LTC3411_15 Datasheet, PDF (10/24 Pages) Linear Technology – 1.25A, 4MHz, Synchronous Step-Down DC/DC Converter

LTC3411
APPLICATIONS INFORMATION
higher ripple current which causes this to occur at lower
load currents. This causes a dip in efficiency in the upper
range of low current operation. In Burst Mode operation,
lower inductance values will cause the burst frequency
to increase.
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 don’t 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 LTC3411 requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3411 applications.
Table 1. Representative Surface Mount Inductors
MANU-
FACTURER PART NUMBER
MAX DC
VALUE CURRENT DCR HEIGHT
Toko
A914BYW-2R2M-D52LC 2.2μH 2.05A 49mΩ 2mm
Toko
A915AY-2ROM-D53LC 2μH 3.3A 22mΩ 3mm
Coilcraft D01608C-222
2.2μH 2.3A 70mΩ 3mm
Coilcraft LP01704-222M
2.2μH 2.4A 120mΩ 1mm
Sumida CDRH4D282R2
2.2μH 2.04A 23mΩ 3mm
Sumida CDC5D232R2
2.2μH 2.16A 30mΩ 2.5mm
Taiyo Yuden N06DB2R2M
2.2μH 3.2A 29mΩ 3.2mm
Taiyo Yuden N05DB2R2M
2.2μH 2.9A 32mΩ 2.8mm
Murata LQN6C2R2M04
2.2μH 3.2A 24mΩ 5mm
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 5, 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 cur-
rent 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 LTC3411 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 (see Board Layout Considerations) to avoid ring-
ing 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_OUT/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 ≈ I_MAX
V_OUT (V_IN − V_OUT )
V_IN
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