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LT1945 Dual Micropower DC/DC Converter with Positive and Negative Outputs
FEATURES
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DESCRIPTIO
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Generates Well-Regulated Positive and Negative Outputs Low Quiescent Current: 20A in Active Mode (per Converter) <1A in Shutdown Mode Operates with VIN as Low as 1.2V Low VCESAT Switch: 250mV at 300mA Uses Small Surface Mount Components High Output Voltage: Up to 34V Tiny 10-Pin MSOP Package
APPLICATIO S
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Small TFT LCD Panels Handheld Computers Battery Backup Digital Cameras
The LT(R)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 20A at no load, which further reduces to 0.5A 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.
TYPICAL APPLICATIO
VIN 2.7V TO 5V L1 10H 8 VIN 2 C1 4.7F 4 SHDN2 3 7 9 SHDN1 LT1945 FB2 6
Dual Output (+12V, -20V) Converter
C4 0.1F D1 -20V 10mA 10 SW1 NFB1 1 D2 5 24.9k C2 1F 100pF 365k
85 +12V OUTPUT 80
EFFICIENCY (%)
90
75 70 65 60
GND PGND PGND SW2
115k C3 1F 12V 20mA
1945 TA01
55 50 0.1 1 10 LOAD CURRENT (mA) 100
1945 TA01a
4.7pF L2 10H C1: TAIYO YUDEN JMK212BJ475 C2, C3: TAIYO YUDEN TMK316BJ105 C4: TAIYO YUDEN EMK107BJ104 D1, D2, D3: ZETEX ZHCS400 L1, L2: MURATA LQH3C100 D3
1M
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Efficiency at VIN = 3.6V
-20V OUTPUT
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LT1945
ABSOLUTE
(Note 1)
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PACKAGE/ORDER I FOR ATIO
TOP VIEW NFB1 SHDN1 GND SHDN2 FB2 1 2 3 4 5 10 9 8 7 6 SW1 PGND VIN PGND SW2
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 ........................................... 125C Operating Temperature Range (Note 2) .. - 40C to 85C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec).................. 300C
ORDER PART NUMBER LT1945EMS MS PART MARKING LTTS
MS PACKAGE 10-LEAD PLASTIC MSOP
TJMAX = 125C, JA = 160C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
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 VCESAT Switch Current Limit SHDN Pin Current SHDN Input Voltage High SHDN Input Voltage Low Switch Leakage Current Switch Off, VSW = 5V VSHDN = 1.2V VSHDN = 5V VFB2 > 1V VFB2 < 0.6V ISW = 300mA 1.2V < VIN < 12V VNFB1 = -1.23V VFB2 = 1.23V Not Switching VSHDN = 0V CONDITIONS
The q denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 1.2V, VSHDN = 1.2V unless otherwise noted.
MIN TYP 20
q q
MAX 1.2 30 1 -1.255 1.255 0.1 2.9 80
UNITS V A A V V mV %/V A nA ns ns s
-1.205 1.205
-1.23 1.23 8 0.05
q q
1.3
2 30 400 400 1.5 250
350 400 3 12 0.25
250
350 2 8
0.9 0.01 5
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 0C to 70C. Specifications over the - 40C to 85C operating
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.
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mV mA A A V V A
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LT1945 TYPICAL PERFOR A CE CHARACTERISTICS
Switch Saturation Voltage (VCESAT)
0.60 0.55 0.50 1.24 40
FEEDBACK VOLTAGE (V)
FEEDBACK VOLTAGE (V)
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
ISWITCH = 500mA
ISWITCH = 300mA
Switch Off Time
550 500 400 350
SWITCH OFF TIME (ns)
450 400 350 300 250 -50 VIN = 1.2V VIN = 12V
PEAK CURRENT (mA)
300 250 200 150 100 50
QUIESCENT CURRENT (A)
-25
0 25 50 TEMPERATURE (C)
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.
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UW
75
1945 G04
FB2 Pin Voltage and Bias Current
1.25 50
-1.25
NFB1 Pin Voltage and Bias Current
5
-1.24 VOLTAGE -1.23
4
BIAS CURRENT (A)
VOLTAGE 1.23 30
BIAS CURRENT (nA)
3
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 Current Limit
VIN = 12V VIN = 1.2V 25
Quiescent Current
VFB = 1.23V NOT SWITCHING 23
21 VIN = 12V 19 VIN = 1.2V 17
100
0 -50
-25
0 25 50 TEMPERATURE (C)
75
100
1945 G05
15 -50
-25
0 25 50 TEMPERATURE (C)
75
100
1945 G06
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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.
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LT1945
BLOCK DIAGRA
VIN C1 VIN
8
2
R5 80k
R6 80k
+ -
Q1 Q2 X10 R3 60k VOUT1 R1 (EXTERNAL) R2 (EXTERNAL) R4 280k
NFB1
1
OPERATIO
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 current in inductors L1 and L2 begins ramping up. Once the switch current reaches 350mA, comparator A2 resets the oneshot, which turns off Q3 for 400ns. L2 continues to deliver current to the output while Q3 is off. Q3 turns on again and
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L1 C3 L2 VOUT1 D1 SHDN1 SW1 C2 VOUT2 C4 SW2 SHDN2 D2 L3 VIN 10 6 4 VIN R6B 40k A1 ENABLE ENABLE A1B R5B 40k
+
VOUT2
-
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)
+
0.12 A2 SWITCHER 1 3 GND 9 PGND PGND 7 0.12
+
42mV
-
-
A2B SWITCHER 2
1945 BD
Figure 1. LT1945 Block Diagram
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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 shortcircuit conditions. When the FB2 pin voltage is less than approximately 600mV, the switch off-time is increased to 1.5s 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.
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LT1945
APPLICATIO S I FOR ATIO
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 recommendations 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 (H) 4.7 10 22 4.7 10 4.7 10 4.7 10 22 MAX DCR () 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 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= VOUT - VIN(MIN) + VD ILIM tOFF
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
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in the above equation. For most regulators with output voltages below 7V, a 4.7H 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 21H inductor is called for with the above equation, but a 10H 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.
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V + VD L = 2 OUT ILIM
tOFF
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:
VOUT + VD L = 2 ILIM
tOFF
where VD = 0.4V (Schottky diode voltage), ILIM = 350mA and tOFF = 400ns.
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LT1945
APPLICATIO S I FOR ATIO
For higher output voltages, the formula above will give large inductance values. For a 2V to 20V converter (typical LCD bias application), a 47H inductor is called for with the above equation, but a 10H or 22H 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 application, 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= VOUT - VIN(MIN) + VD ILIM tOFF
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:
VIN(MAX) - VSAT IPEAK = ILIM + 100ns L
Where VSAT = 0.25V (switch saturation voltage). The current overshoot will be most evident for regulators with
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high input voltages and smaller inductor values. 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 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 efficiency 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 sufficient 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 4.7F input capacitor is sufficient for most applications. Table 2 shows a list of several capacitor manufacturers. Consult the manufacturers for more detailed information and for their entire selection of related parts.
Table 2. Recommended Capacitors
CAPACITOR TYPE Ceramic VENDOR Taiyo Yuden (408) 573-4150 www.t-yuden.com AVX (803) 448-9411 www.avxcorp.com Murata (714) 852-2001 www.murata.com Ceramic Ceramic
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LT1945
APPLICATIO S I FOR ATIO
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. R1 = VOUT -1.23V 1.23V + 2 * 10-6 R2
(
)
Set the output voltage for Switcher 2 (positive output voltage) by choosing the appropriate values for feedback resistors R1B and R2B (see Figure 1).
V R1B = R2B OUT - 1 1.23V
Diode Selection For most LT1945 applications, the Zetex ZHCS400 surface mount Schottky diode (0.4A, 40V) is an ideal choice. Schottky diodes, with their low forward voltage drop and
PACKAGE DESCRIPTIO
0.889 0.127 (.035 .005)
(Reference LTC DWG # 05-08-1661)
5.23 (.206) MIN
3.2 - 3.45 (.126 - .136) 0.254 (.010) GAUGE PLANE
0.50 0.305 0.038 (.0197) (.0120 .0015) BSC TYP RECOMMENDED SOLDER PAD LAYOUT
0.18 (.007) 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 SEATING PLANE 0.17 - 0.27 (.007 - .011) 0.13 0.05 (.005 .002)
MSOP (MS) 0402
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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
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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 Applications section). Adding this small, inexpensive 4.7pF capacitor will greatly reduce the output voltage ripple.
MS Package 10-Lead Plastic MSOP
3.00 0.102 (.118 .004) (NOTE 3) 10 9 8 7 6 0.497 0.076 (.0196 .003) REF DETAIL "A" 0 - 6 TYP 12345 0.53 0.01 (.021 .006) DETAIL "A" 1.10 (.043) MAX 0.86 (.034) REF 4.88 0.10 (.192 .004) 3.00 0.102 (.118 .004) NOTE 4 0.50 (.0197) TYP
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LT1945
TYPICAL APPLICATIO
L1 10H 8 VIN 2 C1 4.7F 4 SHDN2 3 7 9 SHDN1 LT1945 FB2 6 80.6k C3 1F 32V 5mA (408)573-4150 (408)573-4150 (408)573-4150 (631)543-7100 (814)237-1431
1945 TA02
Dual Output (32V) Converter
VIN 2.7V TO 5V C4 0.1F D1 -32V 5mA 10 SW1 NFB1 1 100pF 604k
D2 5 24.9k
GND PGND PGND SW2
EFFICIENCY (%)
4.7pF L2 10H C1: TAIYO YUDEN JMK212BJ475 C2, C3: TAIYO YUDEN GMK316BJ105 C4: TAIYO YUDEN UMK212BJ104 D1, D2, D3: ZETEX ZHCS400 L1, L2: MURATA LQH3C100 D3
RELATED PARTS
PART NUMBER LT1613 LT1615/LT1615-1 LT1940 LT1944 LT1944-1 LT1949/LT1949-1 DESCRIPTION COMMENTS 550mA ISW, 1.4MHz, High Efficiency Step-Up DC/DC Converter VIN = 0.9V to 10V, VOUT = 34V, IQ = 3mA, ISD = <1A, ThinSOT Package 300mA ISW, Constant Off-Time, High Efficiency Step-Up DC/DC Converter Dual Output 1.4A (IOUT), Constant 1.1MHz, High Efficiency Step-Down DC/DC Converter Dual Output 350mA ISW, Constant Off-Time, High Efficiency Step-Up DC/DC Converter Dual Output 150mA ISW, Constant Off-Time, High Efficiency Step-Up DC/DC Converter 550mA ISW, 600kHz/1.1MHz, High Efficiency Step-Up DC/DC Converter VIN = 1.2V to 15V, VOUT = 34V, IQ = 20A, ISD = <1A, ThinSOT Package VIN = 3V to 25V, VOUT = 1.2V, IQ = 2.5mA, ISD = <1A, TSSOP-16E Package VIN = 1.2V to 15V, VOUT = 34V, IQ = 20A, ISD = <1A, MS Package VIN = 1.2V to 15V, VOUT = 34V, IQ = 20A, ISD = <1A, MS Package VIN = 1.5V to 12V, VOUT = 28V, IQ = 4.5mA, ISD = <25A, S8, MS8 Packages VIN = 0.85V to 5V, VOUT = 5V, IQ = 19A/300A, ISD = <1A, ThinSOT Package VIN = 0.5V to 5V, VOUT = 6V, IQ = 38A, ISD = <1A, MS Package VIN = 0.5V to 5V, VOUT = 6V, IQ = 38A, ISD = <1A, MS Package VIN = 0.5V to 5V, VOUT = 6V, IQ = 38A, ISD = <1A, MS Package VIN = 0.5V to 5V, VOUT = 6V, IQ = 38A, ISD = <1A, MS Package
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LTC3400/LTC3400B 600mA ISW, 1.2MHz, Synchronous Step-Up DC/DC Converter LTC3401 LTC3402 LTC3423 LTC3424 1A ISW, 3MHz, Synchronous Step-Up DC/DC Converter 2A ISW, 3MHz, Synchronous Step-Up DC/DC Converter 1A ISW, 3MHz, Low VOUT, Synchronous Step-Up DC/DC Converter 2A ISW, 3MHz, Low VOUT, Synchronous Step-Up DC/DC Converter
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 q FAX: (408) 434-0507
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Efficiency at VIN = 3.6V
80 75
C2 1F
+32V OUTPUT
70 65 60 55 50 0.1
-32V OUTPUT
2M
1 LOAD CURRENT (mA)
10
1945 TA02a
LT/TP 0802 2K * PRINTED IN USA
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2001

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