LT3579_15 Datasheet, PDF (27/42 Pages) Linear Technology – 6A Boost/Inverting DC/DC Converter with Fault Protection

LT3579/LT3579-1
APPENDIX
Maximum Inductance
Excessive inductance can reduce ripple current to levels
that are difficult for the current comparator (A4 in the Block
Diagram) to cleanly discriminate, thus causing duty cycle
jitter and/or poor regulation. The maximum inductance
can be calculated by:
( ) _{LMAX =}
V_IN − V_CESAT • DC
f_OSC • 0.5A
where:
L_MAX = L1 for Boost Topologies (see Figure 6)
L_MAX = L1 = L2 for Coupled Dual Inductor
Topologies (see Figures 7 and 8)
L_MAX = L1 || L2 for Uncoupled Dual Inductor
Topologies (see Figures 7 and 8)
Inductor Current Rating
The inductor(s) must have a rating greater than its (their)
peak operating current to prevent inductor saturation, which
would result in catastrophic failure and efficiency losses.
The maximum inductor current (considering start-up and
steady-state conditions) is given by:
I_{L _ PEAK}
= I_LIM
+
V_IN
•
T_{MIN _ PROP}
L
where:
I_{L_PEAK} = Peak Inductor Current in L1 for a Boost
Topology, or the sum of the Peak
Inductor Currents in L1 and L2 for Dual
Inductor Topologies.
I_LIM
= For Hard-Saturation Inductors, 9.4A with
SW1 and SW2 Tied Together, or 5.1A
with just SW1 used. For Soft-Saturation
Inductors, 6A with SW1 and SW2 Tied
Together, or 3.4A with just SW1 used.
T_{MIN_PROP} = 100ns (Propagation Delay through the
Current Feedback Loop).
Note that these equations offer conservative results for
the required inductor current ratings. The current ratings
could be lower for applications with light loads, provided
the SS capacitor is sized appropriately to limit inductor
currents at start-up.
DIODE SELECTION
Schottky diodes, with their low forward voltage drops and
fast switching speeds, are recommended for use with the
LT3579. Choose a Schottky with low parasitic capacitance
to reduce reverse current spikes through the power switch
of the LT3579. The Diodes Inc. MBRM360 is a very good
choice with a 60V reverse voltage rating and an average
forward current of 3A.
OUTPUT CAPACITOR SELECTION
Low ESR (equivalent series resistance) capacitors should
be used at the output to minimize the output ripple voltage.
Multilayer ceramic capacitors are an excellent choice, as
they have an extremely low ESR and are available in very
small packages. X5R or X7R type are preferred, as these
materials retain their capacitance over wide voltage and
temperature ranges. A 22μF to 47μF output capacitor is
sufficient for most applications, but systems with low
output currents may need only 4.7μF to 22μF. Always
use a capacitor with a sufficient voltage rating. Many
ceramic capacitors, particularly 0805 or 0603 case sizes,
have greatly reduced capacitance at the desired output
voltage. Tantalum polymer or OS-CON capacitors can be
used, but it is likely that these capacitors will occupy more
board area than a ceramic, and will have higher ESR with
greater output ripple.
INPUT CAPACITOR SELECTION
Ceramic capacitors make a good choice for the input
decoupling capacitor, C_VIN, which should be placed as
close as possible to the V_IN pin of the LT3579. This ensures
that the voltage seen at the V_IN pin of the LT3579 remains
a nearly flat DC voltage. A 1μF to 4.7μF input capacitor is
sufficient for most applications.
A ceramic bypass capacitor, C_PWR, should also be placed
as close as possible to the input of the inductor. This
ensures that the inductor ripple current is supplied from
the bypass capacitor and provides a nearly flat DC voltage
to the input of the voltage converter. A 4.7µF to 10µF input
power capacitor is sufficient for most applications.
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