5

LT1931/LT1931A

1931fa

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:

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

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

µH to 15µH

coupled inductor or a 15

µH to 22µH uncoupled inductor

will usually be the best choice for most LT1931 designs.

For the LT1931A, a 2.2

µH to 4.7µH coupled inductor or a

3.3

µH 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

4.7

7.6

× 4.8 × 1.8

Sumida

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