6
LT1613
OPERATIO
U
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
a phase lead capacitor (C
PL
) and isolating resistor (R3),
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 22µH Coupled
CD43-220 22µH
Murata (404) 436-1300 www.murata.com LQH3C-220 22µH, 2mm Height
LQH3C-100 10µH
LQH3C-4R7 4.7µH
Coiltronics (407) 241-7876 www.coiltronics.com CTX20-1 20µH 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
