LTC3703-5
21
37035fa
BOOST values greater than 60
°
usually require Type 3
loops for satisfactory performance.
Finally, choose a convenient resistor value for R1 (10k is
usually a good value). Now calculate the remaining values:
(K is a constant used in the calculations)
f = chosen crossover frequency
G = 10
(GAIN/20)
(this converts GAIN in dB to G in
absolute gain)
TYPE 2 Loop:
TYPE 3 Loop:
APPLICATIO S I FOR ATIO
W
U
U
U
Boost Converter Design
The following sections discuss the use of the LTC3703-5
as a step-up (boost) converter. In boost mode, the
LTC3703-5 can step-up output voltages as high as 60V.
These sections 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 section. An example of a boost converter circuit is
shown in the Typical Applications section. To operate the
LTC3703-5 in boost mode, the INV pin should be tied to
the V
CC
voltage (or a voltage above 2V). Note that in boost
mode, pulse-skip operation and the line feedforward com-
pensation are disabled.
For a boost converter, the duty cycle of the main switch is:
D
V
V
V
OUT
IN
OUT
=
–
For high V
OUT
to V
IN
ratios, the maximum V
OUT
is limited
by the LTC3703-5’s maximum duty cycle which is typically
93%. The maximum output voltage is therefore:
V
V
D
V
OUT MAX
IN MIN
MAX
IN MIN
(
)
(
)
(
)
–
=
≅
1
14
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:
I
I
D
I
V
V
L MAX
O MAX
MAX
O MAX
O
IN MIN
(
)
(
)
(
)
(
)
•
=
−
=
1
As with a buck converter, choose the ripple current to be
20% to 40% of I
L(MAX)
. The ripple current amplitude then
determines the inductor value as follows:
L
V
I
f
D
IN MIN
L
MAX
=
∆
(
)
•
•
The minimum required saturation current for the inductor
is:
I
L(SAT)
> I
L(MAX)
+
∆
I
L
/2
K
BOOST
C
f G K R
C
C K
R
K
f C
R
V
R
V
V
B
REF
OUT
REF
=
+ °
⎛
⎝⎜
⎞
⎠⎟
=
=
−
( )
=
=
−
tan
• • • •
• •
( )
2
45
2
1
2
1
1
2
1
2
2
1
1
2
π
π
K
BOOST
C
f G R
C
C K
R
K
f C
R
R
K
C
f K R
R
V
R
V
V
B
REF
OUT
REF
=
+ °
⎛
⎝⎜
⎞
⎠⎟
=
=
−
( )
=
=
−
=
=
−
tan
• • •
• •
•
( )
2
4
45
2
1
2
1
1
2
1
2
2
1
3
1
1
3
1
2
3
1
π
π
π
