LT3471
6
3471fb
OPERATION
APPLICATIONS INFORMATION
Duty Cycle
The typical maximum duty cycle of the LT3471 is 94%.
The duty cycle for a given application is given by:
DC =
|V
OUT
|+|V
D
|–|V
IN
|
|V
OUT
|+|V
D
|–|V
CESAT
|
Where V
D
is the diode forward voltage drop and V
CESAT
is in the worst case 330mV (at 1.3A)
The LT3471 can be used at higher duty cycles, but it must
be operated in the discontinuous conduction mode so that
the actual duty cycle is reduced.
Setting Output Voltage
Setting the output voltage depends on the topology used.
For normal noninverting boost regulator topologies:
V
OUT
= V
FBP
1+
R1
R2
where V
FBN
is connected between R1 and R2 (see the
Typical Applications section for examples).
Select values of R1 and R2 according to the following
equation:
R1= R2
V
OUT
V
REF
–1
A good value for R2 is 15k which sets the current in the
resistor divider chain to 1.00V/15k = 67μA.
V
FBP
is usually just tied to V
REF
= 1.00V, but V
FBP
can also
be tied to a divided down version of V
REF
or some other
voltage as long as the absolute maximum ratings for the
feedback pins are not exceeded (see Absolute Maximum
Ratings).
For inverting topologies, V
FBN
is tied to ground and V
FBP
is connected between R1 and R2. R2 is between V
FBP
and V
REF
and R1 is between V
FBP
and V
OUT
(see the Ap-
plications section for examples). In this case:
V
OUT
= V
REF
R1
R
2
Select values of R1 and R2 according to the following
equation:
R1=R2
V
OUT
V
REF
A good value for R2 is 15k, which sets the current in the
resistor divider chain to 1.00V/15k = 67μA.
Switching Frequency and Inductor Selection
The LT3471 switches at 1.2 MHz, allowing for small valued
inductors to be used. 4.7μH or 10μH will usually suffi ce.
Choose an inductor that can handle at least 1.4A without
saturating, and ensure that the inductor has a low DCR
(copper-wire resistance) to minimize I
2
R power losses.
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. For better effi ciency, use similar
valued inductors with a larger volume. Many different sizes
and shapes are available from various manufacturers.
Choose a core material that has low losses at 1.2 MHz,
such as ferrite core.
Table 1. Inductor Manufacturers
Sumida (847) 956-0666 www.sumida.com
TDK (847) 803-6100 www.tdk.com
Murata (714) 852-2001 www.murata.com
current reaches this value, the SR latch is reset regardless
of the state of the comparator A2 (A4). Also not shown
in the Block Diagram is the thermal shutdown circuit. If
the temperature of the part exceeds approximately 160°C,
both latches are reset regardless of the state of compara-
tors A2 and A4. The current limit and thermal shutdown
circuits protect the power switch as well as the external
components connected to the LT3471.
