Datasheet

LTC3548
8
3548fc
required. Several capacitors may also be paralleled to meet
the size or height requirements of the design. An additional
0.1μF to 1μF ceramic capacitor is also recommended on
V
IN
for high frequency decoupling, when not using an all
ceramic capacitor solution.
Table 1. Representation Surface Mount Inductors
PART
NUMBER
VALUE
(μH)
DCR
(Ω MAX)
MAX DC
CURRENT (A)
SIZE
W × L × H (mm
3
)
Sumida
CDRH3D16
2.2
3.3
4.7
0.075
0.110
0.162
1.20
1.10
0.90
3.8 × 3.8 × 1.8
Sumida
CDRH2D11
1.5
2.2
0.068
0.170
0.900
0.780
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
0.079
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
Output Capacitor (C
OUT
) Selection
The selection of C
OUT
is driven by the required ESR to
minimize voltage ripple and load step transients. Typically,
once the ESR requirement is satisfi ed, the capacitance
is adequate for fi ltering. The output ripple (ΔV
OUT
) is
determined by:
ΔV
OUT
≈ ΔI
L
ESR +
1
8f
O
C
OUT
⎛
⎝
⎜
⎞
⎠
⎟
where f = operating frequency, C
OUT
= output capacitance
and ΔI
L
= ripple current in the inductor. The output ripple
is highest at maximum input voltage since ΔI
L
increases
with input voltage. With ΔI
L
= 0.3 • I
OUT(MAX)
the output
ripple will be less than 100mV at maximum V
IN
and
f
O
= 2.25MHz with:
ESR
COUT
< 150mΩ
Once the ESR requirements for C
OUT
have been met, the
RMS current rating generally far exceeds the I
RIPPLE(P-P)
requirement, except for an all ceramic solution.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the capacitance, ESR or
RMS current handling requirement of the application.
Aluminum electrolytic, special polymer, ceramic and dry
The inductor value will also have an effect on Burst Mode
operation. The transition from low current operation
begins when the peak inductor current falls below a level
set by the burst clamp. Lower inductor values result in
higher ripple current which causes this to occur at lower
load currents. This causes 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 inductor. Toroid
or shielded pot cores in ferrite or permalloy materials are
small and do not radiate much energy, but generally cost
more than powdered iron core inductors with similar elec-
trical 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
LTC3548 requires to operate. Table 1 shows some typi-
cal surface mount inductors that work well in LTC3548
applications.
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
where the maximum average output current I
MAX
equals
the peak current minus half the peak-to-peak ripple cur-
rent, I
MAX
= I
LIM
– ΔI
L
/2.
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
= I
OUT
/2. This simple worst-case is commonly used to
design because even signifi cant deviations do not offer
much relief. Note that capacitor manufacturer’s 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
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