lt3580ems8e-trpbf Linear Technology Corporation, lt3580ems8e-trpbf Datasheet - Page 10

• Manufacturer Part Number: lt3580ems8e-trpbf
• Description: Boost/inverting Dc/dc Converter With 2a Switch, Soft-start, And Synchronization
• Manufacturer: Linear Technology Corporation

LT3580
APPLICATIONS INFORMATION
for boost, coupled inductor SEPIC and coupled inductor
inverting topologies, or:
for the uncoupled inductor SEPIC and uncoupled inductor
inverting topologies.
where:
Negative values of L
current I
of the LT3580.
Avoiding Subharmonic Oscillations : The LT3580’s internal
slope compensation circuit will prevent subharmonic oscil-
lations that can occur when the duty cycle is greater than
50%, provided that the inductance exceeds a minimum
value. In applications that operate with duty cycles greater
than 50%, the inductance must be at least:
for boost, coupled inductor SEPIC, and coupled inductor
inverting topologies, or:
for the uncoupled inductor SEPIC and uncoupled inductor
inverting topologies.
Maximum Inductance : Excessive inductance can reduce
current ripple to levels that are diffi cult for the current com-
parator (A3 in the Block Diagram) to cleanly discriminate,
10
DC = switch duty cycle (see previous section)
I
duty cycle (see the Typical Performance Characteristics
section).
η = power conversion effi ciency (typically 88% for
boost and 75% for dual inductor topologies at high
currents).
f = switching frequency
L1 L2 >
L >
L1 L2 >
LIM
= switch current limit, typically about 2.4A at 50%
(1 DC) • (f) • 0.8
V
IN
OUT
• 2 • DC – 1
2(f) I
(1 DC) • (f) • 0.8
V
(
IN
exceeds the switch current limit capability
• 2 • DC – 1
(
LIM
MIN1
)
DC • V
V
OUT
V
indicate that the output load
)
IN
IN
• I
•
OUT
I
OUT
thus causing duty cycle jitter and/or poor regulation. The
maximum inductance can be calculated by:
where L
good choice for I
Current Rating : Finally, the inductor(s) must have a rating
greater than its peak operating current to prevent inductor
saturation resulting in effi ciency loss. In steady state, the
peak input inductor current (continuous conduction mode
only) is given by:
for the boost, uncoupled inductor SEPIC and uncoupled
inductor inverting topologies, or:
for the coupled inductor SEPIC and coupled inductor
inverting topology.
For dual inductor topologies, the peak output inductor
current is given by:
for the uncoupled inductor topologies, or
for the coupled inductor topologies
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 dielectrics are preferred, as
these materials retain their capacitance over wider voltage
I
I
I
I
L
L1 PEAK
L1 PEAK
L2 PEAK
L2 PEAK
MAX
MAX
=
V
I
=
=I
MIN RIPPLE
IN
is L1||L2 for dual inductor topologies and a
| V
1– DC
OUT
V
– V
I
V
OUT
OUT
IN
OUT
MIN-RIPPLE
V
CESAT
IN
• • DC
+
•I
•
•I
+
V
OUT
OUT
OUT
V
•
2 • L2 • f
OUT
DC
|
2 • L2 • f
f
+
• 1– DC
+
is 300mA.
(
V
2 • L1• f
• 1– DC
V
2 • L1• f
IN
(
IN
• DC
• DC
)
)
3580fa