Datasheet

LTC3549
14
3549f
Figure 4b. Burst Mode Effi ciency, V
OUT
= 1.5V Figure 4c. Load Step Response
Design Example
As a design example, assume the LTC3549 is used in a
2-alkaline cell battery-powered application. The V
IN
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. Effi 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
V
fI
V
V
OUT
L
OUT
IN
=
∆
⎛
⎝
⎜
⎞
⎠
⎟
•
–1
(3)
Substituting V
OUT
= 1.5V, V
IN
= 3.1V, ΔI
L
= 100mA and
f = 2.25MHz in Equation 3 gives:
L
MHz mA
H=
⎛
⎝
⎜
⎞
⎠
⎟
≅µ
1
2 25 100
15 1
15
31
33
.•
.–
.
.
.
For best effi ciency choose a 350mA or greater inductor
with less than 0.3Ω series resistance. C
IN
will require an
RMS current rating of at least 0.125A ≅ I
LOAD(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 effi ciency curve.
APPLICATIO S I FOR ATIO
WUU
U
Figure 4a. High Effi ciency Step-Down Regulator
RUN
C
IN
4.7µF
CERAMIC
V
IN
1.8V TO
3.1V
LTC3549
V
IN
MODE
L1
3.3µH*
CL
22pF
R1
200k
R2
137k
3549 F04a
SW
V
FB
GND
C
OUT
4.7µF
CERAMIC
V
OUT
1.5V
*TDK VLF3012AT-3R3MR87
LOAD CURRENT (mA)
30
EFFICIENCY (%)
90
100
20
10
80
50
70
60
40
0.1 10 100 1000
3549 F04b
0
1
V
IN
= 1.8
V
IN
= 3.1
V
IN
= 2.5
20µs/DIV
V
OUT
100mV/DIV
AC COUPLED
I
LOAD
200mA/DIV
I
L
200mA/DIV
3549 F04c
V
IN
= 2.5V
V
OUT
= 1.5V
I
LOAD
= 100mA to 250mA