LT8471
15
8471fd
For more information www.linear.com/8471
Power Switch Duty Cycle (Primary Channels)
In order to maintain loop stability and deliver adequate
current to the load, the internal power switches (Q1 and
Q2 in the Block Diagram) cannot remain on for 100% of
each clock cycle. The maximum allowable duty cycle is
given by:
DC
MAX
=
P
(OFF)TIME
T
P
⎛
⎝
⎜
⎞
⎠
⎟
• 100%
where T
P
is the clock period and MIN
(OFF)TIME
is typically
170ns (refer to the Electrical Characteristics section).
The application should be designed so that the steady state
duty cycle does not exceed DC
MAX
. Duty cycle equations
for several common topologies are given below, where V
D
is the diode forward voltage drop and V
CESAT
is typically
300mV at 1.5A.
DC
BOOST
≅
OUT
CC
+
D
V
OUT
+ V
D
– V
CESAT
DC
BUCK
≅
V
OUT
+ V
D
V
CC
+ V
D
– V
CESAT
DC
1L _INV
≅
V
OUT
+ V
D
V
CC
+ V
D
– V
CESAT
+ V
OUT
DC
2L _INV
≅
V
OUT
+ V
D
V
CC
+ V
D
– V
CESAT
+ V
OUT
DC
SEPIC
≅
V
D
+ V
OUT
V
CC
+ V
OUT
+ V
D
– V
CESAT
DC
ZETA
≅
V
D
+ V
OUT
V
CC
+ V
OUT
+ V
D
– V
CESAT
where V
CC
is the positive input voltage to the DC/DC con-
verter. See the Typical Applications section for examples.
The
LT8471 can be used in configurations where the duty
cycle is higher than DC
MAX
, but it must be operated in the
discontinuous conduction mode so that the effective duty
cycle is reduced.
Inductor Selection (Primary Channels)
General Guidelines: The high frequency operation of
the LT8471 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. To improve efficiency, choose inductors
with more volume for a given inductance. The inductor
should have low DCR (copper wire resistance) to reduce
I
2
R 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 a fraction 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
3A range. To minimize radiated noise, use a toroidal or
shielded inductor. Note
that the
inductance of shielded
core types will drop more as current increases, and will
saturate more easily. See Table 1 for a list of inductor
manufacturers. Thorough lab evaluation is recommended
to verify that the following guidelines properly suit the
final application.
Table 1. Inductor Manufacturers
VENDOR PART NUMBER WEB
Coilcraft MSS1038, MSS7341 and
LPS4018
www.coilcraft.com
Coiltronics DR, LD and CD Series www.coiltronics.com
Sumida CDRH105R Series www.sumida.com
Würth Elektronik WE-LHMI and WE-TPC Series www.we-online.com
Minimum Inductance: Although there can be a trade-
off with efficiency, it is often desirable to minimize
board space by choosing smaller inductors. When
choosing an inductor, there are two conditions that
limit the minimum inductance; (1) providing adequate
load current, and (2) avoiding subharmonic oscillation.
Choose an inductance that is high enough to meet both
of these requirements.
applicaTions inForMaTion
