Datasheet

LTC1871
30
1871fe
APPLICATIONS INFORMATION
9. The DC coupling capacitor in a SEPIC converter is cho-
sen based on its RMS current requirement and must be
rated for a minimum voltage of V
IN
plus the AC ripple
voltage. Start with the minimum value which satisfi es
the RMS current requirement and then check the ripple
voltage to ensure that it doesn’t exceed the DC rating.
I
RMS(CI)
I
O(MAX)
•
V
O
+ V
D
V
IN(MIN)
= 1.5A •
12V + 0.5V
5
V
= 2.4A
Figure 18a. 4.5V to 15V Input, 12V/2A Output SEPIC Converter
Figure 18b. SEPIC Effi ciency vs Output Current
RUN
I
TH
FB
FREQ
MODE/SYNC
SENSE
V
IN
INTV
CC
GATE
GND
1
2
3
4
5
10
9
8
7
6
LTC1871
R
T
80.6k
1%
R1
12.1k
1%
R2
105k
1%
R3
1M
C
VCC
4.7µF
X5R
C
IN
47µF
M1
C
IN
, C
OUT1
: KEMET T495X476K020AS
C
DC
, C
OUT2
: TAIYO YUDEN TMK432BJ106MM
D1: INTERNATIONAL RECTIFIER 30BQ040
D1
•
•
L1*
L2*
R
C
33k
C
C1
6.8nF
C
C2
47pF
C
OUT1
47µF
20V
×2
V
IN
4.5V to 15V
V
OUT
12V
1.5A
(2A PEAK)
GND
1871 F018a
+
C
OUT2
10µF
25V
X5R
×2
L1, L2: BH ELECTRONICS BH510-1007 (*COUPLED INDUCTORS)
M1: INTERNATIONAL RECTIFIER IRF7811W
C
DC
10µF
25V
X5R
+
OUTPUT CURRENT (A)
50
EFFICIENCY (%)
55
60
90
85
80
75
70
65
95
0.001 0.1 1 10
1871 F18b
45
0.01
100
V
IN
= 4.5V
V
IN
= 15V
V
IN
= 12V
V
O
= 12V
MODE = INTV
CC
For this design a single 10µF, low ESR (X5R) Taiyo Yuden
ceramic capacitor (TMK432BJ106MM) is adequate.