5
LT1931/LT1931A
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APPLICATIO S I FOR ATIO
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LT1931A AND LT1931 DIFFERENCES:
Switching Frequency
The key difference between the LT1931A and LT1931 is
the faster switching frequency of the LT1931A. At 2.2MHz,
the LT1931A switches at nearly twice the rate of the
LT1931. Care must be taken in deciding which part to use.
The high switching frequency of the LT1931A 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
LT1931. Generally, if efficiency and maximum output
current are critical, the LT1931 should be used. If applica-
tion size and cost are more important, the LT1931A will be
the better choice. In many applications, tiny inexpensive
chip inductors can be used with the LT1931A, reducing
solution cost.
Duty Cycle
The maximum duty cycle (DC) of the LT1931A is 75%
compared to 84% for the LT1931. The duty cycle for a
given application using the dual inductor inverting topol-
ogy is given by:
DC
V
VV
OUT
IN OUT
=
+
||
||| |
For a 5V to –5V application, the DC is 50% indicating that
the LT1931A can be used. A 5V to –16V application has a
DC of 76.2% making the LT1931 the right choice. The
LT1931A 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 LT1931 are listed
in Table 1 and those for the LT1931A are listed in Table 2.
Besides these, there are many other inductors that can be
used. Consult each manufacturer for detailed information
and for their entire selection of related parts. Ferrite core
inductors should be used to obtain the best efficiency, as
core losses at frequencies above 1MHz are much lower for
ferrite cores than for powdered-iron units. When using
coupled inductors, choose one that can handle at least 1A
of current without saturating, and ensure that the inductor
has a low DCR (copper-wire resistance) to minimize I
2
R
power losses. If using uncoupled inductors, each inductor
need only handle one-half of the total switch current so
that 0.5A per inductor is sufficient. A 4.7µH to 15µH
coupled inductor or a 15µH to 22µH uncoupled inductor
will usually be the best choice for most LT1931 designs.
For the LT1931A, a 2.2µH to 4.7µH coupled inductor or a
3.3µH to 10µH uncoupled inductor will usually suffice. In
certain applications such as the “Charge Pump” inverting
DC/DC converter, only a single inductor is used. In this
case, the inductor must carry the entire 1A switch current.
Table 1. Recommended Inductors—LT1931
L Size
PART (µH) (L × W × H) mm VENDOR
CLS62-100 10 6.8 × 6.6 × 2.5 Sumida
CR43-150 15 4.5 × 4.0 × 3.2 (847) 956-0666
CR43-220 22 www.sumida.com
CTX10-1 10 8.9 × 11.4 × 4.2 Coiltronics
CTX15-1 15 (407) 241-7876
www. coiltronics.com
LQH3C100K24 10 3.2 × 2.5 × 2.0 Murata
LQH4C150K04 15 (404) 436-1300
www.murata.com
Table 2. Recommended Inductors—LT1931A
L Size
PART (µH) (L × W × H) mm VENDOR
ELJPC3R3MF 3.3 2.5 × 2.0 × 1.6 Panasonic
ELJPC4R7MF 4.7 (408) 945-5660
www.panasonic.com
CLQ4D10-4R7
1
4.7 7.6 × 4.8 × 1.8 Sumida
CLQ4D10-6R8
2
6.8 (847) 956-0666
www.sumida.com
LB20164R7M 4.7 2.0 × 1.6 × 1.6 Taiyo Yuden
LB20163R3M 3.3 (408) 573-4150
www.t-yuden.com
LQH3C4R7K24 4.7 3.2 × 2.5 × 2.0 Murata
LQH4C100K24 10 (404) 436-1300
www.murata.com
1
Use drawing #5382-T039
2
Use drawing #5382-T041
