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FEATURES
LT1945
Dual Micropower DC/DC
Converter with Positive and
Negative Outputs
U
DESCRIPTIO
■
Generates Well-Regulated Positive and
Negative Outputs
■
Low Quiescent Current:
20µA in Active Mode (per Converter)
<1µA in Shutdown Mode
■
Operates with VIN as Low as 1.2V
■
Low V
■
Uses Small Surface Mount Components
■
High Output Voltage: Up to ±34V
■
Tiny 10-Pin MSOP Package
Switch: 250mV at 300mA
CESAT
U
APPLICATIO S
■
Small TFT LCD Panels
■
Handheld Computers
■
Battery Backup
■
Digital Cameras
U
TYPICAL APPLICATIO
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 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, 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.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Dual Output (+12V, –20V) Converter
L1
V
IN
2.7V
TO 5V
2
C1
4.7µF
C1: TAIYO YUDEN JMK212BJ475
C2, C3: TAIYO YUDEN TMK316BJ105
C4: TAIYO YUDEN EMK107BJ104
D1, D2, D3: ZETEX ZHCS400
L1, L2: MURATA LQH3C100
4
10µH
810
V
IN
SHDN1
LT1945
SHDN2
PGND9PGND
GND
3
7
L2
10µH
SW1
NFB1
FB2
SW2
C4
0.1µF
6
D1
100pF
365k
1
D2
5
4.7pF
D3
24.9k
115k
1M
C2
1µF
C3
1µF
–20V
10mA
12V
20mA
1945 TA01
Efficiency at VIN = 3.6V
90
85
80
75
70
65
EFFICIENCY (%)
60
55
50
0.1
+12V OUTPUT
–20V OUTPUT
1 10 100
LOAD CURRENT (mA)
1945 TA01a
1945f
1
LT1945
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
WW
W
ABSOLUTE AXI U RATI GS
U
UUW
PACKAGE/ORDER I FOR ATIO
(Note 1)
ORDER PART
VIN, 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
T
= 125°C, θJA = 160°C/W
JMAX
NUMBER
LT1945EMS
MS PART MARKING
LTTS
Operating Temperature Range (Note 2) .. – 40°C to 85°C
Storage Temperature Range ................. –65°C to 150°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
Lead Temperature (Soldering, 10 sec).................. 300°C
ELECTRICAL CHARACTERISTICS
temperature range, otherwise specifications are at TA = 25°C. VIN = 1.2V, V
PARAMETER CONDITIONS MIN TYP MAX UNITS
Minimum Input Voltage 1.2 V
Quiescent Current, (per Converter) Not Switching 20 30 µA
= 0V 1 µA
V
SHDN
NFB1 Comparator Trip Point ● –1.205 –1.23 –1.255 V
FB2 Comparator Trip Point ● 1.205 1.23 1.255 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) V
FB2 Pin Bias Current (Note 4) V
Switch Off Time, Switcher 1 (Note 5) 400 ns
Switch Off Time, Switcher 2 (Note 5) V
Switch V
CESAT
Switch Current Limit 250 350 400 mA
SHDN Pin Current V
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
= –1.23V ● 1.3 2 2.9 µA
NFB1
= 1.23V ● 30 80 nA
FB2
> 1V 400 ns
FB2
< 0.6V 1.5 µs
V
FB2
I
= 300mA 250 350 mV
SW
= 1.2V 2 3 µA
SHDN
= 5V 8 12 µA
V
SHDN
The ● denotes the specifications which apply over the full operating
= 1.2V unless otherwise noted.
SHDN
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
2
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.
1945f
UW
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 (µA)
TYPICAL PERFOR A CE CHARACTERISTICS
Switch Saturation Voltage
(V
)
CESAT
0.60
0.55
0.50
0.45
0.40
0.35
0.30
0.25
SWITCH VOLTAGE (V)
0.20
0.15
0.10
–25 0 25 50 75 100
–50
TEMPERATURE (°C)
I
SWITCH
I
SWITCH
= 500mA
= 300mA
1945 G01
FB2 Pin Voltage and
Bias Current
1.25
1.24
1.23
1.22
FEEDBACK VOLTAGE (V)
1.21
1.20
–50
–25 0 25 50 75 100
TEMPERATURE (°C)
VOLTAGE
CURRENT
1945 G02
50
40
BIAS CURRENT (nA)
30
20
10
0
LT1945
NFB1 Pin Voltage and
Bias Current
550
500
450
400
350
SWITCH OFF TIME (ns)
300
250
PI FU CTIO S
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.
Switch Off Time Switch Current Limit
400
VIN = 12V
VIN = 1.2V
VIN = 12V
–50 –25 0 25 50 75 100
TEMPERATURE (°C)
1945 G04
350
300
250
200
150
PEAK CURRENT (mA)
100
50
0
–50 –25 0 25 50 75 100
VIN = 1.2V
TEMPERATURE (°C)
UUU
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.
1945 G05
Quiescent Current
25
VFB = 1.23V
NOT SWITCHING
23
21
19
QUIESCENT CURRENT (µA)
17
15
–50 –25 0 25 50 75 100
VIN = 12V
VIN = 1.2V
TEMPERATURE (°C)
1945 G06
1945f
3
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