參數(shù)資料
型號(hào)： LT3581IMSE#PBF
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： SWITCHING REGULATOR, PDSO16
封裝： LEAD FREE, PLASTIC, MSOP-16
文件頁數(shù)： 15/36頁
文件大小： 498K
代理商： LT3581IMSE#PBF
LT3581
3581f
appenDix
SETTING THE OUTPUT VOLTAGE
The output voltage is set by connecting a resistor (RFB)
from VOUT to the FB pin. RFB is determined by using the
following equation:
R
V
A
FB
OUT
FB
=
|
–
|
.
83 3
where VFB is 1.215V (typical) for non-inverting topologies
(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
BlockDiagram)cannotremain“on”for100%ofeachclock
cycle. The maximum allowable duty cycle is given by:
DC
T
MinOffTime
T
MAX
P
= (
)
–
%
100
where TP is the clock period and MinOffTime (found in the
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:
DC
MinOnTime
T
MIN
P
= (
)
100%
Where TP is the clock period and MinOnTime (found in
the Electrical Characteristics) is typically 100ns.
Theapplicationshouldbedesignedsuchthattheoperating
duty cycle is between DCMIN and DCMAX.
Dutycycleequationsforseveralcommontopologiesaregiven
belowwhereVDisthediodeforwardvoltagedropandVCESAT
is the collector to emitter saturation voltage of the switch.
VCESAT, with SW1 and SW2 tied together, is typically 250mV
when the combined switch current (ISW1 + ISW2) is 2.75A.
For the boost topology (see Figure 5):
DC
V
BOOST
OUT
IN
D
OUT
D
CESAT
+
–
For the SEPIC or Dual Inductor Inverting topology (see
Figures 6 and 7):
DC
V
SEPIC
INVERT
D
OUT
IN
OUT
D
CE
_&_
|
+
SSAT
For the Single Inductor Inverting topology (see Figure 13):
DC
V
SI INVERT
OUT
IN
CESAT
D
OUT
_
|
=
+
+
3
3 VVD
The LT3581 can be used in configurations where the duty
cycle is higher than DCMAX, but it must be operated in
the discontinuous conduction mode so that the effective
duty cycle is reduced.
INDUCTOR SELECTION
General Guidelines: The high frequency operation of the
LT3581allowsfortheuseofsmallsurfacemountinductors.
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 I2R losses, and
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
CDR6D28MN and CDR7D28MN
Series
www.sumida.com
Coilcraft
MSD7342 Series
www.coilcraft.com
Vishay
IHLP-1616BZ-01, IHLP-2020BZ-01
and IHLP-2525CZ-01 Series
www.vishay.com
Taiyo Yuden NR Series
www.t-yuden.com
Wurth
WE-PD Series
www.we-online.com
TDK
VLF, SLF and RLF Series
www.tdk.com
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