Datasheet

12
LTC34 0 6B
3406bfa
APPLICATIO S I FOR ATIO
WUUU
Design Example
As a design example, assume the LTC3406B 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. Efficiency 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
=
()
∆
()
−
⎛
⎝
⎜
⎞
⎠
⎟
1
1
(3)
Figure 5a. LTC3406B Layout Diagram
Figure 6a. LTC3406B Suggested Layout
Substituting V
OUT
= 2.5V, V
IN
= 4.2V, ∆I
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
efficiency 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.
RUN
LTC3406B
GND
SW
L1
R2
R1
C
FWD
BOLD LINES INDICATE HIGH CURRENT PATHS
V
IN
V
OUT
3406B F05a
4
5
1
3
+
–
2
V
FB
V
IN
C
IN
+
–
C
OUT
RUN
LTC3406B-1.8
GND
SW
L1
BOLD LINES INDICATE HIGH CURRENT PATHS
V
IN
V
OUT
3406B F05b
4
5
1
3
+
–
2
V
OUT
V
IN
C
IN
C
OUT
Figure 5b. LTC3406B-1.8 Layout Diagram
LTC3406B
GND
3406B F06a
PIN 1
V
OUT
V
IN
VIA TO V
OUT
SW
VIA TO V
IN
VIA TO GND
C
OUT
C
IN
L1
R2
C
FWD
R1
LTC3406B-1.8
GND
3406B F06b
PIN 1
V
OUT
V
IN
SW
VIA TO V
IN
VIA TO V
OUT
C
OUT
C
IN
L1
Figure 6b. LTC3406B-1.8 Suggested Layout