Datasheet

LTC3544
10
3544fa
The basic LTC3544 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 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 the 300mA regulator is
ΔI
L
= 120mA (40% of 300mA).
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 360mA rated inductor
should be enough for most applications (300mA + 60mA).
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 LTC3544 requires to operate. Table 1
shows typical surface mount inductors that work well in
LTC3544 applications.
APPLICATIONS INFORMATION
Table 1. Representative Surface Mount Inductors
Part Number
Value
(μH)
DCR
(Ω MAX)
MAX DC
CURRENT (A) W × L × H (mm
3
)
Sumida
CDH2D09B
10
6.4
4.7
3.3
0.47
0.32
0.218
0.15
0.48
0.6
0.7
0.85
3.0 × 2.8 × 1.0
Wurth
TPC744029
10
6.8
4.7
3.3
0.50
0.38
0.210
0.155
0.50
0.65
0.80
0.95
2.8 × 2.8 × 1.35
TDK
VLF3010AT
10
6.8
4.7
3.3
0.67
0.39
0.28
0.17
0.49
0.61
0.70
0.87
2.8 × 2.6 × 1.0
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.
The selection of C
OUT
is driven by the required effective
series resistance (ESR). Typically, once the ESR require-
ment for C
OUT
has been met, the RMS current rating
generally far exceeds the I
RIPPLE(P-P)
requirement. The
output ripple ΔV
OUT
is determined by:
ΔΔV I ESR
C
OUT L
OUT
≅+
⎛
⎝
⎜
⎞
⎠
⎟
1
8• •ƒ
where f = operating frequency, C
OUT
= output capacitance
and ΔI
L
= ripple current in the inductor. For a fi xed output