LTC3703-5_15 Datasheet, PDF (21/32 Pages) Linear Technology – 60V Synchronous Switching Regulator Controller

LTC3703-5
APPLICATIO S I FOR ATIO
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:
K
=
⎛
tan_⎝⎜
BOOST
2
+
⎞
45°_⎠⎟
C2 =
1
2π • f •G • K •R1
( ) ^{C1= C2 K2 − 1}
R2 = ^K
2π • f • C1
R_B
=
V_REF (R1)
V_OUT − V_REF
TYPE 3 Loop:
K
=
tan²
⎛
⎝⎜
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 LTC3703-5
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 = ^{VOUT – VIN}
V_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_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)
• ^VO
V_IN(MIN)
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_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
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