LT3461/LT3461A 1.3MHz/3MHz Step-Up DC/DC Converters with Integrated Schottky in ThinSOT U FEATURES DESCRIPTIO ■ The LT®3461/LT3461A are general purpose fixed frequency current mode step-up DC/DC converters. Both devices feature an integrated Schottky and a low VCESAT switch allowing a small converter footprint and lower parts cost. The LT3461 switches at 1.3MHz while the LT3461A switches at 3MHz. These high switching frequencies enable the use of tiny, low cost and low height capacitors and inductors. The constant switching frequency results in predictable output noise that is easy to filter, and the inductor based topology ensures an input free from switching noise typically present with charge pump solutions. The high voltage switch in the LT3461/LT3461A is rated at 40V making the device ideal for boost converters up to 38V. ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Integrated Schottky Rectifier Fixed Frequency 1.3MHz/3MHz Operation High Output Voltage: Up to 38V Low VCESAT Switch: 260mV at 250mA 12V at 70mA from 5V Input 5V at 115mA from 3.3V Input Wide Input Range: 2.5V to 16V Uses Small Surface Mount Components Low Shutdown Current: <1µA Soft-Start Low Profile (1mm) SOT-23 (ThinSOTTM) Package U APPLICATIO S ■ ■ ■ ■ ■ ■ Digital Cameras CCD Bias Supply XDSL Power Supply TFT-LCD Bias Supply Local 5V or 12V Supply Medical Diagnostic Equipment Battery Backup The LT3461/LT3461A are available in a low profile (1mm) SOT-23 package. , LTC and LT are registered trademarks of Linear Technology Corporation. ThinSOT is a trademark of Linear Technology Corporation. U ■ TYPICAL APPLICATIO 5V to 12V, 70mA Step-Up DC/DC Converter C1 1µF OFF ON L1 10µH 1 SW 6 5 VIN VOUT LT3461A 4 3 SHDN FB GND 2 80 261k 30.1k 15pF VOUT 12V 70mA C2 1µF VIN = 5V EFFICIENCY (%) VIN 5V Efficiency 85 75 VIN = 3.3V 70 65 3461 TA01a 60 0 20 60 40 LOAD CURRENT (mA) 80 3461 TAO1b 3461af 1 LT3461/LT3461A U W W W ABSOLUTE AXI U RATI GS U W U PACKAGE/ORDER I FOR ATIO (Note 1) Input Voltage (VIN) .................................................. 16V VOUT, SW Voltage .................................................... 40V FB Voltage ................................................................. 5V SHDN Voltage .......................................................... 16V Operating Ambient Temperature Range (Note 2) .................. – 40°C to 85°C Maximum Junction Temperature .......................... 125°C Storage Temperature Range ................. – 65°C to 150°C Lead Temperature (Soldering, 10 sec).................. 300°C ORDER PART NUMBER TOP VIEW SW 1 GND 2 FB 3 6 VIN LT3461AES6 LT3461ES6 5 VOUT 4 SHDN S6 PACKAGE 6-LEAD PLASTIC TSOT-23 TJMAX = 125°C, θJA = 150°C ON BOARD OVER GROUND PLANE, θJC = 120°C/W S6 PART MARKING LTAHG LTAEB Consult LTC Marketing for parts specified with wider operating temperature ranges. ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C, VIN = 3V, VSHDN = 3V, unless otherwise noted. PARAMETER CONDITIONS MIN Minimum Operating Voltage TYP Feedback Voltage ● 1.235 1.225 Feedback Line Regulation 1.255 16 V 1.275 1.280 V V 0.005 ● FB Pin Bias Current FB = 1.3V, Not Switching SHDN = 0V UNITS V Maximum Operating Voltage Supply Current MAX 2.5 %/V 40 100 nA 2.8 0.1 3.6 0.5 mA µA Switching Frequency (LT3461A) ● 2.1 3.0 3.9 MHz Switching Frequency (LT3461) ● 1.0 1.3 1.7 MHz Maximum Duty Cycle (LT3461A) ● 82 % Maximum Duty Cycle (LT3461) ● 92 % Switch Current Limit 420 600 mA Switch VCESAT ISW = 250mA 300 260 350 mV Switch Leakage Current VSW = 5V 0.01 1 µA Schottky Forward Voltage ISCHOTTKY = 250mA 800 1100 mV Schottky Reverse Leakage VOUT – SW = 40V 0.03 4 µA SHDN Voltage High 1.5 V SHDN Voltage Low SHDN Pin Bias Current Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. 35 0.4 V 50 µA Note 2: The LT3461E/LT3461AE is guaranteed to meet 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. 3461af 2 LT3461/LT3461A U W TYPICAL PERFOR A CE CHARACTERISTICS Oscillator Frequency (LT3461) Current Limit 1.6 FB Pin Voltage 1.28 480 TA = 25°C 1.27 1.4 1.3 1.2 360 FB VOLTAGE (V) CURRENT LIMIT (mA) FREQUENCY (MHz) 1.5 240 1.26 1.25 1.24 120 1.1 1.23 1.0 –40 –20 40 20 60 0 TEMPERATURE (°C) 80 0 10 100 20 30 40 50 60 70 DUTY CYCLE (%) 80 3461a G01 1.22 –40 –20 90 480 320 TA = 25°C 3.0 2.7 2.4 280 SHDN PIN CURRENT (µA) CURRENT LIMIT (mA) FREQUENCY (MHz) TA = 25°C 420 3.6 360 300 240 180 120 60 2.1 –60 –40 –20 0 20 40 60 TEMPERATURE (°C) 80 100 100 SHDN Pin Current Current Limit in Soft-Start Mode Oscillator Frequency (LT3461A) 80 3461a G03 3461a G02 3.9 3.3 40 20 60 0 TEMPERATURE (°C) 0 1.3 240 200 160 120 80 40 1.5 0 1.7 2.1 1.9 SHDN PIN VOLTAGE (V) 2.3 0 4 8 12 3461a G06 3461a G05 3461a G04 16 SHDN PIN VOLTAGE (V) Switching Waveform Circuit of Figure 4 Load Transient Response Circuit of Figure 4 ILOAD 70mA 35mA VSW 5V/DIV VOUT 100mV/DIV VOUT 50mV/DIV ILOAD = 60mA 0.2µs/DIV 3461a G08 50µs/DIV 3461a G09 3461af 3 LT3461/LT3461A U U U PI FU CTIO S SW (Pin 1): Switch Pin. Connect inductor here. Minimize trace at this pin to reduce EMI. SHDN (Pin 4): Shutdown Pin. Tie to 1.5V or higher to enable device; 0.4V or less to disable device. Also functions as soft-start. Use RC filter (47k, 47nF typ) as shown in Figure 1. GND (Pin 2): Ground Pin. Tie directly to local ground plane. VOUT (Pin 5): Output Pin. Connect to resistor divider. Put capacitor close to pin and close to GND plane. FB (Pin 3): Feedback Pin. Reference voltage is 1.255V. Connect resistor divider tap here. Minimize trace area at FB. Set VOUT according to VOUT = 1.255V (1 + R1/R2). VIN (Pin 6): Input Supply Pin. Must be locally bypassed. W BLOCK DIAGRA 1.255V REFERENCE VIN 6 – 5 VOUT Q Q1 S + CC 3 FB FB R A2 RC R1 (EXTERNAL) SW DRIVER – A1 VOUT 1 COMPARATOR + + R2 (EXTERNAL) ∑ 0.1Ω – SHUTDOWN RS (EXTERNAL) 4 SHDN RAMP GENERATOR CS (EXTERNAL) RS, CS OPTIONAL SOFT-START COMPONENTS 3MHz* OSCILLATOR 2 *LT3461 IS 1.3MHz GND 3461a F02 Figure 1. Block Diagram U OPERATIO Layout Hints The high speed operation of the LT3461/LT3461A demands careful attention to board layout. You will not get advertised performance with careless layout. Figure 2 shows the recommended component placement. C1 + GND L1 VIN R2 + The LT3461/LT3461A uses a constant frequency, current mode control scheme to provide excellent line and load regulation. Operation can be best understood by referring to the block diagram in Figure 1. At the start of each oscillator cycle, the SR latch is set, which turns on the power switch Q1. A voltage proportional to the switch current is added to a stabilizing ramp and the resulting sum is fed into the positive terminal of the PWM comparator A2. When this voltage exceeds the level at the negative input of A2, the SR latch is reset turning off the power switch. The level at the negative input of A2 is set by the error amplifier A1, and is simply an amplified version of the difference between the feedback voltage and the reference voltage of 1.255V. In this manner, the error amplifier sets the correct peak current level to keep the output in regulation. If the error amplifier’s output increases, more current is delivered to the output; if it decreases, less current is delivered. R1 C2 VOUT SHUTDOWN C3 3461a F03 Figure 2. Suggested Layout 3461af 4 LT3461/LT3461A U W U U APPLICATIO S I FOR ATIO The LT3461 has a built-in Schottky diode. When supply voltage is applied to the VIN pin, the voltage difference between VIN and VOUT generates inrush current flowing from input through the inductor and the Schottky diode to charge the output capacitor. The maximum nonrepetitive surge current the Schottky diode in the LT3461 can sustain is 1.5A. The selection of inductor and capacitor value should ensure the peak of the inrush current to be below 1.5A. In addition, turn-on of the LT3461 should be delayed until the inrush current is less than the maximum current limit. The peak inrush current can be calculated as follows: numbers can be expected if the LT3461 is supplied from a separate low voltage rail. 160 VIN = 5V VIN = 8V VIN = 12V 120 VIN >15V IOUT (mA) Inrush Current 80 40 0 6 14 22 30 38 VOUT (V) 3461 F01a ⎞ ⎛ ⎜ π ⎟⎟ V – 0.6 • exp ⎜ − IP = IN ⎟ ⎜ L L –1 − 1⎟ ⎜ 2 ⎠ ⎝ C C Figure 3a. LT3461 Operating Region 160 VIN = 5V where L is the inductance, r is the resistance of the inductor and C is the output capacitance. Table 3 gives inrush peak currents for some component selections. IOUT (mA) 120 VIN = 8V VIN = 12V VIN >15V 80 40 Table 3. Inrush Peak Current VIN (V) L (µH) C (µF) IP (A) 5 4.7 1 1.1 5 10 1 0.9 0 6 14 22 30 38 VOUT (V) 3461 F01b Figure 3b. LT3461A Operating Region Thermal Considerations Significant power dissipation can occur on the LT3461 and LT3461A, particularly at high input voltage. Device load, voltage drops in the power path components, and switching losses are the major contributors. It is important to measure device power dissipation in an application to ensure that the LT3461 does not exceed the absolute maximum operating junction temperature of 125°C over the operating ambient temperature range. Generally, for supply voltages below 5V the integrated current limit function provides adequate protection for nonfault conditions. For supply voltages above 5V, Figures 3a and 3b show the recommended operating region of the LT3461 and LT3461A, respectively. These graphs are based on 250mW on-chip dissipation. Improvement of these Switching Frequency The key difference between the LT3461 and LT3461A is the faster switching frequency of the LT3461A. At 3MHz, the LT3461A switches at twice the rate of the LT3461. The higher switching frequency of the LT3461A allows physically smaller inductors and capacitors to be used in a given application, but with a slight decrease in efficiency and maximum output current when compared to the LT3461. Generally if efficiency and maximum output current are crucial, or a high output voltage is being generated, the LT3461 should be used. If application size and cost are more important, the LT3461A will be the better choice. 3461af 5 LT3461/LT3461A U W U U APPLICATIO S I FOR ATIO Inductor Selection The inductors used with the LT3461/LT3461A should have a saturation current rating of 0.3A or greater. If the device is used in an application where the input supply will be hot-plugged, then the saturation current rating should be equal to or greater than the peak inrush current. For the LT3461, an inductor value between 10µH and 47µH, depending upon output voltage, will usually be the best choice for most designs. For the LT3461A, inductor values between 4.7µH and 15µH inductor will suffice for most applications. For best loop stability results, the inductor value selected should provide a ripple current of 70mA or more. For a given VIN and VOUT the inductor value to use with LT3461A is estimated by the formula: L (in microhenries) = D • VIN • VOUT • 1sec 1A • 1V VOUT + 1V – VIN where D = VOUT + 1V Use twice this value for the LT3461. Capacitor Selection Low ESR capacitors should be used at the output to minimize the output voltage ripple. Multilayer ceramic capacitors using X5R/X7R dielectrics are preferred as they have a low ESR and maintain capacitance over wide voltage and temperature range. A 2.2µF output capacitor is sufficient for most applications using the LT3461, while a 1µF capacitor is sufficient for most applications using the LT3461A. High output voltages typically require less capacitance for loop stability. Always use a capacitor with sufficient voltage rating. Either ceramic or solid tantalum capacitors may be used for the input decoupling capacitor, which should be placed as close as possible to the LT3461/LT3461A. A 1µF capacitor is sufficient for most applications. Phase Lead Capacitor A small value capacitor can be added across resistor R1 between the output and the FB pin to reduce output perturbation due to a load step and to improve transient response. This phase lead capacitor introduces a polezero pair to the feedback that boosts phase margin near the cross-over frequency. The following formula is useful to estimate the capacitor value needed: C PL = 500kΩ • 1pF R2 For an application running 50µA in the feedback divider, capacitor values from 10pF to 22pF work well. U TYPICAL APPLICATIO S L1 10µH VIN 5V CONTROL SIGNAL C1 1µF 47k 47nF 1 SW 5 VIN VOUT LT3461A 4 3 SHDN FB GND 2 Input Current and Output Voltage 6 C1, C2: TAIYO YUDEN EMK212BJ105 L1: MURATA LQH32CN100K53 R1 261k R2 30.1k 15pF VOUT 12V 70mA C2 1µF 3461a TA02a Figure 4. 5V to 12V with Soft-Start Circuit (LT3461A) CONTROL SIGNAL 5V/DIV IIN 50mA/DIV VOUT 5V/DIV 1ms/DIV 3461a TA02b 3461af 6 LT3461/LT3461A U TYPICAL APPLICATIO S 3.3V to 5V Step-Up Converter Efficiency 80 3.3V to 5V Step-Up Converter (LT3461A) L1 4.7µH VIN 3.3V C1 1µF OFF ON R1 45.3k EFFICIENCY (%) 75 1 SW 6 5 VIN VOUT LT3461A 4 3 SHDN FB GND 2 VOUT 5V 115mA 15pF C2 1µF R2 15k C1, C2: TAIYO YUDEN X7R LMK212BJ105 L1: MURATA LQH32CN4R7M33 OR EQUIVALENT 70 65 3461a TA03a 60 0 30 60 90 LOAD CURRENT (mA) 120 3461a TA03b U PACKAGE DESCRIPTIO S6 Package 6-Lead Plastic TSOT-23 (Reference LTC DWG # 05-08-1636) 0.62 MAX 2.90 BSC (NOTE 4) 0.95 REF 1.22 REF 1.4 MIN 3.85 MAX 2.62 REF 2.80 BSC 1.50 – 1.75 (NOTE 4) PIN ONE ID RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR 0.30 – 0.45 6 PLCS (NOTE 3) 0.95 BSC 0.80 – 0.90 0.20 BSC 0.01 – 0.10 1.00 MAX DATUM ‘A’ 0.30 – 0.50 REF NOTE: 1. DIMENSIONS ARE IN MILLIMETERS 2. DRAWING NOT TO SCALE 3. DIMENSIONS ARE INCLUSIVE OF PLATING 4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR 5. MOLD FLASH SHALL NOT EXCEED 0.254mm 6. JEDEC PACKAGE REFERENCE IS MO-193 0.09 – 0.20 (NOTE 3) 1.90 BSC S6 TSOT-23 0302 3461af 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. 7 LT3461/LT3461A U TYPICAL APPLICATIO S Low Profile (1mm) 3.3V to 15V Step-Up Converter 3.3V to 15V Efficiency L1 10µH C1 1µF OFF ON 1 SW 5 VIN VOUT LT3461A 4 3 SHDN FB GND 2 70 VOUT 15V 25mA 6 332k EFFICIENCY (%) VIN 3.3V 75 22pF C2 2.2µF 30.1k C1: TAIYO YUDEN LMK107BJ105KA C2: TAIYO YUDEN EMK316BJ225KD (X5R) L1: MURATA LQH2MCN100K02 3461a TA04a 65 60 55 50 0 5 5V to 36V Step-Up Converter (LT3461) 10 15 20 LOAD CURRENT (mA) 25 30 3461a TA04b 5V to 36V Efficiency 80 L1 47µH C1 1µF OFF ON 1 SW 6 5 VIN VOUT LT3461 4 3 SHDN FB GND 2 75 280k 22pF VOUT 36V 18mA EFFICIENCY (%) VIN 5V C2 0.47µF 50V 10k C1: TAIYO YUDEN X7R LMK212BJ105 C2: MURATA GRM42-6X7R474K50 L1: MURATA LQH32CN470 70 65 60 55 3461 TA05a 50 0 2 4 6 8 10 12 14 16 18 LOAD CURRENT (mA) 3461 TA05b 3.3V to ±5V Dual Output Converter C3 1µF L1 4.7µH VIN 3.3V 1 SW 5 VIN VOUT LT3461A 4 3 SHDN FB GND 2 VOUT 5V 100mA 6 C1 1µF OFF ON 45.3k 15pF C2 1µF 15k D1 C1, C2, C3, C4: TAIYO YUDEN JMK107BJ105 D1, D2: PHILIPS PMEG2005EB L1: MURATA LQH2MCN4R7M02 D2 C4 1µF 10Ω –5V 15mA 3461 TA06 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1615/LT1615-1 300mA/80mA (ISW) Constant Off-Time, High Efficiency Step-Up DC/DC Converter VIN: 1.2V to 15V, VOUT(MAX) = 34V, IQ = 20µA, ISD <1µA, ThinSOT Package LT1944/LT1944-1 Dual Output 350mA/100mA (ISW), Constant Off-Time, High Efficiency Step-Up DC/DC Converter VIN: 1.2V to 15V, VOUT(MAX) = 34V, IQ = 20µA, ISD <1µA, MS Package LTC3400/LTC3400B 600mA (ISW), 1.2MHz, Synchronous Step-Up DC/DC Converter VIN: 0.85V to 5V, VOUT(MAX) = 5V, IQ = 19µA/300µA, ISD <1µA, ThinSOT LT3460 0.32A (ISW), 1.3MHz, High Efficiency Step-Up DC/DC Converter VIN: 2.5V to 16V, VOUT(MAX) = 36V, IQ = 2mA, ISD <1µA, SC70, ThinSOT Packages LT3465/LT3465A Constant Current, 1.2MHz/2.7MHz, High Efficiency White LED Boost Regulator with Integrated Schottky Diode VIN: 2.7V to 16V, VOUT(MAX) = 34V, IQ = 1.9mA, ISD <1µA, ThinSOT Package 3461af 8 Linear Technology Corporation LT/TP 1004 1K • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2003