Datasheet

13
LTC3405
3405fa
APPLICATIO S I FOR ATIO
WUUU
Figure 11a
Figure 11b
V
IN
C
IN
†
2.2µF
CER
V
IN
2.7V
TO 4.2V
LTC3405
RUN
MODE
3
6.8µH*
22pF
887k
412k
3405 F11a
5
4
6
1
2
SW
V
FB
GND
C
OUT
**
33µF
TANT
V
OUT
2.5V
+
*SUMIDA CMD4D11-6R8MC
** AVX TPSB336K006R0600
†
TAIYO YUDEN LMK212BJ225MG
TYPICAL APPLICATIO S
U
4.7µH*
22pF
200k
1M
C
OUT
†
4.7µF
CER
332k
V
IN
C
IN
**
1µF
CER
*
**
†
MURATA LQH3C4R7M34
TAIYO YUDEN CERAMIC JMK107BJ105MA
TAIYO YUDEN CERAMIC JMK212BJ475MG
V
IN
2.7V
TO 4.2V
LTC3405
RUN
MODE
3
3405 TA01a
5
4
6
1
2
SW
V
FB
GND
V
OUT
1.8V
Single Li-Ion to 1.8V/300mA Regulator
Optimized for Small Footprint and High Efficiency
Substituting V
OUT
= 2.5V, V
IN
= 4.2V, ∆I
L
= 100mA and
f = 1.5MHz in equation (3) gives:
L
V
MHz mA
V
V
H= −
⎛
⎝
⎜
⎞
⎠
⎟
≅ µ
25
1 5 100
1
25
42
68
.
.( )
.
.
.
For best efficiency choose a 300mA or greater inductor
with less than 0.3Ω series resistance.
C
IN
will require an RMS current rating of at least 0.125A ≅
I
LOAD(MAX)
/2 at temperature and C
OUT
will require an ESR
of less than 0.6Ω and greater than 0.1Ω. In most cases,
a tantalum capacitor will satisfy this requirement.
OUTPUT CURRENT (mA)
0.1
EFFICIENCY (%)
10
1000
100
90
80
70
60
50
40
30
3405 F11b
1 100
V
IN
= 2.7V
V
IN
= 4.2V
V
IN
= 3.6V
OUTPUT CURRENT (mA)
0.1
EFFICIENCY (%)
10
1000
100
90
80
70
60
50
40
30
3405 TA01b
1 100
V
IN
= 2.7V
V
IN
= 4.2V
V
IN
= 3.6V
3405 TA01c
40µs/DIV
V
IN
= 3.6V
I
LOAD
= 100mA TO 250mA
V
OUT
100mV/DIV
AC COUPLED
I
LOAD
200mA/DIV
I
L
200mA/DIV
For the feedback resistors, choose R1 = 412k. R2 can
then be calculated from equation (2) to be:
R
V
R k use
OUT
2
08
1 1 875 5 8= −
⎛
⎝
⎜
⎞
⎠
⎟
=
.
. ; 87k
Figure 11 shows the complete circuit along with its
efficiency curve.