LTC3446
12
3446ff
applicaTions inForMaTion
Inductor Core Selection
Different core materials and shapes will change the
size/current and price/current relationship of an induc-
tor. Toroid or shielded pot cores in ferrite or permalloy
materials are small and dont radiate much energy, but
generally cost more than powdered iron core inductors
with similar electrical characteristics. The choice of which
style inductor to use often depends more on the price vs
size requirements and any radiated field/EMI requirements
than on what the LTC3446 requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3446 applications.
Table 1. Representative Surface Mount Inductors
MANU-
FACTURER
PART NUMBER
VALUE
MAX DC
CURRENT
DCR
HEIGHT
Toko A914BYW-2R2M-
D52LC
2.2礖 2.05A 49m?nbsp 2mm
Toko A915AY-2ROM-
D53LC
2礖 3.3A 22m?nbsp 3mm
Coilcraft D01608C-222 2.2礖 2.3A 70m?nbsp 3mm
Coilcraft LP01704-222M 2.2礖 2.4A 120m?nbsp 1mm
Sumida CDRH4D282R2 2.2礖 2.04A 23m?nbsp 3mm
Sumida CDC5D232R2 2.2礖 2.16A 30m?nbsp 2.5mm
Taiyo
Yuden
N06DB2R2M 2.2礖 3.2A 29m?nbsp 3.2mm
Taiyo
Yuden
N05DB2R2M 2.2礖 2.9A 32m?nbsp 2.8mm
Murata LQN6C2R2M04 2.2礖 3.2A 24m?nbsp 5mm
W黵th 744042001
1礖 2.6A 20m?nbsp 2mm
Catch Diode Selection
Although unnecessary in most applications, a small
improvement in efficiency can be obtained in a few ap-
plications by including the optional diode D1 shown in
Figure 1, which conducts when the synchronous switch
is off. When using Burst Mode operation or pulse skip
mode, the synchronous switch is turned off at a low current
and the remaining current will be carried by the optional
diode. It is important to adequately specify the diode peak
current and average power dissipation so as not to exceed
the diode ratings. The main problem with Schottky diodes
is that their parasitic capacitance reduces the efficiency,
usually negating the possible benefits for LTC3446 circuits.
Another problem that a Schottky diode can introduce is
higher leakage current at high temperatures, which could
reduce the low current efficiency.
Remember to keep lead lengths short and observe proper
grounding to avoid ringing and increased dissipation when
using a catch diode.
Input Capacitor (C
IN
) Selection
In continuous mode, the input current of the converter is a
square wave with a duty cycle of approximately V
OUTB
/V
IN
.
To prevent large voltage transients, a low equivalent series
resistance (ESR) input capacitor sized for the maximum
RMS current must be used. The maximum RMS capacitor
current is given by:
I
RMS
HI
MAX
V
OUTB
(V
IN
V
OUTB
)
V
IN
where the maximum average output current I
MAX
equals
the peak current minus half the peak-to-peak ripple cur-
rent, I
MAX
= I
MAXP
I
L
/2.
This formula has a maximum at V
IN
= 2V
OUTB
, where I
RMS
= I
OUT
/2. This simple worst case is commonly used to
design because even significant deviations do not offer
much relief. Note that capacitor manufacturers ripple cur-
rent ratings are often based on only 2000 hours lifetime.
This makes it advisable to further derate the capacitor,
or choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to meet
the size or height requirements of the design. An additional
0.1礔 to 1礔 ceramic capacitor is also recommended on
V
IN
for high frequency decoupling, when not using an all
ceramic capacitor solution.
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