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LT1945 Datasheet

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Datasheet

LT1945
1
1945fa
TYPICAL APPLICATION
DESCRIPTION
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 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, fi xed off-time control scheme
conserves operating current, resulting in high effi ciency
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 profi le
inductors and capacitors to minimize footprint and cost
in space-conscious portable applications.
Dual Output (+12V, –20V) Converter
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
FEATURES
APPLICATIONS
n Small TFT LCD Panels
n Handheld Computers
n Battery Backup
n Digital Cameras
n Generates Well-Regulated Positive and
Negative Outputs
n Low Quiescent Current:
20μA in Active Mode (per Converter)
<1μA in Shutdown Mode
n Operates with VIN as Low as 1.2V
n Low VCESAT Switch: 250mV at 300mA
n Uses Small Surface Mount Components
n High Output Voltage: Up to ±34V
n Tiny 10-Pin MSOP Package
Effi ciency at VIN = 3.6V
VIN SW1
FB2
LT1945
4
3
L1
10μH D1
D2
SHDN2 24.9k
365k
C2
F
–20V
10mA
VIN
2.7V
TO 5V
1945 TA01
GND
7
PGND
9
PGND
6
810
SW2
4.7pF
C1
4.7μF
C4
0.1μF
NFB1
2
5
1
SHDN1
L2
10μH D3 1M
115k
C3
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
LOAD CURRENT (mA)
0.1
EFFICIENCY (%)
1 10 100
1945 TA01a
90
85
80
75
70
65
60
55
50
+12V OUTPUT
–20V OUTPUT
LT1945
2
1945fa
PIN CONFIGURATION ABSOLUTE MAXIMUM RATINGS
VIN, 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
(Note 1)
1
2
3
4
5
NFB1
SHDN1
GND
SHDN2
FB2
10
9
8
7
6
SW1
PGND
VIN
PGND
SW2
TOP VIEW
MS PACKAGE
10-LEAD PLASTIC MSOP
TJMAX = 125°C, θJA = 160°C/W
ORDER INFORMATION
LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LT1945EMS#PBF LT1945EMS#TRPBF LTTS 10-Lead Plastic MSOP –40°C to 85°C
LT1945IMS#PBF LT1945IMS#TRPBF LTTS 10-Lead Plastic MSOP –40°C to 125°C
LEAD BASED FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LT1945EMS LT1945EMS#TR LTTS 10-Lead Plastic MSOP –40°C to 85°C
LT1945IMS LT1945IMS#TR LTTS 10-Lead Plastic MSOP –40°C to 125°C
Consult LTC Marketing for parts specifi ed with wider operating temperature ranges. *The temperature grade is identifi ed 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 specifi
cations, go to: http://www.linear.com/tapeandreel/
PARAMETER CONDITIONS MIN TYP MAX UNITS
Minimum Input Voltage 1.2 V
Quiescent Current, (per Converter) Not Switching
VSHDN = 0V
20 30
1
μA
μA
NFB1 Comparator Trip Point –40°C < TJ < 85°C
–40°C < TJ < 125°C
–1.205
–1.195
–1.23 –1.255
1.255
V
V
FB2 Comparator Trip Point –40°C < TJ < 85°C
–40°C < TJ < 125°C
1.205
1.195
1.23 1.255
1.255
V
V
FB Comparator Hysteresis 8mV
NFB1, FB2 Voltage Line Regulation 1.2V < VIN < 12V 0.05 0.1 %/V
NFB1 Pin Bias Current (Note 3) VNFB1= –1.23V l1.3 2 2.9 μA
FB2 Pin Bias Current (Note 4) –40°C < TJ < 85°C
–40°C < TJ < 125°C
30 80
300
nA
nA
Switch Off Time, Switcher 1 (Note 5) 400 ns
Switch Off Time, Switcher 2 (Note 5) VFB2 > 1V
VFB2 < 0.6V
400
1.5
ns
μs
Switch VCESAT ISW = 300mA 250 350 mV
Switch Current Limit 250 350 400 mA
ELECTRICAL CHARACTERISTICS
The l denotes the specifi cations which apply over the full operating
temperature range, otherwise specifi cations are at TA = 25°C. VIN = 1.2V, VSHDN = 1.2V unless otherwise noted.
LT1945
3
1945fa
ELECTRICAL CHARACTERISTICS
The l denotes the specifi cations which apply over the full operating
temperature range, otherwise specifi cations are at TA = 25°C. VIN = 1.2V, VSHDN = 1.2V unless otherwise noted.
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 specifi cations
from 0°C to 70°C junction temperature. Specifi cations 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 fl ows out of the NFB1 pin.
Note 4: Bias current fl ows into the FB2 pin.
Note 5: See Figure 1 for Switcher 1 and Switcher 2 locations.
PARAMETER CONDITIONS MIN TYP MAX UNITS
SHDN Pin Current VSHDN = 1.2V
VSHDN = 5V
2
8
3
12
μA
μA
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
TYPICAL PERFORMANCE CHARACTERISTICS
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
ISWITCH = 500mA
ISWITCH = 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 (mA)
TEMPERATURE (°C)
SWITCH OFF TIME (ns)
1945 G04
550
500
450
400
350
300
250
–50 –25 0 25 50 75 100
VIN = 1.2V
VIN = 12V
TEMPERATURE (°C)
PEAK CURRENT (mA)
1945 G05
400
350
300
250
200
150
100
50
0
–50 –25 0 25 50 75 100
VIN = 12V
VIN = 1.2V
TEMPERATURE (°C)
QUIESCENT CURRENT (μA)
1945 G06
25
23
21
19
17
15
–50 –25 0 25 50 75 100
VIN = 12V
VIN = 1.2V
VFB = 1.23V
NOT SWITCHING
Switch Off Time Switch Current Limit Quiescent Current
Switch Saturation Voltage
(VCESAT)
FB2 Pin Voltage and
Bias Current
NFB1 Pin Voltage and
Bias Current
LT1945
4
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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.
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.
BLOCK DIAGRAM
+
+
8
400ns
ONE-SHOT
DRIVER
RESET RESET
ENABLE
42mV
0.12Ω
A2
A1
Q3
9
3
R4
280k
R3
60k
R6
80k
R5
80k
Q2
X10
Q1
1
VIN
NFB1
2SHDN1 10 SW1
PGND
GND
1945 BD
L1 C3
C2
VOUT1
VIN
D1
R2
(EXTERNAL)
R1
(EXTERNAL)
VOUT1
C1
+
+
400ns
ONE-SHOT
ENABLE
42mV
0.12Ω
A2B
A1B
Q3B
7
R4B
140k
R3B
30k
R6B
40k
R5B
40k
Q2B
X10
Q1B
5
VIN
FB2
4
SHDN2
6
SW2
PGND
L3L2
C4
VOUT2 VIN
D2
R2B
(EXTERNAL)
R1B
(EXTERNAL)
VOUT2
SWITCHER 1 SWITCHER 2
DRIVER
Figure 1. LT1945 Block Diagram
OPERATION
The LT1945 uses a constant off-time control scheme
to provide high effi ciencies 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
LT1945
5
1945fa
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 fi nd the correct inductance
value for your design.
Table 1. Recommended Inductors
PART VALUE (μH) MAX DCR (Ω) VENDOR
LQH3C4R7
LQH3C100
LQH3C220
4.7
10
22
0.26
0.30
0.92
Murata
(714) 852-2001
www.murata.com
CD43-4R7
CD43-100
CDRH4D18-4R7
CDRH4D18-100
4.7
10
4.7
10
0.11
0.18
0.16
0.20
Sumida
(847) 956-0666
www.sumida.com
DO1608-472
DO1608-103
DO1608-223
4.7
10
22
0.09
0.16
0.37
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 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.
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
VV V
It
OUT IN MIN D
LIM OFF
=−+
()
where VD = 0.4V (Schottky diode voltage), ILIM = 350mA
and tOFF = 400ns; for designs with varying VIN such as
battery powered applications, use the minimum VIN 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.
LVV
It
OUT D
LIM OFF
=+
2
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.
LT1945
6
1945fa
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:
II VV
Lns
PEAK LIM
IN MAX SAT
=+
() 100
Where VSAT = 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 benefi cial 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
effi ciency will suffer. Best results will be obtained when IPEAK
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 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 suffi cient 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 4.7μF 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
APPLICATIONS INFORMATION
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:
LVV
It
OUT D
LIM OFF
=+
2
where VD = 0.4V (Schottky diode voltage), ILIM = 350mA
and tOFF = 400ns.
For higher output voltages, the formula above will give
large inductance values. For a 2V to 20V converter (typical
LCD bias application), a 47μH inductor is called for with the
above equation, but a 10μH or 22μH 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 ap-
plication, 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 VIN such as battery powered applications, use the
minimum VIN value in the equation below.
L
VV V
It
OUT IN MIN D
LIM OFF
=−+
()
LT1945
7
1945fa
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 representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
APPLICATIONS INFORMATION
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
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.
RVV
V
R
OUT
1123
123
22106
=
+
()
–.
.
Set the output voltage for Switcher 2 (positive output
voltage) by choosing the appropriate values for feedback
resistors R1B and R2B (see Figure 1).
RR VOUT
12
123 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 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 LT1945 (see the circuits in the Typical Ap-
plications section). Adding this small, inexpensive 4.7pF
capacitor will greatly reduce the output voltage ripple.
PACKAGE DESCRIPTION
MS Package
10-Lead Plastic MSOP
(Reference LTC DWG # 05-08-1661)
MSOP (MS) 0402
0.53 p 0.01
(.021 p .006)
SEATING
PLANE
0.18
(.007)
1.10
(.043)
MAX
0.17 –0.27
(.007 – .011)
0.13 p 0.05
(.005 p .002)
0.86
(.034)
REF
0.50
(.0197)
TYP
12345
4.88 p 0.10
(.192 p .004)
0.497 p 0.076
(.0196 p .003)
REF
8910 76
3.00 p 0.102
(.118 p .004)
(NOTE 3)
3.00 p 0.102
(.118 p .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) 0o – 6o TYP
DETAIL “A”
DETAIL “A”
GAUGE PLANE
5.23
(.206)
MIN
3.2 – 3.45
(.126 – .136)
0.889 p 0.127
(.035 p .005)
RECOMMENDED SOLDER PAD LAYOUT
0.305 p 0.038
(.0120 p .0015)
TYP
0.50
(.0197)
BSC
LT1945
8
1945fa
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2001
LT 1208 REV A • PRINTED IN USA
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DC/DC Converter
VIN = 0.5V to 5V, VOUT = 6V, IQ = 38μA, ISD = <1μA, MS Package
LTC3424 2A ISW, 3MHz, Low VOUT, Synchronous Step-Up
DC/DC Converter
VIN = 0.5V to 5V, VOUT = 6V, IQ = 38μA, ISD = <1μA, MS Package
TYPICAL APPLICATIONS
VIN SW1
FB2
LT1945
4
3
L1
10μH D1
D2
SHDN2 24.9k
604k
C2
F
–32V
5mA
VIN
2.7V
TO 5V
1945 TA02
GND
7
PGND
9
PGND
6
810
SW2
4.7pF
C1
4.7μF
NFB1
2
5
1
SHDN1
L2
10μH D3
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
0.1μF
(408)573-4150
(408)573-4150
(408)573-4150
(631)543-7100
(814)237-1431
2M
80.6k
C3
F
Dual Output (±32V) Converter
LOAD CURRENT (mA)
0.1
50
EFFICIENCY (%)
55
60
65
70
80
110
1945 TA02a
75 +32V OUTPUT
–32V OUTPUT
Effi ciency at VIN = 3.6V

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