LTC3549 Datasheet, PDF (9/16 Pages) Linear Technology – 250mA Low VIN Buck Regulator in 2mm × 3mm DFN

LTC3549
OPERATION
be defined 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
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.
APPLICATIO S I FOR ATIO
The basic LTC3549 application circuit is shown on the first
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_L
=
V_OUT
f •L
⎛
⎝⎜
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 300mA rated
inductor should be enough for most applications (250mA
+ 50mA). For better efficiency, 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
efficiency 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 field/EMI requirements
than on what the LTC3549 requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3549 applications.
Table 1. Representative Surface Mount Inductors
MANU-
FACTURER
PART NUMBER
MAX DC
VALUE CURRENT
HEIGHT
(µH) (A) DCR (mm)
Taiyo
Yuden
LB2016T2R2M
2.2
315 0.13 1.6
LB2012T2R2M
2.2
240 0.23 1.25
LB2016T3R3M
3.3
280 0.2 1.6
LB2016T4R7M
4.7
210 0.25 1.6
Panasonic ELT5KT4R7M
4.7 950 0.2 1.2
Murata
LQH32CN4R7M34 4.7 450 0.2 2
TDK
VLF3012AT2R2M1R0 2.2
1 0.088 1.2
VLF3012AT3R3MR87 3.3 0.87 0.11 1.2
VLF3012AT4R7MR74 4.7 0.74 0.16 1.2
VLF3010AT2R2M1R0 2.2
1
0.10 1.0
VLF3010AT3R3MR87 3.3 0.87 0.15 1.0
VLF3010AT4R7MR70 4.7 0.74 0.24 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:
[ ^{( )}] _{CIN Required IRMS ≅ IOUT(MAX)}
V_OUT V_IN – V_OUT
V_IN
1/2
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-
3549f
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