Datasheet

LTC3549
9
3549f
be defi ned by the combination of the current needed to
charge the output capacitance and the current provided
to the load as the output voltage ramps up. The start-up
APPLICATIO S I FOR ATIO
WUU
U
The basic LTC3549 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 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
= 100mA
(40% of 250mA).
∆=
⎛
⎝
⎜
⎞
⎠
⎟
I
V
fL
V
V
L
OUT OUT
IN
•
–1
(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 300mA rated
inductor should be enough for most applications (250mA
+ 50mA). 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
100mA. 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 LTC3549 requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3549 applications.
waveform, shown in the Typical Performance Character-
istics, shows the output voltage start-up from 0V to 1.2V
with a 1kΩ load and V
IN
= 3.6V.
OPERATION
Table 1. Representative Surface Mount Inductors
MANU-
FACTURER PART NUMBER
VALUE
(µH)
MAX DC
CURRENT
(A) DCR
HEIGHT
(mm)
Taiyo
Yuden
LB2016T2R2M
LB2012T2R2M
LB2016T3R3M
LB2016T4R7M
2.2
2.2
3.3
4.7
315
240
280
210
0.13
0.23
0.2
0.25
1.6
1.25
1.6
1.6
Panasonic ELT5KT4R7M 4.7 950 0.2 1.2
Murata LQH32CN4R7M34 4.7 450 0.2 2
TDK VLF3012AT2R2M1R0
VLF3012AT3R3MR87
VLF3012AT4R7MR74
VLF3010AT2R2M1R0
VLF3010AT3R3MR87
VLF3010AT4R7MR70
2.2
3.3
4.7
2.2
3.3
4.7
1
0.87
0.74
1
0.87
0.74
0.088
0.11
0.16
0.10
0.15
0.24
1.2
1.2
1.2
1.0
1.0
1.0
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 quired I I
VVV
V
IN RMS OUT MAX
OUT IN OUT
IN
Re
–
()
/
≅
()
[]
12
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT
/2. This simple worst-case condition is com-
monly used for design because even significant devia-