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 ap-
plications. 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.
L,
LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
n
n
n
n
n
Generates Well-Regulated Positive and
Negative Outputs
Low Quiescent Current:
20μA in Active Mode (per Converter)
<1μA 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
APPLICATIONS
n
n
n
n
Small TFT LCD Panels
Handheld Computers
Battery Backup
Digital Cameras
TYPICAL APPLICATION
Dual Output (+12V, –20V) Converter
V
IN
2.7V
TO 5V
2
C1
4.7μF
4
SHDN2
3
7
9
L1
10μH
8
V
IN
SHDN1
LT1945
FB2
6
115k
C3
1μF
1M
12V
20mA
1945 TA01
Efficiency at V
IN
= 3.6V
90
85
+12V OUTPUT
80
EFFICIENCY (%)
–20V OUTPUT
75
70
65
60
55
50
0.1
1
10
LOAD CURRENT (mA)
100
1945 TA01a
C4
0.1μF
D1
–20V
10mA
10
SW1
NFB1
1
D2
5
24.9k
C2
1μF
100pF
365k
GND PGND PGND SW2
4.7pF
L2
10μH
C1: TAIYO YUDEN JMK212BJ475
C2, C3: TAIYO YUDEN TMK316BJ105
C4: TAIYO YUDEN EMK107BJ104
D1, D2, D3: ZETEX ZHCS400
L1, L2: MURATA LQH3C100
D3
1945fa
1
LT1945
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW
NFB1
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
NFB1 Voltage ............................................................–3V
FB2 Voltage ............................................................... VIN
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
MS PACKAGE
10-LEAD PLASTIC MSOP
T
JMAX
= 125°C,
θ
JA
= 160°C/W
ORDER INFORMATION
LEAD FREE FINISH
LT1945EMS#PBF
LT1945IMS#PBF
LEAD BASED FINISH
LT1945EMS
LT1945IMS
TAPE AND REEL
LT1945EMS#TRPBF
LT1945IMS#TRPBF
TAPE AND REEL
LT1945EMS#TR
LT1945IMS#TR
PART MARKING*
LTTS
LTTS
PART MARKING*
LTTS
LTTS
PACKAGE DESCRIPTION
10-Lead Plastic MSOP
10-Lead Plastic MSOP
PACKAGE DESCRIPTION
10-Lead Plastic MSOP
10-Lead Plastic MSOP
TEMPERATURE RANGE
–40°C to 85°C
–40°C to 125°C
TEMPERATURE RANGE
–40°C to 85°C
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
For more information on lead free part marking, go to:
http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to:
http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
PARAMETER
Minimum Input Voltage
Quiescent Current, (per Converter)
NFB1 Comparator Trip Point
FB2 Comparator Trip Point
FB Comparator Hysteresis
NFB1, FB2 Voltage Line Regulation
NFB1 Pin Bias Current (Note 3)
FB2 Pin Bias Current (Note 4)
Switch Off Time, Switcher 1 (Note 5)
Switch Off Time, Switcher 2 (Note 5)
Switch V
CESAT
Switch Current Limit
V
FB2
> 1V
V
FB2
< 0.6V
I
SW
= 300mA
1.2V < V
IN
< 12V
V
NFB1
= –1.23V
–40°C < T
J
< 85°C
–40°C < T
J
< 125°C
Not Switching
V
SHDN
= 0V
–40°C < T
J
< 85°C
–40°C < T
J
< 125°C
–40°C < T
J
< 85°C
–40°C < T
J
< 125°C
CONDITIONS
The
l
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
–1.205
–1.195
1.205
1.195
–1.23
1.23
8
0.05
l
MAX
1.2
30
1
–1.255
1.255
1.255
1.255
0.1
2.9
80
300
UNITS
V
μA
μA
V
V
V
V
mV
%/V
μA
nA
nA
ns
ns
μs
1.3
2
30
400
400
1.5
250
350
400
mV
mA
1945fa
250
350
2
LT1945
ELECTRICAL CHARACTERISTICS
PARAMETER
SHDN
Pin Current
SHDN
Input Voltage High
SHDN
Input Voltage Low
Switch Leakage Current
Switch Off, V
SW
= 5V
0.01
CONDITIONS
V
SHDN
= 1.2V
V
SHDN
= 5V
0.9
0.25
5
The
l
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
2
8
MAX
3
12
UNITS
μA
μA
V
V
μA
Note 1:
Stresses beyond those listed under Absolute Maximum Ratings may
cause permanent damage to the device. Exposure to any Absolute Maximum
Rating condition for extended periods may affect device reliability and lifetime.
Note 2:
The LT1945E is guaranteed to meet performance specifications
from 0°C to 70°C junction temperature. Specifications over the –40°C
to 85°C operating junction temperature range are assured by design,
characterization and correlation with statistical process controls. The
LT1945I is guaranteed over the full –40°C to 125°C operating junction
temperature range.
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.
TYPICAL PERFORMANCE CHARACTERISTICS
Switch Saturation Voltage
(VCESAT)
0.60
0.55
0.50
SWITCH VOLTAGE (V)
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
1945 G01
FB2 Pin Voltage and
Bias Current
1.25
50
–1.25
NFB1 Pin Voltage and
Bias Current
5
1.24
FEEDBACK VOLTAGE (V)
I
SWITCH
= 500mA
VOLTAGE
40
FEEDBACK VOLTAGE (V)
BIAS CURRENT (nA)
–1.24
VOLTAGE
–1.23
4
BIAS CURRENT (mA)
1.23
30
3
I
SWITCH
= 300mA
1.22
CURRENT
20
–1.22
CURRENT
–1.21
2
1.21
10
1
1.20
–50
–25
0
25
50
TEMPERATURE (°C)
75
0
100
1945 G02
–1.20
–50
–25
0
25
50
TEMPERATURE (°C)
75
0
100
1945 G03
Switch Off Time
550
500
SWITCH OFF TIME (ns)
450
400
350
300
250
–50
V
IN
= 1.2V
V
IN
= 12V
PEAK CURRENT (mA)
400
350
300
250
200
150
100
50
–25
0
25
50
TEMPERATURE (°C)
75
100
1945 G04
Switch Current Limit
V
IN
= 12V
QUIESCENT CURRENT (μA)
V
IN
= 1.2V
25
Quiescent Current
V
FB
= 1.23V
NOT SWITCHING
23
21
V
IN
= 12V
19
V
IN
= 1.2V
17
0
–50
–25
0
25
50
TEMPERATURE (°C)
75
100
1945 G05
15
–50
–25
0
25
50
TEMPERATURE (°C)
75
100
1945 G06
1945fa
3
LT1945
PIN FUNCTIONS
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.
BLOCK DIAGRAM
L1
V
IN
C1
V
IN
SHDN1
SW1
D1
C3
L2
V
OUT1
C2
V
OUT2
C4
SW2
SHDN2
D2
L3
V
IN
8
2
10
6
4
V
IN
R5
80k
R6
80k
R6B
40k
R5B
40k
+
–
Q1
Q2
X10
R3
60k
V
OUT1
R1
(EXTERNAL)
R2
(EXTERNAL)
R4
280k
A1
ENABLE
ENABLE
A1B
+
V
OUT2
–
Q1B
400ns
ONE-SHOT
DRIVER
RESET
Q3
Q3B
DRIVER
RESET
R3B
30k
R4B
140k
42mV
400ns
ONE-SHOT
Q2B
X10
5
FB2
R1B
(EXTERNAL)
R2B
(EXTERNAL)
+
0.12Ω
A2
NFB1
1
SWITCHER 1
3
GND
9
PGND
PGND
7
0.12Ω
+
42mV
–
–
A2B
SWITCHER 2
1945 BD
Figure 1. LT1945 Block Diagram
OPERATION
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
1945fa
4
LT1945
OPERATION
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 operation
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 approxi-
mately 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.
APPLICATIONS INFORMATION
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
LQH3C4R7
LQH3C100
LQH3C220
CD43-4R7
CD43-100
CDRH4D18-4R7
CDRH4D18-100
DO1608-472
DO1608-103
DO1608-223
VALUE (μH)
4.7
10
22
4.7
10
4.7
10
4.7
10
22
MAX DCR (Ω)
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
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 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.
⎛
V
+
V
⎞
L
=
2
⎜
OUT D
⎟
t
OFF
⎝
I
LIM
⎠
1945fa
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.
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