Datasheet
LTC1871-1
29
18711fb
APPLICATIONS INFORMATION
And so the inductor value is:
L =
V
IN(MIN)
2•I
L
• f
•D
MAX
=
5
2 • 1.5 • 300k
• 0.714 = 4μH
T
he component chosen is a BH Electronics BH510-
1007, which has a saturation current of 8A.
5. With an minimum input voltage of 5V, only logic-level
power MOSFETs should be considered. Because the
maximum duty cycle is 71.4%, the maximum SENSE
pin threshold voltage is reduced from its low duty
cycle typical value of 150mV to approximately 120mV.
Assuming a MOSFET junction temperature of 125°C,
the room temperature MOSFET R
DS(ON)
should be less
than:
R
DS(ON)
V
SENSE(MAX)
I
O(MAX)
•
1
1+
2
•
T
•
1
V
O
+ V
D
V
IN(MIN)
+
1
=
0.12
1.5
•
1
1.2 • 1.5
•
1
12.5
5
+ 1
= 12.7m
For a SEPIC converter, the switch BV
DSS
rating must be
greater than V
IN(MAX)
+ V
O
, or 27V. This comes close to
an IRF7811W, which is rated to 30V, and has a maximum
room temperature R
DS(ON)
of 12m at V
GS
= 4.5V.
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
LTC1871-1
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
18711 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
18711 F18b
45
0.01
100
V
IN
= 4.5V
V
IN
= 15V
V
IN
= 12V
V
O
= 12V
MODE = INTV
CC