Datasheet LT1945 Datasheet (LINEAR TECHNOLOGY)

Download

Page 1

查询CD43-100供应商

FEATURES

LT1945

Dual Micropower DC/DC

Converter with Positive and

Negative Outputs

DESCRIPTIO

■

Generates Well-Regulated Positive and Negative Outputs

■

Low Quiescent Current:

20µA in Active Mode (per Converter) <1µA in Shutdown Mode

■

Operates with VIN as Low as 1.2V

■

Low V

■

Uses Small Surface Mount Components

■

High Output Voltage: Up to ±34V

■

Tiny 10-Pin MSOP Package

Switch: 250mV at 300mA

CESAT

APPLICATIO S

■

Small TFT LCD Panels

■

Handheld Computers

■

Battery Backup

■

Digital Cameras

TYPICAL APPLICATIO

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. A current limited, fixed off-time control scheme conserves operating current, resulting in high efficiency 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 profile inductors and capacitors to minimize footprint and cost in space-conscious portable applications.

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

Dual Output (+12V, –20V) Converter

2.7V

TO 5V

4.7µF

C1: TAIYO YUDEN JMK212BJ475 C2, C3: TAIYO YUDEN TMK316BJ105 C4: TAIYO YUDEN EMK107BJ104 D1, D2, D3: ZETEX ZHCS400 L1, L2: MURATA LQH3C100

10µH

810

SHDN1

LT1945

SHDN2

PGND9PGND

GND

10µH

SW1

NFB1

FB2

SW2

0.1µF

100pF

365k

4.7pF

24.9k

115k

C2 1µF

C3 1µF

–20V 10mA

12V 20mA

1945 TA01

Efficiency at VIN = 3.6V

EFFICIENCY (%)

0.1

+12V OUTPUT

–20V OUTPUT

1 10 100

LOAD CURRENT (mA)

1945 TA01a

1945f

Page 2

LT1945

1 2 3 4 5

NFB1

SHDN1

GND

SHDN2

FB2

10 9 8 7 6

SW1 PGND V

PGND SW2

TOP VIEW

MS PACKAGE

10-LEAD PLASTIC MSOP

ABSOLUTE AXI U RATI GS

UUW

PACKAGE/ORDER I FOR ATIO

(Note 1)

ORDER PART

VIN, SHDN1, SHDN2 Voltage ................................... 15V

SW1, SW2 Voltage .................................................. 36V

NFB1 Voltage ........................................................... –3V

FB2 Voltage ...............................................................V

Current into NFB1 Pin ........................................... –1mA

Current into FB2 Pin .............................................. 1mA

Junction Temperature........................................... 125°C

= 125°C, θJA = 160°C/W

JMAX

NUMBER

LT1945EMS

MS PART MARKING

LTTS

Operating Temperature Range (Note 2) .. – 40°C to 85°C

Storage Temperature Range ................. –65°C to 150°C

Consult LTC Marketing for parts specified with wider operating temperature ranges.

Lead Temperature (Soldering, 10 sec).................. 300°C

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at TA = 25°C. VIN = 1.2V, V

PARAMETER CONDITIONS MIN TYP MAX UNITS

Minimum Input Voltage 1.2 V Quiescent Current, (per Converter) Not Switching 20 30 µA

= 0V 1 µA

SHDN

NFB1 Comparator Trip Point ● –1.205 –1.23 –1.255 V FB2 Comparator Trip Point ● 1.205 1.23 1.255 V FB Comparator Hysteresis 8mV NFB1, FB2 Voltage Line Regulation 1.2V < VIN < 12V 0.05 0.1 %/V NFB1 Pin Bias Current (Note 3) V FB2 Pin Bias Current (Note 4) V Switch Off Time, Switcher 1 (Note 5) 400 ns Switch Off Time, Switcher 2 (Note 5) V

Switch V

CESAT

Switch Current Limit 250 350 400 mA SHDN Pin Current V

SHDN Input Voltage High 0.9 V SHDN Input Voltage Low 0.25 V Switch Leakage Current Switch Off, VSW = 5V 0.01 5 µA

= –1.23V ● 1.3 2 2.9 µA

NFB1

= 1.23V ● 30 80 nA

FB2

> 1V 400 ns

FB2

< 0.6V 1.5 µs

FB2

= 300mA 250 350 mV

= 1.2V 2 3 µA

SHDN

= 5V 8 12 µA

SHDN

The ● denotes the specifications which apply over the full operating

= 1.2V unless otherwise noted.

SHDN

Note 1: Absolute Maximum Ratings are those values beyond which the life

of a device may be impaired. Note 2: The LT1945 is guaranteed to meet performance specifications

from 0°C to 70°C. Specifications over the –40°C to 85°C operating

temperature range are assured by design, characterization and correlation with statistical process controls.

Note 3: Bias current flows out of the NFB1 pin. Note 4: Bias current flows into the FB2 pin. Note 5: See Figure 1 for Switcher 1 and Switcher 2 locations.

1945f

Page 3

TEMPERATURE (°C)

FEEDBACK VOLTAGE (V)

1945 G03

–1.25

–1.24

–1.23

–1.22

–1.21

–1.20

–50 –25 0

25 50 75 100

VOLTAGE

CURRENT

BIAS CURRENT (µA)

TYPICAL PERFOR A CE CHARACTERISTICS

Switch Saturation Voltage (V

)

CESAT

0.60

0.55

0.50

0.45

0.40

0.35

0.30

0.25

SWITCH VOLTAGE (V)

0.20

0.15

0.10 –25 0 25 50 75 100

–50

TEMPERATURE (°C)

SWITCH

= 500mA

= 300mA

1945 G01

FB2 Pin Voltage and Bias Current

1.25

1.24

1.23

1.22

FEEDBACK VOLTAGE (V)

1.21

1.20 –50

–25 0 25 50 75 100

TEMPERATURE (°C)

VOLTAGE

CURRENT

1945 G02

BIAS CURRENT (nA)

LT1945

NFB1 Pin Voltage and Bias Current

550

500

450

400

350

SWITCH OFF TIME (ns)

300

250

PI FU CTIO S

NFB1 (Pin 1): Feedback Pin for Switcher 1. Set the output voltage by selecting values for R1 and R2.

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.

GND (Pin 3): Ground. Tie this pin directly to the local ground plane.

SHDN2 (Pin 4): Shutdown Pin for Switcher 2. Tie this pin to 0.9V or higher to enable device. Tie below 0.25V to turn it off.

FB2 (Pin 5): Feedback Pin for Switcher 2. Set the output voltage by selecting values for R1B and R2B.

Switch Off Time Switch Current Limit

400

VIN = 12V

VIN = 1.2V

VIN = 12V

–50 –25 0 25 50 75 100

TEMPERATURE (°C)

1945 G04

350

300

250

200

150

PEAK CURRENT (mA)

100

–50 –25 0 25 50 75 100

VIN = 1.2V

TEMPERATURE (°C)

UUU

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.

PGND (Pins 7, 9): Power Ground. Tie these pins directly to the local ground plane. Both pins must be tied.

VIN (Pin 8): Input Supply Pin. Bypass this pin with a capacitor as close to the device as possible.

SW1 (Pin 10): Switch Pin for Switcher 1. This is the collector of the internal NPN power switch. Minimize the metal trace area connected to the pin to minimize EMI.

1945 G05

Quiescent Current

VFB = 1.23V NOT SWITCHING

QUIESCENT CURRENT (µA)

–50 –25 0 25 50 75 100

VIN = 12V

VIN = 1.2V

TEMPERATURE (°C)

1945 G06

1945f

Page 4

LT1945

BLOCK DIAGRA

(EXTERNAL)

R6 80k

SHDN1

R5 80k

ENABLE

SW1

OUT1

OUT2

–

Q2 X10

R3 60k

OUT1

NFB1

R4 280k

400ns

ONE-SHOT

RESET RESET

DRIVER

Q3B

0.12Ω

42mV

–

SWITCHER 1 SWITCHER 2

GND

PGND

0.12Ω

42mV

DRIVER

SW2

L3L2

SHDN2

A1B

R6B

R5B

40k

ENABLE

–

400ns

ONE-SHOT

–

A2B

Q2B

R3B

30k

R4B

140k

X10

Q1B

1945 BD

OUT2

R1B (EXTERNAL)

FB2

R2B (EXTERNAL)

Figure 1. LT1945 Block Diagram

OPERATIO

The LT1945 uses a constant off-time control scheme to provide high efficiencies over a wide range of output current. Operation can be best understood by referring to the block diagram in Figure 1. Q1 and Q2 along with R3 and R4 form a bandgap reference used to regulate the output voltage. When the voltage at the NFB1 pin is slightly below –1.23V, comparator A1 disables most of the internal circuitry. Output current is then provided by capacitor C2, which slowly discharges until the voltage at the NFB1 pin goes above the hysteresis point of A1 (typical hysteresis at the NFB1 pin is 8mV). A1 then enables the internal circuitry, turns on power switch Q3, and the current in inductors L1 and L2 begins ramping up. Once the switch current reaches 350mA, comparator A2 resets the oneshot, 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.

The second switching regulator is a step-up converter (which generates a positive output) but the basic operation is the same.The LT1945 contains additional circuitry to provide protection during start-up and under shortcircuit conditions. When the FB2 pin voltage is less than approximately 600mV, the switch off-time is increased to

1.5µs and the current limit is reduced to around 250mA (70% of its normal value). This reduces the average inductor current and helps minimize the power dissipation in the power switch and in the external inductor and diode.

1945f

Page 5

LT1945

WUU

APPLICATIO S I FOR ATIO

Choosing an Inductor

Several recommended inductors that work well with the LT1945 are listed in Table 1, although there are many other manufacturers and devices that can be used. Consult each manufacturer for more detailed information and for their entire selection of related parts. Many different sizes and shapes are available. Use the equations and recommendations in the next few sections to find the correct inductance value for your design.

Table 1. Recommended Inductors

PART VALUE (µH) MAX DCR (Ω) VENDOR

LQH3C4R7 4.7 0.26 Murata LQH3C100 10 0.30 (714) 852-2001 LQH3C220 22 0.92 www.murata.com

CD43-4R7 4.7 0.11 Sumida CD43-100 10 0.18 (847) 956-0666 CDRH4D18-4R7 4.7 0.16 www.sumida.com CDRH4D18-100 10 0.20

DO1608-472 4.7 0.09 Coilcraft DO1608-103 10 0.16 (847) 639-6400 DO1608-223 22 0.37 www.coilcraft.com

Inductor Selection—Boost Regulator

The formula below calculates the appropriate inductor value to be used for a boost 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. A larger value can be used to slightly increase the available output current, but limit it to around twice the value calculated below, as too large of an inductance will increase the output voltage ripple without providing much additional output current. A smaller value can be used (especially for systems with output voltages greater than 12V) to give a smaller physical size. Inductance can be calculated as:

VV V

−+

OUT

IN MIN

()

LIM

where VD = 0.4V (Schottky diode voltage), I and t

= 400ns; for designs with varying VIN such as

OFF

battery powered applications, use the minimum VIN value

OFF

= 350mA

LIM

in the above equation. For most regulators with output voltages below 7V, a 4.7µH inductor is the best choice, even though the equation above might specify a smaller value. This is due to the inductor current overshoot that occurs when very small inductor values are used (see Current Limit Overshoot section).

For higher output voltages, the formula above will give large inductance values. For a 2V to 20V converter (typical LCD Bias application), a 21µH inductor is called for with the above equation, but a 10µH inductor could be used without excessive reduction in maximum output current.

Inductor Selection—SEPIC Regulator

The formula below calculates the approximate inductor value to be used for a SEPIC regulator using the LT1945. As for the boost inductor selection, a larger or smaller value can be used.







OUT D

LIM



OFF





Inductor Selection—Inverting Regulator

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). A larger value can be used to slightly increase the available output current, but limit it to around twice the value calculated below, as too large of an inductance will increase the output voltage ripple without providing much additional output current. A smaller value can be used (especially for systems with output voltages greater than 12V) to give a smaller physical size. Inductance can be calculated as:



OUT D

 

LIM



where VD = 0.4V (Schottky diode voltage), I and t

OFF

= 400ns.





OFF





= 350mA

LIM

1945f

Page 6

LT1945

WUU

APPLICATIO S I FOR ATIO

For higher output voltages, the formula above will give large inductance values. For a 2V to 20V converter (typical LCD bias application), a 47µH inductor is called for with the above equation, but a 10µH or 22µH inductor could be used without excessive reduction in maximum output current.

Inductor Selection—Inverting Charge Pump Regulator

For the inverting regulator, the voltage seen by the internal power switch is equal to the sum of the absolute value of the input and output voltages, so that generating high output voltages from a high input voltage source will often exceed the 36V maximum switch rating. For instance, a 12V to – 30V converter using the inverting topology would generate 42V on the SW pin, exceeding its maximum rating. For this application, an inverting charge pump is the best topology.

The formula below calculates the approximate inductor value to be used for an inverting charge pump regulator using the LT1945. As for the boost inductor selection, a larger or smaller value can be used. For designs with varying VIN such as battery powered applications, use the minimum VIN value in the equation below.

VV V

−+

OUT

IN MIN

()

LIM

Current Limit Overshoot

For the constant off-time control scheme of the LT1945, the power switch is turned off only after the 350mA current limit is reached. There is a 100ns delay between the time when the current limit is reached and when the switch actually turns off. During this delay, the inductor current exceeds the current limit by a small amount. The peak inductor current can be calculated by:



PEAK LIM

IN MAX SAT

 

Where V

= 0.25V (switch saturation voltage). The

SAT

current overshoot will be most evident for regulators with

−

()

OFF

 



100

high input voltages and smaller inductor values. This overshoot can be beneficial as it helps increase the amount of available output current for smaller inductor values. This will be the peak current seen by the inductor (and the diode) during normal operation. For designs using small inductance values (especially at input voltages greater than 5V), the current limit overshoot can be quite high. Although it is internally current limited to 350mA, the power switch of the LT1945 can handle larger currents without problem, but the overall efficiency will suffer. Best results will be obtained when I

is kept below 700mA

PEAK

for the LT1945.

Capacitor Selection

Low ESR (Equivalent Series Resistance) capacitors should be used at the output to minimize the output ripple voltage. X5R or X7R multilayer ceramic capacitors are the best choice, as they have a very low ESR and are available in very small packages. Y5V ceramics are not recommended. Their small size makes them a good companion to the LT1945’s MS10 package. Solid tantalum capacitors (like the AVX TPS, Sprague 593D families) or OS-CON capacitors can be used, but they will occupy more board area than a ceramic and will have a higher ESR. Always use a capacitor with a sufficient voltage rating.

Ceramic capacitors also make a good choice for the input decoupling capacitor, which should be placed as close as possible to the LT1945. A 4.7µF input capacitor is suffi- cient for most applications. Table 2 shows a list of several capacitor manufacturers. Consult the manufacturers for more detailed information and for their entire selection 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

1945f

Page 7

LT1945

WUU

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.

123

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 RB

Diode Selection

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

123

–.

OUT

()



OUT



123



210

•

−



1=−





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 Applications section). Adding this small, inexpensive 4.7pF capacitor will greatly reduce the output voltage ripple.

PACKAGE DESCRIPTIO

MS Package

10-Lead Plastic MSOP

0.889

0.127

(.035 ± .005)

5.23

(.206)

MIN

0.305 ± 0.038

(.0120 ± .0015)

TYP

RECOMMENDED SOLDER PAD LAYOUT

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

3.2 – 3.45

(.126 – .136)

0.50

(.0197)

BSC

(Reference LTC DWG # 05-08-1661)

0.254 (.010)

GAUGE PLANE

0.18

(.007)

DETAIL “A”

° – 6° TYP

(.021 ± .006)

0.53 ± 0.01

SEATING

PLANE

3.00 ± 0.102 (.118 ± .004)

(NOTE 3)

4.88 ± 0.10

(.192 ± .004)

(.043)

0.17 – 0.27

(.007 – .011)

1.10

MAX

0.50

(.0197)

TYP

8910

0.497 ± 0.076

(.0196 ± .003)

REF

3.00 ± 0.102 (.118 ± .004)

NOTE 4

0.86

(.034)

REF

0.13 ± 0.05

(.005 ± .002)

MSOP (MS) 0402

1945f

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

Page 8

LT1945

TYPICAL APPLICATIO

Dual Output (±32V) Converter

2.7V

TO 5V

4.7µF

C1: TAIYO YUDEN JMK212BJ475 C2, C3: TAIYO YUDEN GMK316BJ105 C4: TAIYO YUDEN UMK212BJ104 D1, D2, D3: ZETEX ZHCS400 L1, L2: MURATA LQH3C100

10µH

810

SHDN1

LT1945

SHDN2

PGND9PGND

GND

10µH

0.1µF

SW1

NFB1

FB2

SW2

(408)573-4150 (408)573-4150 (408)573-4150 (631)543-7100 (814)237-1431

4.7pF

100pF

604k

24.9k

80.6k

C2 1µF

C3 1µF

1945 TA02

–32V 5mA

32V 5mA

Efficiency at VIN = 3.6V

EFFICIENCY (%)

0.1

+32V OUTPUT

–32V OUTPUT

110

LOAD CURRENT (mA)

1945 TA02a

RELATED PARTS

PART NUMBER DESCRIPTION COMMENTS

LT1613 550mA ISW, 1.4MHz, High Efficiency Step-Up DC/DC Converter VIN = 0.9V to 10V, V

ThinSOT Package

LT1615/LT1615-1 300mA ISW, Constant Off-Time, High Efficiency VIN = 1.2V to 15V, V

Step-Up DC/DC Converter ThinSOT Package

LT1940 Dual Output 1.4A (I

Step-Down DC/DC Converter TSSOP-16E Package

LT1944 Dual Output 350mA ISW, Constant Off-Time, High Efficiency VIN = 1.2V to 15V, V

Step-Up DC/DC Converter

LT1944-1 Dual Output 150mA ISW, Constant Off-Time, High Efficiency VIN = 1.2V to 15V, V

Step-Up DC/DC Converter

LT1949/LT1949-1 550mA ISW, 600kHz/1.1MHz, High Efficiency VIN = 1.5V to 12V, V

Step-Up DC/DC Converter S8, MS8 Packages

LTC3400/LTC3400B 600mA ISW, 1.2MHz, Synchronous Step-Up DC/DC Converter VIN = 0.85V to 5V, V

LTC3401 1A ISW, 3MHz, Synchronous Step-Up DC/DC Converter VIN = 0.5V to 5V, V LTC3402 2A ISW, 3MHz, Synchronous Step-Up DC/DC Converter VIN = 0.5V to 5V, V LTC3423 1A ISW, 3MHz, Low V

DC/DC Converter

LTC3424 2A ISW, 3MHz, Low V

DC/DC Converter

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

(408) 432-1900 ● FAX: (408) 434-0507

), Constant 1.1MHz, High Efficiency VIN = 3V to 25V, V

OUT

ThinSOT Package

Synchronous Step-Up VIN = 0.5V to 5V, V

OUT,

Synchronous Step-Up VIN = 0.5V to 5V, V

OUT,

●

www.linear.com

OUT

= 34V, IQ = 3mA, ISD = <1µA,

OUT

= 34V, IQ = 20µA, ISD = <1µA,

OUT

= 1.2V, IQ = 2.5mA, ISD = <1µA,

= 34V, IQ = 20µA, ISD = <1µA, MS Package

OUT

= 34V, IQ = 20µA, ISD = <1µA, MS Package

OUT

= 28V, IQ = 4.5mA, ISD = <25µA,

OUT

= 5V, IQ = 19µA/300µA, ISD = <1µA,

OUT

= 6V, IQ = 38µA, ISD = <1µA, MS Package

OUT

= 6V, IQ = 38µA, ISD = <1µA, MS Package

OUT

= 6V, IQ = 38µA, ISD = <1µA, MS Package

OUT

= 6V, IQ = 38µA, ISD = <1µA, MS Package

OUT

LT/TP 0802 2K • PRINTED IN USA

 LINEAR TECHNOLOGY CORPORATION 2001

1945f