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| LTC3549 Datasheet, PDF (14/16 Pages) Linear Technology – 250mA Low VIN Buck Regulator in 2mm × 3mm DFN | |||
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LTC3549
APPLICATIO S I FOR ATIO
Design Example
As a design example, assume the LTC3549 is used in a
2-alkaline cell battery-powered application. The VIN will be
operating from a maximum of 3.1V down to about 1.8V.
The load current requirement is a maximum of 250mA
but most of the time it will be in standby mode, requiring
only 2mA. Efï¬ciency at both low and high load currents
is important. Output voltage is 1.5V. With this information
we can calculate L using Equation 3:
L
=
VOUT
f ⢠âIL
â
ââ
1â
VOUT
VIN
â
â â
(3)
Substituting VOUT = 1.5V, VIN = 3.1V, ÎIL = 100mA and
f = 2.25MHz in Equation 3 gives:
L
=
1
2.25MHz ⢠100mA
1.5
â
ââ
1
â
1.5â
3.1â â
â
3.3µH
For best efï¬ciency choose a 350mA or greater inductor
with less than 0.3Ω series resistance. CIN will require an
RMS current rating of at least 0.125A â
ILOAD(MAX)/2 at
temperature.
For the feedback resistors, choose R2 = 137k. Then, from
Equation 3, R1 is 200k. Figure 4 shows the complete circuit
along with its efï¬ciency curve.
VIN
1.8V TO
3.1V
CIN
4.7µF
CERAMIC
RUN
SW
LTC3549
VIN
MODE VFB
GND
L1
3.3µH*
CL
22pF
R1
200k
R2
137k
VOUT
1.5V
COUT
4.7µF
CERAMIC
*TDK VLF3012AT-3R3MR87
3549 F04a
Figure 4a. High Efï¬ciency Step-Down Regulator
100
90
VIN = 1.8
80
VIN = 3.1
70
60
VIN = 2.5
50
40
30
20
10
0
0.1
1
10
100
1000
LOAD CURRENT (mA)
3549 F04b
Figure 4b. Burst Mode Efï¬ciency, VOUT = 1.5V
14
VOUT
100mV/DIV
AC COUPLED
ILOAD
200mA/DIV
IL
200mA/DIV
20µs/DIV
VIN = 2.5V
VOUT = 1.5V
ILOAD = 100mA to 250mA
3549 F04c
Figure 4c. Load Step Response
3549f
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