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LT1945IMS

Dual Micropower DC/DC Converter with Positive and Negative Outputs

Linear Technology 

LT1945

APPLICATIONS INFORMATION

of related parts.

Table 2. Recommended Capacitors

CAPACITOR TYPE

VENDOR

Ceramic

Taiyo Yuden

(408) 573-4150

www.t-yuden.com

Ceramic

AVX

(803) 448-9411

www.avxcorp.com

Ceramic

Murata

(714) 852-2001

www.murata.com

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.

( ) R1=

VOUT –1.23V

1.23V + 2 • 10−6

R2

Set the output voltage for Switcher 2 (positive output

voltage) by choosing the appropriate values for feedback

resistors R1B and R2B (see Figure 1).

R1=

R2

⎛

⎝⎜

VOUT

1.23

− 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 provides

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 capacitor in the feedback

network of the LT1945 (see the circuits in the Typical Ap-

plications section). Adding this small, inexpensive 4.7pF

capacitor will greatly reduce the output voltage ripple.

PACKAGE DESCRIPTION

0.889 p 0.127

(.035 p .005)

MS Package

10-Lead Plastic MSOP

(Reference LTC DWG # 05-08-1661)

3.00 p 0.102

(.118 p .004)

(NOTE 3)

10 9 8 7 6

0.497 p 0.076

(.0196 p .003)

REF

5.23

(.206)

MIN

3.2 – 3.45

(.126 – .136)

0.254

(.010)

DETAIL “A”

0o – 6o TYP

4.88 p 0.10

(.192 p .004)

0.305 p 0.038

(.0120 p .0015)

TYP

0.50

(.0197)

BSC

RECOMMENDED SOLDER PAD LAYOUT

GAUGE PLANE

0.53 p 0.01

(.021 p .006)

DETAIL “A”

0.18

(.007)

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

SEATING

PLANE

12345

1.10

(.043)

MAX

0.17 – 0.27

(.007 – .011)

0.50

(.0197)

TYP

3.00 p 0.102

(.118 p .004)

NOTE 4

0.86

(.034)

REF

0.13 p 0.05

(.005 p .002)

MSOP (MS) 0402

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 representa-

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

1945fa

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