LTC3409_15 Datasheet, PDF (14/16 Pages) Linear Technology – 600mA Low VIN Buck Regulator in 3mm 3mm DFN

LTC3409
APPLICATIONS INFORMATION
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. Efficiency 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
=
f
•
1

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
=
1.7MHz
1
•
240mA
1.5^{⎛⎝⎜⎜⎜}1–
13..52^{⎞⎠⎟⎟⎟}

2.2μH
For best efficiency 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 efficiency curve.
Table 2 below gives 1% resistor values for selected output
voltages.
V_OUT
R1
R2
0.85V
51.1k
133k
1.2V
127k
133k
1.5V
191k
133k
1.8V
255k
133k
V_IN
1.6V TO 5.5V
C_IN
4.7μF
R2
133k
LTC3409
V_FB SYNC
GND RUN
V_IN
SW
V_IN MODE
L1
2.2μH
R1
191k
3409 F04
L1: SUMIDA CDRH2D18/LD
C_OUT
10μF
CER
V_OUT
1.5V
0.6A
C1
10pF
Figure 4
Burst Mode Efficiency, 1.5V_OUT
100
90 _1.8VIN
80
3.1V_IN
70
2.5V_IN
60
50
40
30
20
10
0
0.1
1
10
100
1000
LOAD CURRENT (mA)
3409 F04b
3409fc
14