

Table 2. Capacitor Vendors

VENDOR

PHONE

URL

PART

COMMENT

Taiyo Yuden

(408) 573-4150

www.t-yuden.com

Ceramic Caps

X5R Dielectric

AVX

(803) 448-9411

www.avxcorp.com

Ceramic Caps

Tantalum Caps

Murata

(404) 436-1300

www.murata.com

Ceramic Caps