LTC3703
applicaTions inForMaTion
Finally, choose a convenient resistor value for R1 (10k is
usuallyagoodvalue).Nowcalculatetheremainingvalues:
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
(K is a constant used in the calculations)
f = chosen crossover frequency
(GAIN/20)
G = 10
(this converts GAIN in dB to G in
absolute gain)
to the V voltage (or a voltage above 2V). Note that in
CC
boostmode,pulse-skipoperationandthelinefeedforward
compensation are disabled.
TYPE 2 Loop:
BOOST
For a boost converter, the duty cycle of the main switch is:
K = tan
+ 45°
2
VOUT – V
IN
D=
1
C2=
VOUT
2π •f•G•K •R1
C1= C2 K2 − 1
For high V
to V ratios, the maximum V
is limited
OUT
OUT
IN
(
)
by the LTC3703’s maximum duty cycle which is typically
93%. The maximum output voltage is therefore:
K
R2=
RB =
2π •f•C1
V
IN(MIN)
VOUT(MAX)
=
≅ 14V
IN(MIN)
VREF(R1)
1–DMAX
VOUT − VREF
Boost Converter: Inductor Selection
TYPE 3 Loop:
In a boost converter, the average inductor current equals
the average input current. Thus, the maximum average
inductor current can be calculated from:
BOOST
K = tan2
+ 45°
4
1
C2=
IO(MAX)
VO
2π •f•G•R1
IL(MAX)
=
= IO(MAX) •
1− DMAX
V
IN(MIN)
C1= C2 K− 1
(
)
K
2π •f•C1
R1
Similar to a buck converter, choose the ripple current to
R2=
R3=
C3=
be 20% to 40% of I
. The ripple current amplitude
L(MAX)
then determines the inductor value as follows:
K− 1
V
L = IN(MIN) •DMAX
1
∆IL •f
2πf K • R3
Theminimumrequiredsaturationcurrentfortheinductoris:
VREF(R1)
RB =
I
> I
+ ∆I /2
L(MAX) L
VOUT − VREF
L(SAT)
Boost Converter: Power MOSFET Selection
Boost Converter Design
For information about choosing power MOSFETs for a
boostconverter, seethePowerMOSFETSelectionsection
for the buck converter, since MOSFET selection is similar.
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.
3703fc
21
Linear Systems [ Linear Systems ]
