Datasheet

12
LTC1707
OUTPUT CURRENT (mA)
EFFICIENCY (%)
1 100 1000
1707 F08b
10
100
90
80
70
60
50
V
OUT
= 2.5V
L = 22µH
Burst Mode OPERATION
V
IN
= 3.6V
V
IN
= 4.2V
C
SS
0.1µF
C
OUT
†
100µF
6.3V
C
IN
††
22µF
16V
22µH*
* SUMIDA CD54-220
†
AVX TPSC107M006R0150
††
AVX TPSC226M016R0375
R2
169k
1%
R1
80.6k
1%
C
ITH
47pF
1
2
3
4
8
7
6
5
V
REF
SYNC/MODE
V
IN
SW
I
TH
RUN/SS
V
FB
GND
LTC1707
V
IN
2.85V TO
4.5V
V
OUT
2.5V
0.3A
1707 F08a
+
+
Figure 8. Single Lithium-Ion to 2.5V/0.3A Regulator from Design Example
Design Example
As a design example, assume the LTC1707 is used in a
single lithium-ion battery-powered cellular phone applica-
tion. The V
IN
will be operating from a maximum of 4.2V
down to about 2.85V. The load current requirement is a
maximum of 0.3A 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
= 120mA and
f = 350kHz in equation (3) gives:
L
V
kHz mA
V
V
H=
()()
−






=
25
350 120
1
25
42
24 1
..
.
. µ
A 22µH inductor works well for this application. For best
efficiency choose a 1A inductor with less than 0.25Ω
series resistance.
C
IN
will require an RMS current rating of at least 0.15A at
temperature and C
OUT
will require an ESR of less than
0.25Ω. In most applications, the requirements for these
capacitors are fairly similar.
For the feedback resistors, choose R1 = 80.6k. R2 can then
be calculated from equation (2) to be:
R
V
Rk
OUT
2
08
1 1 171=−






=
.
; use 169k
Figure 8 shows the complete circuit along with its effi-
ciency curve.
APPLICATIO S I FOR ATIO
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