LT1944EMS#TRPBF

LT1944 Dual Micropower Step-Up DC/DC Converter U FEATURES ■ ■ ■ ■ ■ ■ DESCRIPTIO The LT®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 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 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. Low Quiescent Current: 20µA in Active Mode 1V VFB < 0.6V 400 1.5 Switch VCESAT ISW = 300mA 250 350 mV 350 400 mA 2 8 3 12 µA µA Switch Current Limit SHDN Pin Current ● 250 VSHDN = 1.2V VSHDN = 5V SHDN Input Voltage High 0.05 0.1 %/V 30 80 nA 0.9 V SHDN Input Voltage Low Switch Leakage Current Switch Off, VSW = 5V 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 0°C to 70°C. Specifications over the – 40°C to 85°C operating temperature range are assured by design, characterization and correlation with statistical process controls. Note 3: Bias current flows into the FB pin. 2 ns µs 0.01 0.25 V 5 µA LT1944 U W TYPICAL PERFOR A CE CHARACTERISTICS Switch Saturation Voltage (VCESAT) Feedback Pin Voltage and Bias Current 0.60 Quiescent Current 1.25 25 50 VFB = 1.23V NOT SWITCHING FEEDBACK VOLTAGE (V) 1.24 0.45 ISWITCH = 500mA 0.40 0.35 0.30 ISWITCH = 300mA 0.25 0.20 40 VOLTAGE 1.23 30 CURRENT 1.22 20 1.21 10 BIAS CURRENT (nA) SWITCH VOLTAGE (V) 0.50 QUIESCENT CURRENT (µA) 0.55 23 21 VIN = 12V 19 VIN = 1.2V 17 0.15 0.10 –50 –25 0 25 50 TEMPERATURE (°C) 75 1.20 –50 100 –25 0 25 50 TEMPERATURE (°C) 75 1944 G01 Switch Off Time VIN = 12V SHUTDOWN PIN CURRENT (µA) 350 300 250 200 150 100 300 250 –50 100 Shutdown Pin Current VIN = 1.2V PEAK CURRENT (mA) SWITCH OFF TIME (ns) 500 VIN = 12V 75 25 350 VIN = 1.2V 0 25 50 TEMPERATURE (°C) 1944 G03 Switch Current Limit 400 450 –25 1944 G02 550 400 15 –50 0 100 20 15 25°C 10 100°C 5 50 –25 0 25 50 TEMPERATURE (°C) 75 100 0 –50 0 –25 0 25 50 TEMPERATURE (°C) 1944 G04 75 100 1944 G05 0 5 10 SHUTDOWN PIN VOLTAGE (V) 15 1944 G03 U U U 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. 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. GND (Pin 3): Ground. Tie this pin directly to the local ground plane. VIN (Pin 8): Input Supply Pin. Bypass this pin with a capacitor as close to the device as possible. 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. 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. FB2 (Pin 5): Feedback Pin for Switcher 2. Set the output voltage by selecting values for R1B and R2B. 3 LT1944 W BLOCK DIAGRA D2 D1 L1 VOUT1 VIN 8 VIN 2 SHDN1 10 VIN C3 C2 C1 L2 VOUT2 SW1 SW2 SHDN2 6 4 VIN R5 40k R6 40k R6B 40k + A1 A1B ENABLE ENABLE R5B 40k + VOUT1 VOUT2 – R1 (EXTERNAL) FB1 – Q1B 1 Q1 Q2 X10 R2 (EXTERNAL) 400ns ONE-SHOT Q3 DRIVER R3 30k 400ns ONE-SHOT Q3B RESET + GND 3 FB2 R1B (EXTERNAL) R2B (EXTERNAL) R3B 30k + R4 140k 0.12Ω – 5 DRIVER RESET A2 Q2B X10 R4B 140k 0.12Ω 42mV 42mV 9 PGND PGND – A2B 7 1944 BD Figure 1. LT1944 Block Diagram U 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 4 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 1.5µs 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. The second switching regulator operates in the same manner. LT1944 U U W U 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 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 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 LT1944. As for the boost inductor selection, a larger or smaller value can be used. V +V L = 2  OUT D  ILIM   tOFF  Inductor Selection—Boost Regulator Current Limit Overshoot 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: 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: 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  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 5 LT1944 U U W U 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 4.7µF 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 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 6 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. LT1944 U TYPICAL APPLICATIO S 2-Cell Dual Output (3.3V, 5V) Boost Converter L1 4.7µH VIN 1.8V TO 3V D1 5V 40mA 8 10 VIN 2 4.7pF SW1 SHDN1 C1 4.7µF FB1 1M 1 C2 10µF LT1944 4 SHDN2 FB2 5 324k GND PGND PGND SW2 7 3 9 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 4.7pF L2 4.7µH 1M C3 10µF D2 3.3V 80mA 1944 TA02 2-Cell to 5V Efficiency 2-Cell to 3.3V Efficiency 90 90 85 75 80 EFFICIENCY (%) EFFICIENCY (%) 80 VIN = 1.8V 70 65 75 70 60 55 55 1 10 LOAD CURRENT (mA) 50 0.1 100 VIN = 1.8V 65 60 50 0.1 VIN = 3V 85 VIN = 3V 1 10 LOAD CURRENT (mA) 1944 TA02a 100 1944 TA02b U PACKAGE DESCRIPTIO MS10 Package 10-Lead Plastic MSOP (LTC DWG # 05-08-1661) 0.034 (0.86) REF 0.043 (1.10) MAX 0.007 (0.18) 0.118 ± 0.004* (3.00 ± 0.102) 10 9 8 7 6 0° – 6° TYP 0.021 ± 0.006 (0.53 ± 0.015) SEATING PLANE 0.007 – 0.011 (0.17 – 0.27) 0.0197 (0.50) BSC 0.005 ± 0.002 (0.13 ± 0.05) 0.118 ± 0.004** (3.00 ± 0.102) 0.193 ± 0.006 (4.90 ± 0.15) * 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. MSOP (MS10) 1100 1 2 3 4 5 7 LT1944 U TYPICAL APPLICATIO Four Output Power Supply for Color LCD Displays Q1 D3A C6 2.2µF C7 0.1µF –6.5V 500µA Q2 140k D3B D2B C3 0.1µF C4 0.1µF L1 10µH VIN 2.7V TO 4.2V D2A D1 10V 5mA 8 10 VIN 2 20V C5 500µA 1µF 1M SW1 SHDN1 C1 4.7µF 1 FB1 C2 2.2µF LT1944 4 SHDN2 5 FB2 140k GND PGND PGND SW2 3 7 9 6 C8 1µF L2 10µH 82.5Ω 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 1944 TA03 (408) 573-4150 (408) 573-4150 (408) 573-4150 (408) 573-4150 (800) 282-9855 (631) 543-7100 (847) 956-0666 (800) 282-9855 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1307 Single-Cell Micropower 600kHz PWM DC/DC Converter 3.3V at 75mA from One Cell, MSOP Package LT1316 Burst Mode® Operation DC/DC with Programmable Current Limit 1.5V Minimum, Precise Control of Peak Current Limit LT1317 2-Cell Micropower DC/DC with Low-Battery Detector 3.3V at 200mA from Two Cells, 600kHz Fixed Frequency LT1610 Single-Cell Micropower DC/DC Converter 3V at 30mA from 1V, 1.7MHz Fixed Frequency LT1611 1.4MHz Inverting Switching Regulator in 5-Lead SOT-23 – 5V at 150mA from 5V Input, Tiny SOT-23 Package LT1613 1.4MHz Switching Regulator in 5-Lead SOT-23 5V at 200mA from 3.3V Input, Tiny SOT-23 Package LT1615 Micropower DC/DC Converter in 5-Lead SOT-23 20V at 12mA from 2.5V Input, Tiny SOT-23 Package LT1617 Micropower Inverting DC/DC Converter in 5-Lead SOT-23 –15V at 12mA from 2.5V Input, Tiny SOT-23 Package LT1930A 2.2MHz Boost DC/DC Converter in SOT-23 5V at 450mA from 3.3V, Tiny SOT-23 Package Burst Mode is a registered trademark of Linear Technology Corporation 8 Linear Technology Corporation 1944f LT/TP 1001 2K • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com  LINEAR TECHNOLOGY CORPORATION 2001