LINEAR TECHNOLOGY LT1944 Technical data

FEATURES

LT1944

Dual Micropower Step-Up

DC/DC Converter

U

DESCRIPTIO

■

Low Quiescent Current:

20µA in Active Mode
<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

U

APPLICATIO S

■

LCD Bias

■

Handheld Computers

■

Battery Backup

■

Digital Cameras

U

TYPICAL APPLICATIO

The LT®1944 is a dual micropower step-up 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 LT1944 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 in a simple boost topology without the use of
costly transformers. The LT1944’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 (5V, 30V) Boost Converter 5V Output Efficiency

V

IN

2.7V

TO 4.2V

C1

4.7µF

C1: TAIYO YUDEN JMK212BJ475
C2: TAIYO YUDEN JMK316BJ106
C3: TAIYO YUDEN GMK316BJ105
D1, D2: ON SEMI MBR0540
L1: MURATA LQH3C4R7
L2: MURATA LQH3C100

810

V

IN

2

SHDN1

4

SHDN2

PGND9PGND

GND

3

L2

10µH

L1

4.7µH

LT1944

7

SW1

FB1

FB2

SW2

D1

4.7pF

1

5

6

4.7pF

D2

1M

324k

86.6k

2M

5V
80mA

C2
10µF

C3
1µF

30V
8mA

1944 TA01

90

85

VIN = 4.2V

80

75

70

65

EFFICIENCY (%)

60

55

50

0.1

VIN = 2.7V

1 10 100

LOAD CURRENT (mA)

1944 TA01a

1

LT1944

1
2
3
4
5

FB1

SHDN1

GND

SHDN2

FB2

10
9
8
7
6

SW1
PGND
V

IN

PGND
SW2

TOP VIEW

MS10 PACKAGE

10-LEAD PLASTIC MSOP

WW

W

ABSOLUTE AXI U RATI GS

U

UUW

PACKAGE/ORDER I FOR ATIO

(Note 1)

ORDER PART

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

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

FB1, FB2 Voltage .......................................................V

IN

NUMBER

LT1944EMS

Current into FB1, FB2 Pins ..................................... 1mA

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

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

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

T

= 125°C, θJA = 160°C/W

JMAX

MS10 PART

MARKING

LTTR

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

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

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, Each Switcher Not Switching 20 30 µA

= 0V 1 µA

V

SHDN

FB Comparator Trip Point ● 1.205 1.23 1.255 V
FB Comparator Hysteresis 8mV
FB Voltage Line Regulation 1.2V < VIN < 12V 0.05 0.1 %/V
FB Pin Bias Current (Note 3) VFB = 1.23V ● 30 80 nA
Switch Off Time VFB > 1V 400 ns

< 0.6V 1.5 µs

V

FB

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

Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.

Note 2: The LT1944 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 into the FB pin.

I

= 300mA 250 350 mV

SW

= 1.2V 2 3 µA

SHDN

= 5V 8 12 µA

V

SHDN

The ● denotes the specifications which apply over the full operating

= 1.2V unless otherwise noted.

SHDN

2

UW

TYPICAL PERFOR A CE CHARACTERISTICS

Switch Saturation Voltage
(V

) Quiescent Current

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

I

= 500mA

SWITCH

I

= 300mA

SWITCH

TEMPERATURE (°C)

1944 G01

Switch Off Time Shutdown Pin CurrentSwitch Current Limit

550

500

450

400

350

SWITCH OFF TIME (ns)

300

250

–50 –25 0 25 50 75 100

VIN = 1.2V

VIN = 12V

TEMPERATURE (°C)

1944 G04

Feedback Pin Voltage and
Bias Current

1.25

1.24
VOLTAGE

1.23

1.22

FEEDBACK VOLTAGE (V)

1.21

1.20

–50

–25 0 25 50 75 100

400

VIN = 12V

350

300

250

200

150

PEAK CURRENT (mA)

100

50

0

–50 –25 0 25 50 75 100

CURRENT

TEMPERATURE (°C)

VIN = 1.2V

TEMPERATURE (°C)

1944 G02

1944 G05

50

40

BIAS CURRENT (nA)

30

20

10

0

QUIESCENT CURRENT (µA)

SHUTDOWN PIN CURRENT (µA)

LT1944

25

VFB = 1.23V
NOT SWITCHING

23

21

19

17

15

–50 –25 0 25 50 75 100

25

20

15

10

5

0

0 5 10 15

VIN = 12V

VIN = 1.2V

TEMPERATURE (°C)

1944 G03

25°C

100°C

SHUTDOWN PIN VOLTAGE (V)

1944 G03

UUU

PI FU CTIO S

FB1 (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.

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.

3

LT1944

BLOCK DIAGRA

W

(EXTERNAL)

(EXTERNAL)

D2

C3

SW2

6

Q3B

DRIVER

+

42mV

7

–

A2B

L2

ENABLE

400ns

ONE-SHOT

RESET

SHDN2

A1B

V

IN

4

V

IN

R6B

R5B

40k

40k

+

V

1944 BD

OUT2

R1B
(EXTERNAL)

FB2

5

R2B
(EXTERNAL)

–

Q2B

R3B

30k

R4B

140k

X10

Q1B

10

DRIVER

+

–

SW1

D1

42mV

C2

Q3

0.12Ω

V

OUT1

9

V

OUT2

0.12Ω

PGNDGND

PGND

A1

L1

ENABLE

400ns

ONE-SHOT

RESET

A2

V

IN

C1

V

IN

8

R5
40k

R6
40k

SHDN1

2

+

V

OUT1

R1

FB1

R2

Q1

1

–

Q2
X10

R3
30k

R4
140k

3

Figure 1. LT1944 Block Diagram

U

OPERATIO

The LT1944 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 FB1 pin is slightly above

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 FB1 pin
drops below the lower hysteresis point of A1 (typical
hysteresis at the FB pin is 8mV). A1 then enables the
internal circuitry, turns on power switch Q3, and the
current in inductor L1 begins ramping up. Once the switch
current reaches 350mA, comparator A2 resets the one­shot, which turns off Q3 for 400ns. L1 then delivers
current to the output through diode D1 as the inductor

current ramps down. Q3 turns on again and the inductor
current ramps back up to 350mA, then A2 resets the one­shot, again allowing L1 to deliver current to the output.
This switching action continues until the output voltage is
charged up (until the FB1 pin reaches 1.23V), then A1
turns off the internal circuitry and the cycle repeats. The
LT1944 contains additional circuitry to provide protection
during start-up and under short-circuit conditions. When
the FB1 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. The second switching
regulator operates in the same manner.

4

LT1944

U

WUU

APPLICATIO S I FOR ATIO

Choosing an Inductor

Several recommended inductors that work well with the
LT1944 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 [phone]
LQH3C220 22 0.92 www.murata.com

CD43-4R7 4.7 0.11 Sumida
CD43-100 10 0.18 [phone]
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 [phone]
DO1608-223 22 0.37 www.coilcraft.com

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 LT1944.
As for the boost inductor selection, a larger or smaller
value can be used.



VV

=

2





OUT D

I

LIM

L



+

t

OFF





Inductor Selection—Boost Regulator

The formula below calculates the appropriate inductor
value to be used for a boost regulator using the LT1944 (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

L

=

IN MIN

()

I

LIM

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

= 400ns; for designs with varying VIN such as

OFF

D

t

OFF

= 350mA

LIM

battery powered applications, use the minimum VIN value
in the above equation. For most systems with output

Current Limit Overshoot

For the constant off-time control scheme of the LT1944,
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:

II

=+

PEAK LIM

Where V

SAT



VV

IN MAX SAT




−

()

L

= 0.25V (switch saturation voltage). The






100

ns

current overshoot will be most evident for systems with
high input voltages and for systems where smaller induc­tor values are used. 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

5

LT1944

U

WUU

APPLICATIO S I FOR ATIO

limited to 350mA, the power switch of the LT1944 can
handle larger currents without problem, but the overall
efficiency will suffer. Best results will be obtained when
I

is kept below 700mA for the LT1944.

PEAK

Capacitor Selection

Low ESR (Equivalent Series Resistance) capacitors should
be used at the output to minimize the output ripple voltage.
Multilayer ceramic capacitors are the best choice, as they
have a very low ESR and are available in very small
packages. Their small size makes them a good companion
to the LT1944’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 LT1944. 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

[phone]
www.t-yuden.com

Ceramic AVX

[phone]
www.avxcorp.com

Ceramic Murata

[phone]
www.murata.com

Setting the Output Voltage

Set the output voltage for each switching regulator by
choosing the appropriate values for feedback resistors R1
and R2 (see Figure 1).

V



RR

12

Diode Selection

For most LT1944 applications, the Motorola MBR0520
surface mount Schottky diode (0.5A, 20V) is an ideal
choice. Schottky diodes, with their low forward voltage
drop and fast switching speed, are the best match for the
LT1944. For higher output voltage applications the 30V
MBR0530 or 40V MBR0540 can 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 LT1944 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 LT1944 (see the circuits
in the Typical Applications section). Adding this small,
inexpensive 4.7pF capacitor will greatly reduce the output
voltage ripple.





123

OUT

.



1=−





V

6

U

TYPICAL APPLICATIO S

2-Cell Dual Output (3.3V, 5V) Boost Converter

V

1.8V

TO 3V

4.7µF

L1

IN

2

C1

4

4.7µH

810

V

IN

SHDN1

LT1944

SHDN2
GND

PGND9PGND

7

3

SW1

FB1

FB2

SW2

D1

4.7pF

1

5

6

1M

324k

604k

C2
10µF

5V
40mA

LT1944

C1: TAIYO YUDEN JMK212BJ475
C2, C3: TAIYO YUDEN JMK316BJ106
D1, D2: ON SEMI MBR0520
L1, L2: MURATA LQH3C4R7

2-Cell to 5V Efficiency 2-Cell to 3.3V Efficiency

90

85

80

75

70

65

EFFICIENCY (%)

60

55

50

0.1

VIN = 3V

VIN = 1.8V

110100

LOAD CURRENT (mA)

PACKAGE DESCRIPTIO

[phone]
[phone]
[phone]
[phone]

1944 TA02a

U

L2

4.7µH

4.7pF

D2

90

85

80

75

70

65

EFFICIENCY (%)

60

55

50

0.1

C3

1M

10µF

3.3V
80mA

1944 TA02

VIN = 3V

VIN = 1.8V

1 10 100

LOAD CURRENT (mA)

1944 TA02b

MS10 Package

10-Lead Plastic MSOP

(LTC DWG # 05-08-1661)

0.043

(1.10)

0.007
(0.18)

0.021
(0.53 ± 0.015)

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

PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE

** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.

INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE

± 0.006

° – 6° TYP

0

SEATING

PLANE

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.

MAX

0.007 – 0.011
(0.17 – 0.27)

0.0197
(0.50)

BSC

0.034

(0.86)

REF

0.005

± 0.002

(0.13 ± 0.05)

0.118 ± 0.004*
(3.00 ± 0.102)

0.193 ± 0.006

(4.90 ± 0.15)

12

8910

7

45

3

6

0.118 ± 0.004**
(3.00 ± 0.102)

MSOP (MS10) 1100

7

LT1944

TYPICAL APPLICATIO

U

Four Output Power Supply for Color LCD Displays

V

2.7V

TO 4.2V

4.7µF

Q1

C6

2.2µF

D3A

D3B

C3

0.1µF

L1

IN

2

C1

4

10µH

810

V

IN

SHDN1

LT1944

SHDN2

PGND9PGND

GND

7

3

L2

10µH

SW1

FB1

FB2

SW2

C4

0.1µF

1

5

6

C8

1µF

D4

D1

5 WHITE LEDs

140k

D2B

D2A

1M

140k

C7

0.1µF

Q2

C5
1µF

C2

2.2µF

82.5Ω

15mA

–6.5V
500µA

20V
500µA

10V
5mA

1944 TA03

C1: TAIYO YUDEN JMK212BJ475
C2, C6: TAIYO YUDEN LMK212BJ225
C3, C4, C7: TAIYO YUDEN EMK107BJ104
C5, C8: TAIYO YUDEN TMK316BJ105
D1, D4: ON SEMI MBR0530
D2, D3: ZETEX BAT54S
L1, L2: SUMIDA CLQ4D10-100
Q1, Q2: ON SEMI MMBT3906

[phone]
[phone]
[phone]
[phone]
[phone]
[phone]
[phone]
[phone]

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Burst Mode is a registered trademark of Linear Technology Corporation

8

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone] ● FAX: [phone]

●

www.linear.com

1944f LT/TP 1001 2K • PRINTED IN USA

 LINEAR TECHNOLOGY CORPORATION 2001