LT3581 LINER [Linear Technology], LT3581 Datasheet - Page 22

• Manufacturer Part Number: LT3581
• Description: 3.3A Boost/Inverting DC/DC Converter with Fault Protection
• Manufacturer: LINER [Linear Technology]

LT3581
appenDix
SETTING THE OUTPUT VOLTAGE
The output voltage is set by connecting a resistor (R
from V
following equation:
where V
(i.e. boost and SEPIC regulators) and 5mV (typical) for
inverting topologies.
POWER SWITCH DUTY CYCLE
In order to maintain loop stability and deliver adequate
current to the load, the power NPNs (Q1 and Q2 in the
Block Diagram) cannot remain “on” for 100% of each clock
cycle. The maximum allowable duty cycle is given by:
where T
Electrical Characteristics) is typically 60ns.
Conversely, the power NPNs (Q1 and Q2 in the Block Dia-
gram) cannot remain “off” for 100% of each clock cycle,
and will turn on for a minimum on time (MinOnTime) when
in regulation. This MinOnTime governs the minimum al-
lowable duty cycle given by:
Where T
the Electrical Characteristics) is typically 100ns.
The application should be designed such that the operating
duty cycle is between DC
Duty cycle equations for several common topologies are given
below where V
is the collector to emitter saturation voltage of the switch.
V
when the combined switch current (I
For the boost topology (see Figure 5):

CESAT
R
DC
DC
DC
FB
MAX
MIN
BOOST
, with SW1 and SW2 tied together, is typically 250mV
OUT
=
P
FB
P
is the clock period and MinOffTime (found in the
|
=
is 1.215V (typical) for non-inverting topologies
is the clock period and MinOnTime (found in
V
=
to the FB pin. R
OUT
83 3
(
≅
(
MinOnTime
D
T
.
P
is the diode forward voltage drop and V
V
–
OUT
µA
V
–
T
V
OUT
MinOffTime
P
FB
+
T
|
P
–
V
D
V
MIN
)
IN
–
FB
• 100%
V
+
and DC
CESAT
is determined by using the
V
)
D
•
100
SW1
MAX
%
+ I
.
SW2
) is 2.75A.
CESAT
FB
)
For the SEPIC or Dual Inductor Inverting topology (see
Figures 6 and 7):
For the Single Inductor Inverting topology (see Figure 13):
The LT3581 can be used in configurations where the duty
cycle is higher than DC
the discontinuous conduction mode so that the effective
duty cycle is reduced.
INDUCTOR SELECTION
General Guidelines: The high frequency operation of the
LT3581 allows for the use of small surface mount inductors.
For high efficiency, choose inductors with high frequency
core material, such as ferrite, to reduce core losses. Also
to improve efficiency, choose inductors with more volume
for a given inductance. The inductor should have low
DCR (copper-wire resistance) to reduce I
must be able to handle the peak inductor current without
saturating. Note that in some applications, the current
handling requirements of the inductor can be lower, such
as in the SEPIC topology where each inductor only carries
one half of the total switch current. Molded chokes or chip
inductors usually do not have enough core area to support
peak inductor currents in the 2A to 6A range. To minimize
radiated noise, use a toroidal or shielded inductor. See
Table 5 for a list of inductor manufacturers.
Table 5. Inductor Manufacturers
Sumida
Coilcraft
Vishay
Taiyo Yuden NR Series
Wurth
TDK
DC
DC
SEPIC
SI INVERT
_
CDR6D28MN and CDR7D28MN
Series
MSD7342 Series
IHLP-1616BZ-01, IHLP-2020BZ-01
and IHLP-2525CZ-01 Series
WE-PD Series
VLF , SLF and RLF Series
_&_
INVERT
=
|
V
OUT
≅
MAX
V
|
−
IN
|
V
V
, but it must be operated in
OUT
IN
+
|
+
V
|
V
OUT
V
+ •
D
CESAT
3 V V
+
|
|
+
V
V
OUT
D
www.sumida.com
www.coilcraft.com
www.vishay.com
www.t-yuden.com
www.we-online.com
www.tdk.com
D
+ •
2
R losses, and
−
3
|
V
V
CE
D
S S AT
3581f