LTC3703IGN Datasheet, PDF (21/34 Pages) Linear Integrated Systems – 100V Synchronous Switching Regulator

LTC3703
Applications Information
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
=
2π
•
f
•
1
G•K
•
R1
( ) ^{C1= C2 K2 −1}
R2
=
2π
K
•f
•
C1
R_B

=
V_REF (R1)
V_OUT − V_REF
TYPE 3 Loop:
K
=
tan²


BOOST
4
+
45°
C2
=
2π
•
f
1
•G
•R1
C1= C2(K −1)
R2
=
2π
•
K
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.
3703fc
21