Datasheet

LTC1871-7
13
18717fd
applicaTions inForMaTion
Application Circuits
A basic LTC1871-7 application circuit is shown in Figure 9.
External component selection is driven by the characteris-
tics of the load and the input supply. The first topology to
be analyzed will be the boost converter, followed by SEPIC
(single-ended primary inductance converter).
Boost Converter: Duty Cycle Considerations
For a boost converter operating in a continuous conduc-
tion mode (CCM), the duty cycle of the main switch is:
D =
V
O
+ V
D
– V
IN
V
O
+ V
D






where V
D
is the forward voltage of the boost diode. For
converters where the input voltage is close to the output
voltage, the duty cycle is low and for converters that develop
a high output voltage from a low voltage input supply,
the duty cycle is high. The maximum output voltage for a
boost converter operating in CCM is:
V
O(MAX)
=
V
IN(MIN)
1– D
MAX
( )
– V
D
The maximum duty cycle capability of the LTC1871-7 is
typically 92%. This allows the user to obtain high output
voltages from low input supply voltages.
Boost Converter: The Peak and Average Input Currents
The control circuit in the LTC1871-7 is measuring the input
current typically using a sense resistor in the MOSFET
source, so the output current needs to be reflected back
to the input in order to dimension the power MOSFET
properly. Based on the fact that, ideally, the output power
is equal to the input power, the maximum average input
current is:
I
IN(MAX)
=
I
O(MAX)
1– D
MAX
The peak input current is :
I
IN(PEAK)
= 1+
χ
2






•
I
O(MAX)
1– D
MAX
The maximum duty cycle, D
MAX
, should be calculated at
minimum V
IN
.
Figure 9. A High Efficiency 42V, 1.5A Automotive Boost Converter
RUN
I
TH
FB
FREQ
MODE/SYNC
SENSE
V
IN
INTV
CC
GATE
GND
1
2
3
4
5
10
9
8
7
6
LTC1871-7
f = 250kHz
R
T
100k
1%
R1
12.4k
1%
R2
412k
1%
R3
1M
C
VCC
4.7µF
X5R
R
SENSE
0.005Ω
1W
C
IN2
10µF
50V
X5R
×2
M1
D1
L1
6.8µH
R
C
24k
C
C1
2.2nF
C
C2
100pF
C
OUT1
68µF
100V
×2
V
IN
8V TO 28V
V
OUT
42V
1.5A
GND
18717 F09
+
C
IN1
*
560µF
50V
+
C
OUT2
10µF
50V
X5R
×2
C
IN1
: SANYO 50MV560AXL (*RECOMMENDED FOR LAB EVALUATION
FOR SUPPLY LEAD LENGTHS GREATER THAN A FEW INCHES)
C
IN2
: TDK C5750X5R1H106M
C
OUT1
: SANYO 100CV68FS
C
OUT2
: TDK C5750X5R1H106M
D1: DIODES INC B360B
L1: COOPER DR127-6R8
M1: SILICONIX/VISHAY Si7370DP