LT1944
Dual Micropower Step-Up
DC/DC Converter
FEATURES
s
DESCRIPTIO
s
s
s
s
s
Low Quiescent Current:
20
µ
A in Active Mode
<1
µ
A in Shutdown Mode
Operates with V
IN
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
APPLICATIO S
s
s
s
s
LCD Bias
Handheld Computers
Battery Backup
Digital Cameras
The LT
®
1944 is a dual micropower step-up DC/DC con-
verter 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 quies-
cent 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 capaci-
tors to minimize footprint and cost in space-conscious
portable applications.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
L1
4.7µH
8
V
IN
2
C1
4.7µF
4
SHDN2
3
7
9
SHDN1
LT1944
FB2
6
5
10
Dual Output (5V, 30V) Boost Converter
90
D1
5V
80mA
EFFICIENCY (%)
V
IN
2.7V
TO 4.2V
85
80
V
IN
= 4.2V
V
IN
= 2.7V
SW1
FB1
1
4.7pF
1M
75
70
65
60
55
C2
10µF
324k
GND PGND PGND SW2
86.6k
C3
1µF
30V
8mA
1944 TA01
50
0.1
4.7pF
L2
10µH
C1: TAIYO YUDEN JMK212BJ475
C2: TAIYO YUDEN JMK316BJ106
C3: TAIYO YUDEN GMK316BJ105
D1, D2: ON SEMI MBR0540
L1: MURATA LQH3C4R7
L2: MURATA LQH3C100
D2
2M
U
5V Output Efficiency
1
10
LOAD CURRENT (mA)
100
1944 TA01a
U
U
1
LT1944
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
FB1
SHDN1
GND
SHDN2
FB2
1
2
3
4
5
10
9
8
7
6
SW1
PGND
V
IN
PGND
SW2
V
IN
, SHDN1, SHDN2 Voltage ................................... 15V
SW1, SW2 Voltage .................................................. 36V
FB1, FB2 Voltage .......................................................V
IN
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
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
LT1944EMS
MS10 PART
MARKING
LTTR
MS10 PACKAGE
10-LEAD PLASTIC MSOP
T
JMAX
= 125°C,
θ
JA
= 160°C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
PARAMETER
Minimum Input Voltage
Quiescent Current, Each Switcher
FB Comparator Trip Point
FB Comparator Hysteresis
FB Voltage Line Regulation
FB Pin Bias Current (Note 3)
Switch Off Time
Switch V
CESAT
Switch Current Limit
SHDN Pin Current
SHDN Input Voltage High
SHDN Input Voltage Low
Switch Leakage Current
Switch Off, V
SW
= 5V
V
SHDN
= 1.2V
V
SHDN
= 5V
1.2V < V
IN
< 12V
V
FB
= 1.23V
V
FB
> 1V
V
FB
< 0.6V
I
SW
= 300mA
Not Switching
V
SHDN
= 0V
CONDITIONS
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 1.2V, V
SHDN
= 1.2V unless otherwise noted.
MIN
TYP
20
q
MAX
1.2
30
1
1.255
0.1
80
UNITS
V
µA
µA
V
mV
%/V
nA
ns
µs
1.205
1.23
8
0.05
q
30
400
1.5
250
250
350
2
8
0.9
350
400
3
12
0.25
0.01
5
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.
2
U
mV
mA
µA
µA
V
V
µA
W
U
U
W W
W
LT1944
TYPICAL PERFOR A CE CHARACTERISTICS
Switch Saturation Voltage
(V
CESAT
)
0.60
0.55
QUIESCENT CURRENT (µA)
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
–50
–25
0
25
50
TEMPERATURE (°C)
75
100
1944 G01
FEEDBACK VOLTAGE (V)
SWITCH VOLTAGE (V)
I
SWITCH
= 500mA
I
SWITCH
= 300mA
Switch Off Time
550
500
SWITCH OFF TIME (ns)
V
IN
= 1.2V
PEAK CURRENT (mA)
SHUTDOWN PIN CURRENT (µA)
450
400
350
300
250
–50
V
IN
= 1.2V
V
IN
= 12V
–25
0
25
50
TEMPERATURE (°C)
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.
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.
U W
75
Feedback Pin Voltage and
Bias Current
1.25
50
25
Quiescent Current
V
FB
= 1.23V
NOT SWITCHING
1.24
VOLTAGE
1.23
40
BIAS CURRENT (nA)
23
30
21
V
IN
= 12V
19
V
IN
= 1.2V
17
1.22
CURRENT
20
1.21
10
1.20
–50
–25
0
25
50
TEMPERATURE (°C)
75
0
100
1944 G02
15
–50
–25
0
25
50
TEMPERATURE (°C)
75
100
1944 G03
Switch Current Limit
400
350
300
250
200
150
100
50
100
1944 G04
Shutdown Pin Current
25
V
IN
= 12V
20
15
25°C
10
100°C
5
0
–50
0
–25
0
25
50
TEMPERATURE (°C)
75
100
1944 G05
0
5
10
SHUTDOWN PIN VOLTAGE (V)
15
1944 G03
U
U
U
3
LT1944
BLOCK DIAGRA
V
IN
C1
V
IN
8
2
R5
40k
R6
40k
+
V
OUT1
–
R1
(EXTERNAL)
R2
(EXTERNAL)
FB1
1
Q1
Q2
X10
R3
30k
R4
140k
A2
400ns
ONE-SHOT
DRIVER
RESET
Q3
Q3B
DRIVER
RESET
400ns
ONE-SHOT
GND
3
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 cir-
cuitry. 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 one-
shot, which turns off Q3 for 400ns. L1 then delivers
current to the output through diode D1 as the inductor
4
W
L1
D1
V
OUT1
C2
SHDN1
SW1
V
OUT2
C3
SW2
SHDN2
D2
L2
V
IN
10
6
4
V
IN
R6B
40k
A1
ENABLE
ENABLE
A1B
R5B
40k
+
V
OUT2
–
Q1B
Q2B
X10
R3B
30k
R4B
140k
42mV
5
FB2
R1B
(EXTERNAL)
R2B
(EXTERNAL)
+
0.12Ω
0.12Ω
+
42mV
–
–
A2B
9
PGND
PGND
7
1944 BD
Figure 1. LT1944 Block Diagram
U
current ramps down. Q3 turns on again and the inductor
current ramps back up to 350mA, then A2 resets the one-
shot, 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.
LT1944
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 recommenda-
tions in the next few sections to find the correct inductance
value for your design.
Table 1. Recommended Inductors
PART
VALUE (
µ
H)
MAX DCR (
Ω
)
LQH3C4R7
LQH3C100
LQH3C220
CD43-4R7
CD43-100
CDRH4D18-4R7
CDRH4D18-100
DO1608-472
DO1608-103
DO1608-223
4.7
10
22
4.7
10
4.7
10
4.7
10
22
0.26
0.30
0.92
0.11
0.18
0.16
0.20
0.09
0.16
0.37
VENDOR
Murata
(714) 852-2001
www.murata.com
Sumida
(847) 956-0666
www.sumida.com
Coilcraft
(847) 639-6400
www.coilcraft.com
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 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
OUT
−
V
IN
(
MIN
)
+
V
D
I
LIM
t
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
in the above equation. For most systems with output
U
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.
V
+
V
L
=
2
OUT D
I
LIM
t
OFF
W
U
U
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:
I
PEAK
V
IN(MAX)
−
V
SAT
=
I
LIM
+
100ns
L
Where V
SAT
= 0.25V (switch saturation voltage). The
current overshoot will be most evident for systems with
high input voltages and for systems where smaller induc-
tor 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 over-
shoot can be quite high. Although it is internally current
5
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