LTC1871 Datasheet, PDF (29/36 Pages) Linear Technology – Wide Input Range, No RSENSE Current Mode Boost, Flyback and SEPIC Controller

LTC1871
APPLICATIO S I FOR ATIO
And so the inductor value is:
L
=
V_IN(MIN)
2 • ∆I_L • f
• D_MAX
=
2
•
5
1.5 •
300k
•
0.714
=
4µH
The 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
χ
2

• ρ_T
•


1
V_O + V_D ^
V_{IN(MIN) }
+1
=
0.12
1.5
•
1
1.2 • 1.5
•

1
125.5
+
1
=
12.7mΩ
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 maxi-
mum room temperature R_DS(ON) of 12mΩ at V_GS = 4.5V.
C_C2
47pF
R_C
33k
C_C1
R1
6.8nF 12.1k
R2 ^1%
105k
1%
R3
1M
R_T
80.6k
1%
1
RUN
2
I_TH
10
SENSE
9
V_IN
LTC1871
3
FB
4
FREQ
8
INTV_CC
7
GATE
5
6
MODE/SYNC GND
•
L1* ^CDC
10µF
25V
X5R _D1
C_VCC +
4.7µF
X5R
M1
C_IN
47µF
L2*
•
V_IN
4.5V to 15V
+ ^COUT1
47µF
20V
×2
V_OUT
12V
1.5A
(2A PEAK)
C_OUT2
10µF
25V
X5R
×2
GND
1871 F018a
C_IN, C_OUT1: KEMET T495X476K020AS
C_DC, C_OUT2: TAIYO YUDEN TMK432BJ106MM
D1:
INTERNATIONAL RECTIFIER 30BQ040
L1, L2: BH ELECTRONICS BH510-1007 (*COUPLED INDUCTORS)
M1: INTERNATIONAL RECTIFIER IRF7811W
Figure 18a. 4.5V to 15V Input, 12V/2A Output SEPIC Converter
100
95
90
V_IN = 12V
85
V_IN = 4.5V
80
75
V_IN = 15V
70
65
60
55
50
45
0.001
V_O = 12V
MODE = INTV_CC
0.01
0.1
1
10
OUTPUT CURRENT (A)
1871 F18b
Figure 18b. SEPIC Efficiency vs Output Current
29