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LT1944 Dual Micropower Step-Up DC/DC Converter
FEATURES
s
DESCRIPTIO
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Low Quiescent Current: 20A in Active Mode <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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LCD Bias Handheld Computers Battery Backup Digital Cameras
The LT(R)1944 is a dual micropower step-up 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 LT1944 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 in a simple boost topology without the use of costly transformers. The LT1944'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
L1 4.7H 8 VIN 2 C1 4.7F 4 SHDN2 3 7 9 SHDN1 LT1944 FB2 6 5 10
Dual Output (5V, 30V) Boost Converter
90 D1 5V 80mA
EFFICIENCY (%)
VIN 2.7V TO 4.2V
85 80 VIN = 4.2V VIN = 2.7V
SW1 FB1 1
4.7pF
1M
75 70 65 60 55
C2 10F 324k
GND PGND PGND SW2
86.6k C3 1F 30V 8mA
1944 TA01
50 0.1
4.7pF L2 10H C1: TAIYO YUDEN JMK212BJ475 C2: TAIYO YUDEN JMK316BJ106 C3: TAIYO YUDEN GMK316BJ105 D1, D2: ON SEMI MBR0540 L1: MURATA LQH3C4R7 L2: MURATA LQH3C100 D2
2M
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5V Output Efficiency
1 10 LOAD CURRENT (mA) 100
1944 TA01a
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LT1944
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 VIN PGND SW2
VIN, SHDN1, SHDN2 Voltage ................................... 15V SW1, SW2 Voltage .................................................. 36V FB1, FB2 Voltage .......................................................VIN Current into FB1, FB2 Pins ..................................... 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 LT1944EMS MS10 PART MARKING LTTR
MS10 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, Each Switcher FB Comparator Trip Point FB Comparator Hysteresis FB Voltage Line Regulation FB Pin Bias Current (Note 3) Switch Off Time 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 1.2V < VIN < 12V VFB = 1.23V VFB > 1V VFB < 0.6V ISW = 300mA 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
MAX 1.2 30 1 1.255 0.1 80
UNITS V A A V mV %/V nA ns s
1.205
1.23 8 0.05
q
30 400 1.5 250 250 350 2 8 0.9
350 400 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 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 into the FB pin.
2
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mV mA A A V V A
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LT1944 TYPICAL PERFOR A CE CHARACTERISTICS
Switch Saturation Voltage (VCESAT)
0.60 0.55 QUIESCENT CURRENT (A) 0.50 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
1944 G01
FEEDBACK VOLTAGE (V)
SWITCH VOLTAGE (V)
ISWITCH = 500mA
ISWITCH = 300mA
Switch Off Time
550 500
SWITCH OFF TIME (ns)
VIN = 1.2V
PEAK CURRENT (mA)
SHUTDOWN PIN CURRENT (A)
450 400 350 300 250 -50 VIN = 1.2V VIN = 12V
-25
0 25 50 TEMPERATURE (C)
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. 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.
UW
75
Feedback Pin Voltage and Bias Current
1.25 50
25
Quiescent Current
VFB = 1.23V NOT SWITCHING
1.24 VOLTAGE 1.23
40
BIAS CURRENT (nA)
23
30
21 VIN = 12V 19 VIN = 1.2V 17
1.22
CURRENT
20
1.21
10
1.20 -50
-25
0 25 50 TEMPERATURE (C)
75
0 100
1944 G02
15 -50
-25
0 25 50 TEMPERATURE (C)
75
100
1944 G03
Switch Current Limit
400 350 300 250 200 150 100 50 100
1944 G04
Shutdown Pin Current
25
VIN = 12V
20
15 25C 10 100C 5
0 -50
0 -25 0 25 50 TEMPERATURE (C) 75 100
1944 G05
0
5 10 SHUTDOWN PIN VOLTAGE (V)
15
1944 G03
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3
LT1944
BLOCK DIAGRA
VIN C1 VIN
8
2
R5 40k
R6 40k
+
VOUT1
-
R1 (EXTERNAL) R2 (EXTERNAL) FB1 1 Q1 Q2 X10 R3 30k R4 140k A2 400ns ONE-SHOT DRIVER RESET Q3 Q3B DRIVER RESET 400ns ONE-SHOT
GND
3
OPERATIO
The LT1944 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 circuitry. 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 350mA, comparator A2 resets the oneshot, which turns off Q3 for 400ns. L1 then delivers current to the output through diode D1 as the inductor
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L1 D1 VOUT1 C2 SHDN1 SW1 VOUT2 C3 SW2 SHDN2 D2 L2 VIN 10 6 4 VIN R6B 40k A1 ENABLE ENABLE A1B R5B 40k
+
VOUT2
-
Q1B Q2B X10 R3B 30k R4B 140k 42mV 5 FB2 R1B (EXTERNAL) R2B (EXTERNAL)
+
0.12 0.12
+
42mV
-
-
A2B
9
PGND
PGND
7
1944 BD
Figure 1. LT1944 Block Diagram
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current ramps down. Q3 turns on again and the inductor current ramps back up to 350mA, then A2 resets the oneshot, 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 contains additional circuitry to provide protection 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 1.5s 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. The second switching regulator operates in the same manner.
LT1944
APPLICATIO S I FOR ATIO
Choosing an Inductor
Several recommended inductors that work well with the LT1944 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 LT1944 (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 in the above equation. For most systems with output
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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 LT1944. As for the boost inductor selection, a larger or smaller value can be used.
V +V L = 2 OUT D ILIM tOFF
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Current Limit Overshoot For the constant off-time control scheme of the LT1944, 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 systems with high input voltages and for systems where smaller inductor 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 overshoot can be quite high. Although it is internally current
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LT1944
APPLICATIO S I FOR ATIO
limited to 350mA, the power switch of the LT1944 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 LT1944. Capacitor Selection Low ESR (Equivalent Series Resistance) capacitors should be used at the output to minimize the output ripple voltage. Multilayer ceramic capacitors are the best choice, as they have a very low ESR and are available in very small packages. Their small size makes them a good companion to the LT1944'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 LT1944. 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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Setting the Output Voltage Set the output voltage for each switching regulator by choosing the appropriate values for feedback resistors R1 and R2 (see Figure 1). V R1 = R2 OUT - 1 1.23V Diode Selection For most LT1944 applications, the Motorola MBR0520 surface mount Schottky diode (0.5A, 20V) is an ideal choice. Schottky diodes, with their low forward voltage drop and fast switching speed, are the best match for the LT1944. For higher output voltage applications the 30V MBR0530 or 40V MBR0540 can 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 LT1944 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 LT1944 (see the circuits in the Typical Applications section). Adding this small, inexpensive 4.7pF capacitor will greatly reduce the output voltage ripple.
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LT1944
TYPICAL APPLICATIO S
2-Cell Dual Output (3.3V, 5V) Boost Converter
VIN 1.8V TO 3V L1 4.7H 8 VIN 2 C1 4.7F 4 SHDN2 3 7 9 SHDN1 LT1944 FB2 6 604k C1: TAIYO YUDEN JMK212BJ475 C2, C3: TAIYO YUDEN JMK316BJ106 D1, D2: ON SEMI MBR0520 L1, L2: MURATA LQH3C4R7 (408) 573-4150 (408) 573-4150 (800) 282-9855 (814) 237-1431 C3 10F 3.3V 80mA
1944 TA02
2-Cell to 5V Efficiency
90 85 80 VIN = 3V VIN = 1.8V
EFFICIENCY (%)
75 70 65 60 55 50 0.1
EFFICIENCY (%)
1 10 LOAD CURRENT (mA)
PACKAGE DESCRIPTIO
0.007 (0.18) 0.021 0.006 (0.53 0.015)
0 - 6 TYP SEATING PLANE 0.007 - 0.011 (0.17 - 0.27) 0.193 0.006 (4.90 0.15) 0.118 0.004** (3.00 0.102)
* DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE ** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
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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D1 5V 40mA 10 SW1 FB1 1 C2 10F 5 324k 4.7pF 1M
GND PGND PGND SW2
4.7pF L2 4.7H D2
1M
2-Cell to 3.3V Efficiency
90 85 80 75 70 65 60 55
100
1944 TA02a
VIN = 3V
VIN = 1.8V
50 0.1
1 10 LOAD CURRENT (mA)
100
1944 TA02b
MS10 Package 10-Lead Plastic MSOP
(LTC DWG # 05-08-1661)
0.043 (1.10) MAX 0.034 (0.86) REF 0.118 0.004* (3.00 0.102)
10 9 8 7 6
0.0197 (0.50) BSC
0.005 0.002 (0.13 0.05)
MSOP (MS10) 1100
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LT1944
TYPICAL APPLICATIO U
Four Output Power Supply for Color LCD Displays
Q1 C6 2.2F 140k C7 0.1F Q2 -6.5V 500A D3A D3B D2B C3 0.1F VIN 2.7V TO 4.2V L1 10H 8 VIN 2 C1 4.7F 4 SHDN2 3 7 9 SHDN1 LT1944 FB2 6 C8 1F L2 10H D4 15mA 5 WHITE LEDs C1: TAIYO YUDEN JMK212BJ475 C2, C6: TAIYO YUDEN LMK212BJ225 C3, C4, C7: TAIYO YUDEN EMK107BJ104 C5, C8: TAIYO YUDEN TMK316BJ105 D1, D4: ON SEMI MBR0530 D2, D3: ZETEX BAT54S L1, L2: SUMIDA CLQ4D10-100 Q1, Q2: ON SEMI MMBT3906 (408) 573-4150 (408) 573-4150 (408) 573-4150 (408) 573-4150 (800) 282-9855 (631) 543-7100 (847) 956-0666 (800) 282-9855
1944 TA03
C4 0.1F
D2A D1
20V C5 500A 1F 10V 5mA
10 SW1 FB1 1 C2 2.2F 5 140k 1M
GND PGND PGND SW2
82.5
RELATED PARTS
PART NUMBER LT1307 LT1316 LT1317 LT1610 LT1611 LT1613 LT1615 LT1617 LT1930A DESCRIPTION Single-Cell Micropower 600kHz PWM DC/DC Converter Burst Mode(R) Operation DC/DC with Programmable Current Limit 2-Cell Micropower DC/DC with Low-Battery Detector Single-Cell Micropower DC/DC Converter 1.4MHz Inverting Switching Regulator in 5-Lead SOT-23 1.4MHz Switching Regulator in 5-Lead SOT-23 Micropower DC/DC Converter in 5-Lead SOT-23 Micropower Inverting DC/DC Converter in 5-Lead SOT-23 2.2MHz Boost DC/DC Converter in SOT-23 COMMENTS 3.3V at 75mA from One Cell, MSOP Package 1.5V Minimum, Precise Control of Peak Current Limit 3.3V at 200mA from Two Cells, 600kHz Fixed Frequency 3V at 30mA from 1V, 1.7MHz Fixed Frequency - 5V at 150mA from 5V Input, Tiny SOT-23 Package 5V at 200mA from 3.3V Input, Tiny SOT-23 Package 20V at 12mA from 2.5V Input, Tiny SOT-23 Package -15V at 12mA from 2.5V Input, Tiny SOT-23 Package 5V at 450mA from 3.3V, Tiny SOT-23 Package
Burst Mode is a registered trademark of Linear Technology Corporation
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 q FAX: (408) 434-0507
q
1944f LT/TP 1001 2K * PRINTED IN USA
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2001

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