Datasheet

LTC3545/LTC3545-1
12
35451fb
APPLICATIONS INFORMATION
The basic LTC3545/LTC3545-1 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 selection 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 inductor values lower ripple
current and small inductor values 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 for an 800mA regulator is
ΔI
L
= 320mA (40% of 800mA).
ΔI
L
V
V
V
L OUT
OUT
IN
=
()()
⎛
⎝
⎜
⎞
⎠
⎟
1
1
ƒ
–
(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 960mA rated inductor
should be enough for most applications (800mA + 160mA).
For better effi ciency, choose a low DCR inductor.
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 LTC3545/LTC3545-1 require to operate.
Table 1 shows typical surface mount inductors that work
well in LTC3545/LTC3545-1 applications.
C
IN
and C
OUT
Selection
In continuous mode, a worst-case estimate for the input
current ripple can be determined by assuming that the
source current of the top MOSFET is a square wave of
duty cycle V
OUT
/V
IN
, and amplitude I
OUT(MAX)
. 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:
II
VVV
V
RMS OUT MAX
OUT IN OUT
IN
≅
()
()
–
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
= I
OUT
/2. This simple worst-case condition is commonly
used for design. Note that the capacitor manufacturer’s
ripple current ratings are often based on 2000 hours of
life (non-ceramic capacitors). This makes it advisable to
further de-rate the capacitor, or choose a capacitor rated
at a higher temperature than required. Always consult the
manufacturer if there is any question.
Table 1. Representative Surface Mount Inductors
PART
NUMBER
VALUE
(μH)
DCR
(Ω MAX)
MAX DC
CURRENT (A) W × L × H (mm
3
)
Wurth WE-
TPC 744031
1.5
2.5
3.6
0.035
0.045
0.065
1.75
1.45
1.38
3.8 × 3.8 × 1.65
CoilCraft
LPS4012
1
1.5
2.2
3.3
0.06
0.07
0.1
0.1
2.5
2.5
2.1
1.5
4.0 × 4.0 × 1.1
Sumida
CDH38D11/
SLD
1.4
2.4
3.6
0.055
0.094
0.13
1.8
1.3
1.1
4.0 × 4.0 × 1.2
Sumida
CDRH3D16
1.5
2.2
3.3
0.043
0.075
0.11
1.55
1.2
1.1
3.8 × 3.8 × 1.8