LTC3703 Linear Technology, LTC3703 Datasheet - Page 21

• Manufacturer Part Number: LTC3703
• Description: 100V Synchronous Switching Regulator Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Boost Converter Design
The following sections discuss the use of the LTC3703 as
a step-up (boost) converter. In boost mode, the LTC3703
can step-up output voltages as high as 80V. These sec-
tions discuss only the design steps specific to a boost
converter. For the design steps common to both a buck
and a boost, see the applicable section in the buck mode
C
R
TYPE 2 Loop:
R
TYPE 3 Loop:
C
R
C
R
R
C
C
K
B
K
B
2
2
3
3
2
2
1
1
C K
tan
2
2
V
C K
tan
2
2
K
2
V
OUT
2
R
OUT
2
V
V
1
f K R
2
• • • •
• •
REF
• • •
• •
REF
K
1
BOOST
f G K R
f C
f G R
f C
K
2
1
BOOST
1
( )
( )
1
R
•
R
2
V
1
V
1
1
1
1
REF
1
4
REF
3
U
1
45
1
45
U
W
U
section. An example of a boost converter circuit is shown
in the Typical Applications section. To operate the LTC3703
in boost mode, the INV pin should be tied to the V
voltage (or a voltage above 2V). Note that in boost mode,
pulse-skip operation and the line feedforward compensa-
tion are disabled.
For a boost converter, the duty cycle of the main switch is:
For high V
by the LTC3703’s maximum duty cycle which is typically
93%. The maximum output voltage is therefore:
Boost Converter: Inductor Selection
In a boost converter, the average inductor current equals
the average input current. Thus, the maximum average
inductor current can be calculated from:
Similar to a buck converter, choose the ripple current to be
20% to 40% of I
determines the inductor value as follows:
The minimum required saturation current for the inductor
is:
Boost Converter: Power MOSFET Selection
For information about choosing power MOSFETs for a
boost converter, see the Power MOSFET Selection section
for the buck converter, since MOSFET selection is similar.
However, note that the power dissipation equations for the
MOSFETs at maximum output current in a boost converter
are:
I
D
I
V
L
L MAX
L(SAT)
OUT MAX
(
V
V
(
IN MIN
OUT
)
I
> I
OUT
(
V
L
OUT
•
L(MAX)
)
1
–
f
I
)
to V
O MAX
V
(
•
L(MAX)
D
IN
– 1
D
V
MAX
IN
IN MIN
MAX
+ I
D
(
)
ratios, the maximum V
MAX
. The ripple current amplitude then
L
)
/2
I
O MAX
(
14
V
)
IN MIN
•
(
V
IN MIN
V
(
)
O
LTC3703
)
OUT
is limited
21
3703f
CC