LT1931 Linear Technology, LT1931 Datasheet - Page 5

• Manufacturer Part Number: LT1931
• Description: 1.2MHz/2.2MHz Inverting DC/DC Converters in ThinSOT
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
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:
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
DC
|
V
IN
|
| |
V
OUT
V
OUT
U
|
|
U
W
U
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
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 4.7 H to 15 H
coupled inductor or a 15 H to 22 H uncoupled inductor
will usually be the best choice for most LT1931 designs.
For the LT1931A, a 2.2 H to 4.7 H coupled inductor or a
3.3 H to 10 H 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
PART
CLS62-100
CR43-150
CR43-220
CTX10-1
CTX15-1
LQH3C100K24
LQH4C150K04
Table 2. Recommended Inductors—LT1931A
PART
ELJPC3R3MF
ELJPC4R7MF
CLQ4D10-4R7
CLQ4D10-6R8
LB20164R7M
LB20163R3M
LQH3C4R7K24
LQH4C100K24
1
2
Use drawing #5382-T039
Use drawing #5382-T041
1
2
( H)
( H)
3.3
4.7
4.7
6.8
4.7
3.3
4.7
10
15
22
10
15
10
15
10
L
L
(L W H) mm
(L W H) mm
8.9 11.4 4.2
6.8 6.6 2.5
4.5 4.0 3.2
3.2 2.5 2.0
2.5 2.0 1.6
7.6 4.8 1.8
2.0 1.6 1.6
3.2 2.5 2.0
LT1931/LT1931A
Size
Size
VENDOR
Sumida
[phone]
www.sumida.com
Coiltronics
[phone]
www. coiltronics.com
Murata
[phone]
www.murata.com
VENDOR
Panasonic
[phone]
www.panasonic.com
Sumida
[phone]
www.sumida.com
Taiyo Yuden
[phone]
www.t-yuden.com
Murata
[phone]
www.murata.com
5
2
R