Datasheet

LTC3409
14
3409fc
Design Example
As a design example, assume the LTC3409 is used in a
2-alkaline cell battery-powered application. The V
IN
will be
operating from a maximum of 3.2V down to about 1.8V.
The load current requirement is a maximum of 600mA
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 2:
L =
1
f•I
L
V
OUT
1–
V
OUT
V
IN






(2)
Substituting V
OUT
= 1.5V, V
IN
= 3.2V, ΔI
L
= 240mA and
f = 1.7MHz in Equation 2 gives:
L =
1
1.7MHz • 240m
A
1.5 1–
1.5
3.2
⎛
⎝
⎜
⎜
⎜
⎞
⎠
⎟
⎟
⎟
⎟
2.2μH
For best effi ciency choose a 750mA or greater inductor
with less than 0.3Ω series resistance. C
IN
will require
an RMS current rating of at least 0.3A ≅ I
LOAD(MAX)
/2 at
temperature.
For the feedback resistors, choose R2 = 133k. R1 can then
be calculated from Equation 2 at 191K. Figure 4 shows the
complete circuit along with its effi ciency curve.
Table 2 below gives 1% resistor values for selected output
voltages.
V
FB
GND
V
IN
V
IN
SYNC
RUN
SW
MODE
LTC3409
L1
2.2μH
R1
191k
C1
10pF
C
IN
4.7μF
V
IN
1.6V TO 5.5V
R2
133k
3409 F04
V
OUT
1.5V
0.6A
C
OUT
10μF
CER
L1: SUMIDA CDRH2D18/LD
Figure 4
LOAD CURRENT (mA)
30
EFFICIENCY (%)
90
100
20
10
80
50
70
60
40
0.1 10 100 1000
3409 F04b
0
1
1.8V
IN
3.1V
IN
2.5V
IN
Burst Mode Effi ciency, 1.5V
OUT
APPLICATIONS INFORMATION
V
OUT
R1 R2
0.85V 51.1k 133k
1.2V 127k 133k
1.5V 191k 133k
1.8V 255k 133k