Datasheet

LTC3409
10
3409fc
APPLICATIONS INFORMATION
The basic LTC3409 application circuit is shown on the fi rst
page of this data sheet. External component selection is
driven by the load requirement and begins with the selec-
tion of L followed by C
IN
and C
OUT
.
Inductor Selection
For most applications, the value of the inductor will fall
in the range of 1μH to 10μH. Its value is chosen based
on the desired ripple current. Large value inductors
lower ripple current and small value inductors result in
higher ripple currents. Higher V
IN
or V
OUT
also increases
the ripple current as shown in Equation 1. A reasonable
starting point for setting ripple current is ΔI
L
= 240mA
(40% of 600mA).
I
L
=
1
f•L
V
OUT
1–
V
OUT
V
IN






(1)
The DC current rating of the inductor should be at least
equal to the maximum load current plus half the ripple
current to prevent core saturation. Thus, a 720mA rated
inductor should be enough for most applications (600mA +
120mA). For better effi ciency, choose a low DC resistance
inductor. The inductor value also has an effect on Burst
Mode operation. The transition to low current operation be-
gins when the inductor current peaks fall to approximately
200mA. Lower inductor values (higher ΔI
L
) will cause this
to occur at lower load currents, which can cause a dip in
effi ciency 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 fi eld/EMI requirements
than on what the LTC3409 requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3409 applications.
C
IN
and C
OUT
Selection
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle V
OUT
/V
IN
. To prevent large
voltage transients, a low ESR input capacitor sized for the
maximum RMS current must be used. The maximum RMS
capacitor current is given by:
C
IN
Required I
RMS
I
OUT(MAX)
V
OUT
V
IN
–V
OUT
()
⎡
⎣
⎤
⎦
1/ 2
V
IN
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT
/2. This simple worst-case condition is common-
ly used for design because even signifi cant deviations do
not offer much relief. Note that the capacitor manufacturer’s
ripple current ratings are often based on 2000 hours of
life. This makes it advisable to further derate the capacitor,
or choose a capacitor rated at a higher temperature than
required. Always consult the manufacturer if there is any
question. The selection of C
OUT
is driven by the required
effective series resistance (ESR). Typically, once the ESR
requirement for C
OUT
has been met, the RMS current
rating generally far exceeds the I
RIPPLE(P-P)
requirement.
The output ripple DV
OUT
is determined by:
V
OUT
= I
L
ESR +
1
8•f•C
OUT






Table 1. Representative Surface Mount Inductors
PART
NUMBER
VALUE
(μH)
DCR
(Ω MAX)
MAX DC
CURRENT (A)
SIZE
W × L × H (mm
3
)
Sumida
CDRH2D18/LD
2.2
3.3
0.041
0.054
0.85
0.75
3.2 × 3.2 × 2.0
Sumida
CDRH2D11
1.5
2.2
0.068
0.170
0.90
0.78
3.2 × 3.2 × 1.2
Sumida
CMD4D11
2.2
3.3
0.116
0.174
0.950
0.770
4.4 × 5.8 × 1.2
Murata
LQH32CN
1.0
2.2
0.060
0.097
1.00
0.79
2.5 × 3.2 × 2.0
Toko
D312F
2.2
3.3
0.060
0.260
1.08
0.92
2.5 × 3.2 × 2.0
Panasonic
ELT5KT
3.3
4.7
0.17
0.20
1.00
0.95
4.5 × 5.4 × 1.2