LT1930 Просмотр технического описания (PDF) - Linear Technology
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LT1930 Datasheet PDF : 12 Pages
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LT1930/LT1930A
APPLICATIONS INFORMATION
LT1930 AND LT1930A DIFFERENCES
Switching Frequency
The key difference between the LT1930 and LT1930A is
the faster switching frequency of the LT1930A. At 2.2MHz,
the LT1930A switches at nearly twice the rate of the
LT1930. Care must be taken in deciding which part to use.
The high switching frequency of the LT1930A allows
smaller cheaper inductors and capacitors to be used in a
given application, but with a slight decrease in efficiency
and maximum output current when compared to the
LT1930. Generally, if efficiency and maximum output
current are critical, the LT1930 should be used. If applica-
tion size and cost are more important, the LT1930A will be
the better choice. In many applications, tiny inexpensive
chip inductors can be used with the LT1930A, reducing
solution cost.
Duty Cycle
The maximum duty cycle (DC) of the LT1930A is 75%
compared to 84% for the LT1930. The duty cycle for a
given application using the boost topology is given by:
DC = | VOUT | – | VIN |
| VOUT |
For a 5V to 12V application, the DC is 58.3% indicating that
the LT1930A could be used. A 5V to 24V application has
a DC of 79.2% making the LT1930 the right choice. The
LT1930A can still be used in applications where the DC, as
calculated above, is above 75%. However, the part must
be operated in the discontinuous conduction mode so that
the actual duty cycle is reduced.
INDUCTOR SELECTION
Several inductors that work well with the LT1930 are listed
in Table 1 and those for the LT1930A are listed in Table 2.
These tables are not complete, and there are many other
manufacturers and devices that can be used. Consult each
manufacturer for more detailed information and for their
entire selection of related parts, as many different sizes and
shapes are available. Ferrite core inductors should be used
to obtain the best efficiency, as core losses at 1.2MHz are
much lower for ferrite cores than for cheaper powdered-
iron types. Choose an inductor that can handle at least 1A
without saturating, and ensure that the inductor has a low
DCR (copper-wire resistance) to minimize I2R power losses.
A 4.7µH or 10µH inductor will be the best choice for most
LT1930 designs. For LT1930A designs, a 2.2µH to 4.7µH
inductor will usually suffice. Note that in some applica-
tions, 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.
Table 1. Recommended Inductors – LT1930
PART
MAX
L DCR
(µH) mΩ
SIZE
L×W×H
(mm)
VENDOR
CDRH5D18-4R1 4.1 57 4.5 × 4.7 × 2.0 Sumida
CDRH5D18-100 10 124
(847) 956-0666
CR43-4R7
4.7 109 3.2 × 2.5 × 2.0 www.sumida.com
CR43-100
10 182
DS1608-472
DS1608-103
4.7 60 4.5 × 6.6 × 2.9 Coilcraft
10 75
(847) 639-6400
www.coilcraft.com
ELT5KT4R7M
ELT5KT6R8M
4.7 240 5.2 × 5.2 × 1.1 Panasonic
6.8 360
(408) 945-5660
www.panasonic.com
Table 2. Recommended Inductors – LT1930A
PART
MAX
L DCR
(µH) mΩ
SIZE
L×W×H
(mm)
VENDOR
LQH3C2R2M24 2.2 126 3.2 × 2.5 × 2.0 Murata
LQH3C4R7M24 4.7 195
(404) 573-4150
www.murata.com
CR43-2R2
CR43-3R3
2.2 71 4.5 × 4.0 × 3.0 Sumida
3.3 86
(847) 956-0666
www.sumida.com
1008PS-272
1008PS-332
2.7 100 3.7 × 3.7 × 2.6 Coilcraft
3.3 110
(800) 322-2645
www.coilcraft.com
ELT5KT3R3M
3.3 204 5.2 × 5.2 × 1.1 Panasonic
(408) 945-5660
www.panasonic.com
The inductors shown in Table 2 for use with the LT1930A
were chosen for small size. For better efficiency, use
similar valued inductors with a larger volume. For
example, the Sumida CR43 series in values ranging from
2.2µH to 4.7µH will give an LT1930A application a few
percentage points increase in efficiency, compared to the
smaller Murata LQH3C Series.
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