Datasheet

12
LTC34 06B-2
sn3406b2 3406b2fs
V
IN
C
IN
4.7µF
CER
V
IN
2.7V
TO 4.2V
LTC3406B-2
RUN
3
2.2µH*
22pF
1M
316k
3406B F07a
5
4
1
2
SW
V
FB
GND
C
OUT
**
10µF
CER
V
OUT
2.5V
*MURATA LQH32CN2R2M33
** TAIYO YUDEN JHK316BJ106ML
TAIYO YUDEN JMK212BJ475MG
Figure 7b
Figure 7a
Design Example
As a design example, assume the LTC3406B-2 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)
Substituting V
OUT
= 2.5V, V
IN
= 4.2V, I
L
= 240mA and
f = 2.25MHz in equation (3) gives:
L
V
MHz mA
V
V
H=
25
1 5 240
1
25
42
187
.
.( )
.
.
.
A 2.2µH inductor works well for this application. For best
efficiency choose a 720mA or greater inductor with less
than 0.2 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 0.25. In most cases, a ceramic capacitor will
satisfy this requirement.
For the feedback resistors, choose R1 = 316k. R2 can
then be calculated from equation (2) to be:
R
V
Rk
OUT
2
06
1 1 1000=
=
.
Figure 7 shows the complete circuit along with its effi-
ciency curve.
APPLICATIO S I FOR ATIO
WUUU
OUTPUT CURRENT (mA)
0.1
EFFICIENCY (%)
10
1000
100
90
80
70
60
50
40
30
20
10
3406B G04
1 100
V
OUT
= 2.5V
T
A
= 25°C
V
IN
= 2.7V
V
IN
= 4.2V
V
IN
= 3.6V