1
LT1945
1945f
FEATURES
DESCRIPTIO
U
APPLICATIO S
U
TYPICAL APPLICATIO
U
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 appli-
cations. Both converters feature a quiescent current of
only 20
A at no load, which further reduces to 0.5
A 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.
s
Generates Well-Regulated Positive and
Negative Outputs
s
Low Quiescent Current:
20
A in Active Mode (per Converter)
<1
A in Shutdown Mode
s
Operates with V
IN
as Low as 1.2V
s
Low V
CESAT
Switch: 250mV at 300mA
s
Uses Small Surface Mount Components
s
High Output Voltage: Up to
34V
s
Tiny 10-Pin MSOP Package
s
Small TFT LCD Panels
s
Handheld Computers
s
Battery Backup
s
Digital Cameras
, LTC and LT are registered trademarks of Linear Technology Corporation.
LOAD CURRENT (mA)
0.1
EFFICIENCY (%)
1
10
100
1945 TA01a
90
85
80
75
70
65
60
55
50
+12V OUTPUT
20V OUTPUT
Dual Output (+12V, 20V) Converter
Efficiency at V
IN
= 3.6V
V
IN
SW1
FB2
LT1945
4
3
L1
10
H
D1
D2
SHDN2
24.9k
365k
C2
1
F
20V
10mA
V
IN
2.7V
TO 5V
1945 TA01
GND
7
PGND
9
PGND
6
8
10
SW2
4.7pF
C1
4.7
F
C4
0.1
F
NFB1
2
5
1
SHDN1
L2
10
H
D3
1M
115k
C3
1
F
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
2
LT1945
1945f
ABSOLUTE AXI U
RATI GS
W
W
W
U
PACKAGE/ORDER I FOR ATIO
U
U
W
(Note 1)
V
IN
, SHDN1, SHDN2 Voltage ................................... 15V
SW1, SW2 Voltage .................................................. 36V
NFB1 Voltage ........................................................... 3V
FB2 Voltage ...............................................................V
IN
Current into NFB1 Pin ........................................... 1mA
Current into FB2 Pin .............................................. 1mA
Junction Temperature ........................................... 125
C
Operating Temperature Range (Note 2) .. 40
C to 85
C
Storage Temperature Range ................. 65
C to 150
C
Lead Temperature (Soldering, 10 sec).................. 300
C
ORDER PART
NUMBER
LT1945EMS
MS PART MARKING
ELECTRICAL CHARACTERISTICS
The
q
denotes the specifications which apply over the full operating
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Minimum Input Voltage
1.2
V
Quiescent Current, (per Converter)
Not Switching
20
30
A
V
SHDN
= 0V
1
A
NFB1 Comparator Trip Point
q
1.205
1.23
1.255
V
FB2 Comparator Trip Point
q
1.205
1.23
1.255
V
FB Comparator Hysteresis
8
mV
NFB1, FB2 Voltage Line Regulation
1.2V < V
IN
< 12V
0.05
0.1
%/V
NFB1 Pin Bias Current (Note 3)
V
NFB1
= 1.23V
q
1.3
2
2.9
A
FB2 Pin Bias Current (Note 4)
V
FB2
= 1.23V
q
30
80
nA
Switch Off Time, Switcher 1 (Note 5)
400
ns
Switch Off Time, Switcher 2 (Note 5)
V
FB2
> 1V
400
ns
V
FB2
< 0.6V
1.5
s
Switch V
CESAT
I
SW
= 300mA
250
350
mV
Switch Current Limit
250
350
400
mA
SHDN Pin Current
V
SHDN
= 1.2V
2
3
A
V
SHDN
= 5V
8
12
A
SHDN Input Voltage High
0.9
V
SHDN Input Voltage Low
0.25
V
Switch Leakage Current
Switch Off, V
SW
= 5V
0.01
5
A
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
T
JMAX
= 125
C,
JA
= 160
C/W
temperature range, otherwise specifications are at T
A
= 25
C. V
IN
= 1.2V, V
SHDN
= 1.2V unless otherwise noted.
LTTS
Consult LTC Marketing for parts specified with wider operating temperature ranges.
1
2
3
4
5
NFB1
SHDN1
GND
SHDN2
FB2
10
9
8
7
6
SW1
PGND
V
IN
PGND
SW2
TOP VIEW
MS PACKAGE
10-LEAD PLASTIC MSOP
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.
3
LT1945
1945f
Quiescent Current
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
Switch Saturation Voltage
(V
CESAT
)
FB2 Pin Voltage and
Bias Current
TEMPERATURE (
C)
50
25
0
25
50
75
100
SWITCH VOLTAGE (V)
1945 G01
0.60
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
I
SWITCH
= 500mA
I
SWITCH
= 300mA
TEMPERATURE (
C)
50
FEEDBACK VOLTAGE (V)
1945 G02
1.25
1.24
1.23
1.22
1.21
1.20
BIAS CURRENT (nA)
50
40
30
20
10
0
CURRENT
VOLTAGE
25
0
25
50
75
100
TEMPERATURE (
C)
FEEDBACK VOLTAGE (V)
1945 G03
1.25
1.24
1.23
1.22
1.21
1.20
50
25
0
0
1
2
3
4
5
25
50
75
100
VOLTAGE
CURRENT
BIAS CURRENT (
A)
PI FU CTIO S
U
U
U
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.
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.
V
IN
(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.
TEMPERATURE (
C)
SWITCH OFF TIME (ns)
1945 G04
550
500
450
400
350
300
250
50
25
0
25
50
75
100
V
IN
= 1.2V
V
IN
= 12V
TEMPERATURE (
C)
PEAK CURRENT (mA)
1945 G05
400
350
300
250
200
150
100
50
0
50
25
0
25
50
75
100
V
IN
= 12V
V
IN
= 1.2V
Switch Off Time
Switch Current Limit
NFB1 Pin Voltage and
Bias Current
TEMPERATURE (
C)
QUIESCENT CURRENT (
A)
1945 G06
25
23
21
19
17
15
50
25
0
25
50
75
100
V
IN
= 12V
V
IN
= 1.2V
V
FB
= 1.23V
NOT SWITCHING
4
LT1945
1945f
BLOCK DIAGRA
W
Figure 1. LT1945 Block Diagram
OPERATIO
U
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 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.
The second switching regulator is a step-up converter
(which generates a positive output) but the basic opera-
tion is the same.The LT1945 contains additional circuitry
to provide protection during start-up and under short-
circuit conditions. When the FB2 pin voltage is less than
approximately 600mV, the switch off-time is increased to
1.5
s 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.
+
+
8
400ns
ONE-SHOT
DRIVER
RESET
RESET
ENABLE
42mV
0.12
A2
A1
Q3
9
3
R4
280k
R3
60k
R6
80k
R5
80k
Q2
X10
Q1
1
V
IN
NFB1
2
SHDN1
10
SW1
PGND
GND
1945 BD
L1
C3
C2
V
OUT1
V
IN
D1
R2
(EXTERNAL)
R1
(EXTERNAL)
V
OUT1
C1
+
+
400ns
ONE-SHOT
ENABLE
42mV
0.12
A2B
A1B
Q3B
7
R4B
140k
R3B
30k
R6B
40k
R5B
40k
Q2B
X10
Q1B
5
V
IN
FB2
4
SHDN2
6
SW2
PGND
L3
L2
C4
V
OUT2
V
IN
D2
R2B
(EXTERNAL)
R1B
(EXTERNAL)
V
OUT2
SWITCHER 1
SWITCHER 2
DRIVER
5
LT1945
1945f
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 recommenda-
tions in the next few sections to find the correct inductance
value for your design.
Table 1. Recommended Inductors
PART
VALUE (
H)
MAX DCR (
)
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 pro-
vides a good tradeoff in inductor size and system perfor-
mance. 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 in-
crease 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:
L
V
V
V
I
t
OUT
IN MIN
D
LIM
OFF
=
-
+
( )
where V
D
= 0.4V (Schottky diode voltage), I
LIM
= 350mA
and t
OFF
= 400ns; for designs with varying V
IN
such as
battery powered applications, use the minimum V
IN
value
APPLICATIO S I FOR ATIO
W
U
U
U
in the above equation. For most regulators with output
voltages below 7V, a 4.7
H 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 21
H inductor is called for with
the above equation, but a 10
H 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.
L
V
V
I
t
OUT
D
LIM
OFF
=
+
2
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:
L
V
V
I
t
OUT
D
LIM
OFF
=
+
2
where V
D
= 0.4V (Schottky diode voltage), I
LIM
= 350mA
and t
OFF
= 400ns.
6
LT1945
1945f
APPLICATIO S I FOR ATIO
W
U
U
U
For higher output voltages, the formula above will give
large inductance values. For a 2V to 20V converter (typical
LCD bias application), a 47
H inductor is called for with
the above equation, but a 10
H or 22
H 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 V
IN
such as battery powered applications, use the
minimum V
IN
value in the equation below.
L
V
V
V
I
t
OUT
IN MIN
D
LIM
OFF
=
-
+
( )
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:
I
I
V
V
L
ns
PEAK
LIM
IN MAX
SAT
=
+
-
(
)
100
Where V
SAT
= 0.25V (switch saturation voltage). The
current overshoot will be most evident for regulators with
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
PEAK
is kept below 700mA
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 capaci-
tors 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 4.7
F 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
7
LT1945
1945f
PACKAGE DESCRIPTIO
U
APPLICATIO S I FOR ATIO
W
U
U
U
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
V
V
V
R
OUT
1
1 23
1 23
2
2 10
6
=
+
(
)
-
.
.
Set the output voltage for Switcher 2 (positive output
voltage) by choosing the appropriate values for feedback
resistors R1B and R2B (see Figure 1).
R B
R B
V
V
OUT
1
2
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 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
1 2 3 4 5
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
8
LT1945
1945f
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
q
FAX: (408) 434-0507
q
www.linear.com
LINEAR TECHNOLOGY CORPORATION 2001
LT/TP 0802 2K PRINTED IN USA
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LT1944-1
Dual Output 150mA I
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SD
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LT1949/LT1949-1
550mA I
SW
, 600kHz/1.1MHz, High Efficiency
V
IN
= 1.5V to 12V, V
OUT
= 28V, I
Q
= 4.5mA, I
SD
= <25
A,
Step-Up DC/DC Converter
S8, MS8 Packages
LTC3400/LTC3400B 600mA I
SW
, 1.2MHz, Synchronous Step-Up DC/DC Converter
V
IN
= 0.85V to 5V, V
OUT
= 5V, I
Q
= 19
A/300
A, I
SD
= <1
A,
ThinSOT Package
LTC3401
1A I
SW
, 3MHz, Synchronous Step-Up DC/DC Converter
V
IN
= 0.5V to 5V, V
OUT
= 6V, I
Q
= 38
A, I
SD
= <1
A, MS Package
LTC3402
2A I
SW
, 3MHz, Synchronous Step-Up DC/DC Converter
V
IN
= 0.5V to 5V, V
OUT
= 6V, I
Q
= 38
A, I
SD
= <1
A, MS Package
LTC3423
1A I
SW
, 3MHz, Low V
OUT
,
Synchronous Step-Up
V
IN
= 0.5V to 5V, V
OUT
= 6V, I
Q
= 38
A, I
SD
= <1
A, MS Package
DC/DC Converter
LTC3424
2A I
SW
, 3MHz, Low V
OUT
,
Synchronous Step-Up
V
IN
= 0.5V to 5V, V
OUT
= 6V, I
Q
= 38
A, I
SD
= <1
A, MS Package
DC/DC Converter
Dual Output (
32V) Converter
V
IN
SW1
FB2
LT1945
4
3
L1
10
H
D1
D2
SHDN2
24.9k
604k
C2
1
F
32V
5mA
V
IN
2.7V
TO 5V
1945 TA02
GND
7
PGND
9
PGND
6
8
10
SW2
4.7pF
C1
4.7
F
NFB1
2
5
1
SHDN1
L2
10
H
D3
2M
80.6k
C3
1
F
100pF
C1: TAIYO YUDEN JMK212BJ475
C2, C3: TAIYO YUDEN GMK316BJ105
C4: TAIYO YUDEN UMK212BJ104
D1, D2, D3: ZETEX ZHCS400
L1, L2: MURATA LQH3C100
32V
5mA
C4
0.1
F
(408)573-4150
(408)573-4150
(408)573-4150
(631)543-7100
(814)237-1431
U
TYPICAL APPLICATIO
LOAD CURRENT (mA)
0.1
50
EFFICIENCY (%)
55
60
65
70
80
1
10
1945 TA02a
75
+32V OUTPUT
32V OUTPUT
Efficiency at V
IN
= 3.6V
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