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LT1945

1945fa

TYPICAL APPLICATION

DESCRIPTION

Dual Micropower DC/DC

Converter with Positive and

Negative Outputs

The LT

1945 is a dual micropower DC/DC converter in a

10-pin MSOP package. Each converter is designed with

a 350mA current limit and an input voltage range of 1.2V

to 15V, making the LT1945 ideal for a wide variety of ap-

plications. Both converters feature a quiescent current of

only 20μA at no load, which further reduces to 0.5μA in

shutdown. A current limited, ﬁ xed off-time control scheme

conserves operating current, resulting in high efﬁ ciency

over a broad range of load current. The 36V switch allows

high voltage outputs up to ±34V to be easily generated

without the use of costly transformers. The LT1945’s

low off-time of 400ns permits the use of tiny, low proﬁ le

inductors and capacitors to minimize footprint and cost

in space-conscious portable applications.

Dual Output (+12V, –20V) Converter

L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.

All other trademarks are the property of their respective owners.

FEATURES

APPLICATIONS

Small TFT LCD Panels

Handheld Computers

Battery Backup

Digital Cameras

Generates Well-Regulated Positive and

Negative Outputs

Low Quiescent Current:

20μA in Active Mode (per Converter)

<1μA in Shutdown Mode

Operates with V

as Low as 1.2V

Low V

CESAT

Switch: 250mV at 300mA

Uses Small Surface Mount Components

High Output Voltage: Up to ±34V

Tiny 10-Pin MSOP Package

Efﬁ ciency at V

= 3.6V

SW1

FB2

LT1945

10μH

SHDN2

24.9k

365k

1μF

–20V

10mA

2.7V

TO 5V

1945 TA01

GND

PGND

810

SW2

4.7pF

4.7μF

0.1μF

NFB1

SHDN1

10μH

115k

1μF

100pF

C1: TAIYO YUDEN JMK212BJ475

C2, C3: TAIYO YUDEN TMK316BJ105

C4: TAIYO YUDEN EMK107BJ104

D1, D2, D3: ZETEX ZHCS400

L1, L2: MURATA LQH3C100

12V

20mA

LOAD CURRENT (mA)

0.1

EFFICIENCY (%)

1 10 100

1945 TA01a

+12V OUTPUT

–20V OUTPUT

Summary of content (8 pages)

PAGE 1
LT1945 Dual Micropower DC/DC Converter with Positive and Negative Outputs FEATURES DESCRIPTION n The LT®1945 is a dual micropower DC/DC converter in a 10-pin MSOP package. Each converter is designed with a 350mA current limit and an input voltage range of 1.2V to 15V, making the LT1945 ideal for a wide variety of applications. Both converters feature a quiescent current of only 20μA at no load, which further reduces to 0.5μA in shutdown.
PAGE 2
LT1945 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) TOP VIEW VIN, SHDN1, SHDN2 Voltage.....................................15V SW1, SW2 Voltage ....................................................36V NFB1 Voltage ............................................................–3V FB2 Voltage ............................................................... VIN Current into NFB1 Pin ............................................–1mA Current into FB2 Pin................................................
PAGE 3
LT1945 ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C. VIN = 1.2V, VSHDN = 1.2V unless otherwise noted. PARAMETER CONDITIONS SHDN Pin Current VSHDN = 1.2V VSHDN = 5V MIN SHDN Input Voltage High TYP MAX 2 8 3 12 UNITS μA μA 0.9 V SHDN Input Voltage Low Switch Off, VSW = 5V Switch Leakage Current 0.25 V 5 μA 0.
PAGE 4
LT1945 PIN FUNCTIONS NFB1 (Pin 1): Feedback Pin for Switcher 1. Set the output voltage by selecting values for R1 and R2. SW2 (Pin 6): Switch Pin for Switcher 2. This is the collector of the internal NPN power switch. Minimize the metal trace area connected to the pin to minimize EMI. SHDN1 (Pin 2): Shutdown Pin for Switcher 1. Tie this pin to 0.9V or higher to enable device. Tie below 0.25V to turn it off. PGND (Pins 7, 9): Power Ground. Tie these pins directly to the local ground plane.
PAGE 5
LT1945 OPERATION current in inductors L1 and L2 begins ramping up. Once the switch current reaches 350mA, comparator A2 resets the one-shot, which turns off Q3 for 400ns. L2 continues to deliver current to the output while Q3 is off. Q3 turns on again and the inductor currents ramp back up to 350mA, then A2 again resets the one-shot. This switching action continues until the output voltage is charged up (until the NFB1 pin reaches –1.23V), then A1 turns off the internal circuitry and the cycle repeats.
PAGE 6
LT1945 APPLICATIONS INFORMATION Inductor Selection—Inverting Regulator Current Limit Overshoot The formula below calculates the appropriate inductor value to be used for an inverting regulator using the LT1945 (or at least provides a good starting point). This value provides a good tradeoff in inductor size and system performance. Pick a standard inductor close to this value (both inductors should be the same value).
PAGE 7
LT1945 APPLICATIONS INFORMATION of related parts. Diode Selection Table 2. Recommended Capacitors For most LT1945 applications, the Zetex ZHCS400 surface 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.
PAGE 8
LT1945 TYPICAL APPLICATIONS Dual Output (±32V) Converter C4 0.1μF L1 10μH –32V 5mA 8 100pF SW1 SHDN1 C1 4.7μF NFB1 604k 1 LT1945 4 +32V OUTPUT 75 10 VIN 2 80 D1 C2 1μF D2 SHDN2 FB2 5 24.9k EFFICIENCY (%) VIN 2.7V TO 5V Efﬁciency at VIN = 3.6V 70 –32V OUTPUT 65 60 GND PGND PGND SW2 3 7 9 55 6 50 80.6k C3 1μF 4.7pF L2 10μH C1: TAIYO YUDEN JMK212BJ475 C2, C3: TAIYO YUDEN GMK316BJ105 C4: TAIYO YUDEN UMK212BJ104 D1, D2, D3: ZETEX ZHCS400 L1, L2: MURATA LQH3C100 0.