Datasheet

LTC3703
21
3703fc
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:
K = tan
BOOST
2
+ 45°






C2=
1
2π • f •G•K •R1
C1= C2 K
2
−1
( )
R2=
K
2π • f •C1
R
B
=
V
REF
(R1)
V
OUT
− V
REF
TYPE 3 Loop:
K = tan
2
BOOST
4
+ 45°






C2=
1
2π • f •G•R1
C1= C2 K − 1
( )
R2=
K
2π • f •C1
R3=
R1
K − 1
C3=
1
2πf K •
R3
R
B
=
V
REF
(R1)
V
OUT
− V
REF
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 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 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
compensation are disabled.
For a boost converter, the duty cycle of the main switch is:
D=
V
OUT
– V
IN
V
OUT
For high V
OUT
to V
IN
ratios, the maximum V
OUT
is limited
by the LTC3703’s maximum duty cycle which is typically
93%. The maximum output voltage is therefore:
V
OUT(MAX)
=
V
IN(MIN)
1–D
MAX
≅ 14V
IN(MIN)
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
L(MAX)
=
I
O(MAX)
1−D
MAX
=I
O(MAX)
•
V
O
V
IN(MIN)
Similar to 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
IN(MIN)
∆I
L
• f
•D
MAX
The minimum required saturation current for the inductor is:
I
L(SAT)
> I
L(MAX)
+ ∆I
L
/2
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.
applicaTions inForMaTion