Datasheet
12
LTC1627
3. Does the (+) plate of C
IN
connect to V
IN
as closely as
possible? This capacitor provides the AC current to the
internal power MOSFETs.
4. Keep the switching node SW away from sensitive small-
signal nodes.
Design Example
As a design example, assume the LTC1627 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 2.7V. The load current requirement is a
maximum of 0.5A but most of the time it will be on 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
= 200mA and
f = 350kHz in equation (3) gives:
L
V
kHz mA
V
V
H=
()()
−
=
25
350 200
1
25
42
14 5
..
.
.µ
A 15µH inductor works well for this application. For good
efficiency choose a 1A inductor with less than 0.25Ω
series resistance.
C
IN
will require an RMS current rating of at least 0.25A 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.
C
SS
0.1µF
C
OUT
†
100µF
6.3V
C
IN
††
22µF
16V
C1
0.1µF
C2
0.1µF
D2
D1
BAT54S**
15µH*
* SUMIDA CD54-150
** ZETEX BAT54S
†
AVX TPSC107M006R0150
††
AVX TPSC226M016R0375
R2
169k
1%
R1
80.6k
1%
C
ITH
47pF
1
2
3
4
8
7
6
5
SYNC/FCB
V
DR
V
IN
SW
I
TH
RUN/SS
V
FB
GND
LTC1627
V
IN
2.8V TO
4.5V
V
OUT
2.5V
0.5A
1627 F08a
+
+
OUTPUT CURRENT (mA)
EFFICIENCY (%)
1 100 1000
1627 F08b
10
100
95
90
85
80
75
70
65
60
55
50
45
V
OUT
= 2.5V
V
IN
= 3.6V
V
IN
= 4.2V
Figure 8. Single Lithium-Ion to 2.5V/0.5A Regulator
APPLICATIO S I FOR ATIO
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