LT1944-1
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
Switches: 85mV at 70mA
Uses Small Surface Mount Components
High Output Voltage: Up to 34V
Tiny 10-Pin MSOP Package
APPLICATIO S
s
s
s
s
Small TFT LCD Panels
Handheld Computers
Battery Backup
Digital Cameras
The LT
®
1944-1 is a dual micropower step-up DC/DC
converter in a 10-pin MSOP package. One converter is
designed with a 100mA current limit and a 400ns off-time;
the other with a 175mA current limit and a 1.5µs off-time.
The 1.5µs off-time converter is ideal for generating an
output voltage that is close to the input voltage (i.e. a Li-
Ion to 5V converter, or a two-cell to 3.3V converter). With
an input voltage range of 1.2V to 15V, the LT1944-1 is 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, result-
ing in high efficiency over a broad range of load current.
Tiny, low profile inductors and capacitors can be used to
minimize footprint and cost in space-conscious portable
applications.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
V
IN
2.7V
TO 4.2V
L1
22µH
8
V
IN
4
C1
4.7µF
2
SHDN1
3
7
9
SHDN2
LT1944-1
6
Triple Output Power Supply (5V, 15V, –10V) for LCD Displays
D1
5V
40mA
SW2
FB2
5
EFFICIENCY (%)
C2
4.7µF
FB1
10
178k
C3
1µF
15V
2.5mA
5V
C5
0.1µF
D4
1944-1 TA01
90
85
80
75
70
65
60
55
50
0.1
1
10
LOAD CURRENT (mA)
100
1944-1 TA01a
4.7pF
1M
1
324k
GND PGND PGND SW1
4.7pF
L2
22µH
C1, C2: TAIYO YUDEN JMK212BJ475
C3, C4: TAIYO YUDEN EMK212BJ105
C5: TAIYO YUDEN EMK107BJ104
D1, D2, D3, D4: CENTRAL SEMI CMDSH3
L1, L2: MURATA LQH3C220
D2
D3
2M
C4
1µF
–10V
1mA
U
5V Output Efficiency
V
IN
= 4.2V
V
IN
= 2.7V
U
U
1
LT1944-1
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
LT1944-1EMS
MS10 PART
MARKING
LTTU
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, Switcher 1 (Note 4)
Switch Off Time, Switcher 2 (Note 4)
Switch V
CESAT
Switch Current Limit, Switcher 1
Switch Current Limit, Switcher 2
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
V
FB
> 1V
V
FB
< 0.6V
I
SW
= 70mA
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
µs
µs
1.205
1.23
8
0.05
q
30
400
1.5
1.5
1.5
85
65
130
100
175
2
8
0.9
120
125
225
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 LT1944-1E 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.
Note 4:
See Figure 1 for Switcher 1 and Switcher 2 locations.
2
U
mV
mA
mA
µA
µA
V
V
µA
W
U
U
W W
W
LT1944-1
TYPICAL PERFOR A CE CHARACTERISTICS
Switch Saturation Voltage
(V
CESAT
)
0.15
0.13
0.10
0.08
0.05
0.03
0
–50
I
SWITCH
= 100mA
I
SWITCH
= 70mA
FEEDBACK VOLTAGE (V)
VOLTAGE
1.23
30
QUIESCENT CURRENT (µA)
SWITCH VOLTAGE (V)
–25
0
25
50
TEMPERATURE (°C)
Switch Off Time
2000
1800
1600
SWITCH OFF TIME (ns)
1400
1200
1000
800
600
400
200
0
–50
–25
25
50
0
TEMPERATURE (°C)
75
100
SWITCHER 1
SWITCHER 2
PEAK CURRENT (mA)
175
V
IN
= 1.2V
150
125
V
IN
= 12V
100
V
IN
= 1.2V
75
50
–50
SHUTDOWN PIN CURRENT (µA)
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
1944-1 G01
1944-1 G04
Feedback Pin Voltage and
Bias Current
1.25
50
25
Quiescent Current
V
FB
= 1.23V
NOT SWITCHING
1.24
40
BIAS CURRENT (nA)
23
21
V
IN
= 12V
19
V
IN
= 1.2V
17
1.22
CURRENT
20
1.21
10
100
1.20
–50
–25
0
25
50
TEMPERATURE (°C)
75
0
100
15
–50
–25
0
25
50
TEMPERATURE (°C)
75
100
1944-1 G02
1944-1 G03
Switch Current Limit
200
V
IN
= 12V
SWITCHER 2
20
25
Shutdown Pin Current
15
25°C
10
100°C
5
SWITCHER 1
0
–25
0
25
50
TEMPERATURE (°C)
75
100
0
5
10
SHUTDOWN PIN VOLTAGE (V)
15
1944-1 G03
1944-1 G05
U
U
U
3
LT1944-1
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
SWITCHER 1
GND
3
9
PGND
PGND
7
400ns
ONE-SHOT
DRIVER
RESET
Q3
Q3B
DRIVER
RESET
1.5µs
ONE-SHOT
OPERATIO
The LT1944-1 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 100mA, 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
current ramps down. Q3 turns on again and 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
21mV
5
FB2
R1B
(EXTERNAL)
R2B
(EXTERNAL)
+
0.12Ω
0.12Ω
+
12mV
–
–
A2B
SWITCHER 2
1944-1 BD
Figure 1. LT1944-1 Block Diagram
U
current ramps back up to 100mA, 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
LT1944-1 contains additional circuitry to provide protec-
tion 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 70mA (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, but with a 175mA current limit and an off-time of
1.5µs. With this longer off-time, switcher 2 is ideal for very
low duty cycle applications (i.e. Li-Ion to 5V boost
converters).
LT1944-1
APPLICATIO S I FOR ATIO
Choosing an Inductor
Several recommended inductors that work well with the
LT1944-1 are listed in Table 1, although there are many
other manufacturers and devices that can be used. Con-
sult 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 (
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 LT1944-1
(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 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
= 100mA
(or 175mA) and t
OFF
= 400ns (or 1.5µs); for designs with
varying V
IN
such as battery powered applications, use the
minimum V
IN
value in the above equation. For most
U
systems with output voltages below 7V, a 10µH 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 74µH inductor is called for with
the above equation, but a 22µH 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 LT1944-1.
As for the boost inductor selection, a larger or smaller
value can be used.
V
+
V
D
L
=
2
OUT
I
LIM
t
OFF
W
U
U
Current Limit Overshoot
For the constant off-time control scheme of the LT1944-1,
the power switch is turned off only after the 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
