7

LT1945

1945f

PACKAGE DESCRIPTIO

APPLICATIO S I FOR ATIO

Setting the Output Voltages

Set the output voltage for Switcher 1 (negative output

voltage ) by choosing the appropriate values for feedback

resistors R1 and R2.

R

VV

V

R

OUT

1

123

123

2

=

+

–.

.

•

Set the output voltage for Switcher 2 (positive output

voltage) by choosing the appropriate values for feedback

resistors R1B and R2B (see Figure 1).

RB

R B

V

V

OUT

12

123

1

=−









.

Diode Selection

For most LT1945 applications, the Zetex ZHCS400 sur-

face mount Schottky diode (0.4A, 40V) is an ideal choice.

Schottky diodes, with their low forward voltage drop and

fast switching speed, are the best match for the LT1945.

The Motorola MBR0520, MBR0530, or MBR0540 can also

be used. Many different manufacturers make equivalent

parts, but make sure that the component is rated to handle

at least 0.35A.

Lowering Output Voltage Ripple

Using low ESR capacitors will help minimize the output

ripple voltage, but proper selection of the inductor and the

output capacitor also plays a big role. The LT1945 pro-

vides energy to the load in bursts by ramping up the

inductor current, then delivering that current to the load.

If too large of an inductor value or too small of a capacitor

value is used, the output ripple voltage will increase

because the capacitor will be slightly overcharged each

burst cycle. To reduce the output ripple, increase the

output capacitor value or add a 4.7pF feed-forward capaci-

tor in the feedback network of the LT1945 (see the circuits

in the Typical Applications section). Adding this small,

inexpensive 4.7pF capacitor will greatly reduce the output

voltage ripple.

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-

tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

MS Package

10-Lead Plastic MSOP

(Reference LTC DWG # 05-08-1661)

MSOP (MS) 0402

0.53

± 0.01

(.021

± .006)

SEATING

PLANE

0.18

(.007)

1.10

(.043)

MAX

0.17 – 0.27

(.007 – .011)

0.13

± 0.05

(.005

± .002)

0.86

(.034)

REF

0.50

(.0197)

TYP

12 3 45

4.88

± 0.10

(.192

± .004)

0.497

± 0.076

(.0196

± .003)

REF

8

9

10

7 6

3.00

± 0.102

(.118

± .004)

(NOTE 3)

3.00

± 0.102

(.118

± .004)

NOTE 4

NOTE:

1. DIMENSIONS IN MILLIMETER/(INCH)

2. DRAWING NOT TO SCALE

3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.

MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE

4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.

INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE

5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX

0.254

(.010)

0

° – 6° TYP

DETAIL “A”

DETAIL “A”

GAUGE PLANE

5.23

(.206)

MIN

3.2 – 3.45

(.126 – .136)

0.889

± 0.127

(.035

± .005)

RECOMMENDED SOLDER PAD LAYOUT

0.305

± 0.038

(.0120

± .0015)

TYP

0.50

(.0197)

BSC