Datasheet

LTC3406AB
12
3406abfa
Design Example
As a design example, assume the LTC3406AB is used
in a single lithium-ion battery-powered cellular phone
application. The V
IN
will be operating from a maximum of
4.2V down to about 2.7V. The load current requirement
is a maximum of 0.6A 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 2.5V. With this information we can calculate L using
Equation (1),
L
fI
V
V
V
L
OUT
OUT
IN
=
()
Δ
()
−
⎛
⎝
⎜
⎞
⎠
⎟
1
1
(3)
Substituting V
OUT
= 2.5V, V
IN
= 4.2V, ΔI
L
= 240mA and
f = 1.5MHz in Equation (3) gives:
L
V
MHz mA
V
V
H=−
⎛
⎝
⎜
⎞
⎠
⎟
=μ
25
1 5 240
1
25
42
281
.
.( )
.
.
.
A 2.2μH inductor works well for this application. For best
effi ciency choose a 720mA or greater inductor with less
than 0.2Ω series resistance.
C
IN
will require an RMS current rating of at least 0.3A≅
I
LOAD(MAX)
/2 at temperature and C
OUT
will require an ESR
of less than 0.25Ω. In most cases, a ceramic capacitor
will satisfy this requirement.
Figure 3. LTC3406AB Layout Diagram
Figure 4. LTC3406AB Suggested Layout
APPLICATIONS INFORMATION
RUN
LTC3406AB
GND
SW
L1
R2 R1
C
FWD
BOLD LINES INDICATE HIGH CURRENT PATHS
V
IN
V
OUT
3406AB F05a
4
5
1
3
+
–
2
V
FB
V
IN
C
IN
+
–
C
OUT
LTC3406AB
GND
3406AB F06a
PIN 1
V
OUT
V
IN
VIA TO V
OUT
SW
VIA TO V
IN
C
OUT
C
IN
L1
R2
C
FWD
R1