LTC3409A_15 Datasheet, PDF (14/16 Pages) Linear Technology – 600mA Low VIN Buck Regulator in 3mm 3mm DFN

LTC3409A
APPLICATIONS INFORMATION
Design Example
As a design example, assume the LTC3409A 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, so the minimum frequency setting of 1.7MHz
is chosen. Output voltage is 1.5V. With this information
we can calculate L using Equation 3:
L
=
f
•
1
ΔI_L
V_OUT
⎛
⎝⎜
1–
V_OUT
V_IN
⎞
⎠⎟
(3)
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
1.5V
⎛
⎝⎜
1–
1.5V
3.2V
⎞
⎠⎟
≅
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 = 137k. R1 is then
calculated to be 200k from Equation 2. 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
53.6k
137k
1.2V
137k
143k
1.5V
200k
137k
1.8V
267k
137k
V_IN
1.8V TO 3.2V
C_IN
4.7μF
R2
137k
LTC3409A
V_FB SYNC
GND RUN
V_IN
V_IN
R1
200k
SW
MODE
3409A F04
L1
2.2μH
L1: SUMIDA CDRH2D18/LD
C1
10pF
C_OUT
22μF
s2
V_OUT
1.5V
0.6A
Figure 4
Burst Mode Efficiency, 1.5V_OUT
100
90
1.8V_IN
80
3.2V_IN
70
60
2.5V_IN
50
40
30
20
10
0
0.1
1
10
100
1000
LOAD CURRENT (mA)
3409A F04b
3409af
14