Sample & Buy Product Folder Support & Community Tools & Software Technical Documents LM3481, LM3481-Q1 SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 LM3481 / -Q1 High-Efficiency Controller for Boost, SEPIC and Flyback DC-DC Converters 1 Features 3 Description * The LM3481 device is a versatile Low-Side N-FET high-performance controller for switching regulators. The device is designed for use in Boost, SEPIC and Flyback converters and topologies requiring a lowside FET as the primary switch. The LM3481 device can be operated at very high switching frequencies to reduce the overall solution size. The switching frequency of the LM3481 device can be adjusted to any value between 100kHz and 1MHz by using a single external resistor or by synchronizing it to an external clock. Current mode control provides superior bandwidth and transient response in addition to cycle-by-cycle current limiting. Current limit can be programmed with a single external resistor. 1 * * * * * * * * * * LM3481QMM are Automotive-Grade Products That are AEC-Q100 Grade 1 Qualified (-40C to +125C Operating Junction Temperature) 10-Lead VSSOP Package Internal Push-Pull Driver With 1-A Peak Current Capability Current Limit and Thermal Shutdown Frequency Compensation Optimized With a Capacitor and a Resistor Internal Softstart Current Mode Operation Adjustable Undervoltage Lockout With Hysteresis Pulse Skipping at Light Loads Key Specifications - Wide Supply Voltage Range of 2.97 V to 48 V - 100-kHz to 1-MHz Adjustable and Synchronizable Clock Frequency - 1.5% (Over Temperature) Internal Reference - 10-A Shutdown Current (Over Temperature) Create a Custom Design Using the LM3481 with the WEBENCH Power Designer The LM3481 device has built-in protection features such as thermal shutdown, short-circuit protection and overvoltage protection. Power-saving shutdown mode reduces the total supply current to 5 A and allows power supply sequencing. Internal soft-start limits the inrush current at start-up. Integrated current slope compensation simplifies the design and, if needed for specific applications, can be increased using a single resistor. Device Information(1) PART NUMBER 2 Applications LM3481 * * * LM3481-Q1 * * * * Automotive Start-Stop Applications Automotive ADAS Driver Information One Cell/Two Cell Li-ion Battery Powered Portable Bluetooth Audio Systems Notebooks, PDAs, Digital Cameras, and Other Portable Applications Offline Power Supplies Set-Top Boxes Boost for Audio Amplifiers PACKAGE VSSOP (10) VIN = 5V R8 121 k: ISEN VIN UVLO L1 6.8 PH 1 PF VCC C9 COMP RC 22.6 k: CC 82 nF FB RF2 20 k: RF1 169 k: AGND 3.00 mm x 3.00 mm (1) For all available packages, see the orderable addendum at the end of the data sheet. R7 121 k: C8 390 pF BODY SIZE (NOM) LM3481 Q1 DR D1 + CIN1, CIN2 150 PF VOUT = 12V IOUT = 1.8A + COUT1, COUT2 150 PF PGND FA/SYNC/SD RFA 90.9 k: CSEN 1 nF RSEN 20 m: Figure 1. LM3481 Typical 5V to 12V Boost Converter Application 1 An IMPORTANT NOTICE at the end of this data sheet addresses availability, warranty, changes, use in safety-critical applications, intellectual property matters and other important disclaimers. PRODUCTION DATA. LM3481, LM3481-Q1 SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 www.ti.com Table of Contents 1 2 3 4 5 6 7 Features .................................................................. Applications ........................................................... Description ............................................................. Revision History..................................................... Pin Configuration and Functions ......................... Specifications......................................................... 1 1 1 2 3 4 6.1 6.2 6.3 6.4 6.5 6.6 6.7 4 4 4 4 5 5 8 Absolute Maximum Ratings ...................................... Handling Ratings: LM3481........................................ Handling Ratings: LM3481-Q1.................................. Recommended Operating Ratings............................ Thermal Information .................................................. Electrical Characteristics........................................... Typical Characteristics .............................................. Detailed Description ............................................ 11 7.1 Overview ................................................................. 11 7.2 Functional Block Diagram ....................................... 12 7.3 Feature Description................................................. 12 7.4 Device Functional Modes........................................ 18 8 Application and Implementation ........................ 19 8.1 Application Information............................................ 19 8.2 Typical Applications ................................................ 19 9 Power Supply Recommendations...................... 30 10 Layout................................................................... 30 10.1 Layout Guidelines ................................................. 30 10.2 Layout Example .................................................... 31 11 Device and Documentation Support ................. 32 11.1 11.2 11.3 11.4 11.5 Documentation Support ........................................ Related Links ........................................................ Trademarks ........................................................... Electrostatic Discharge Caution ............................ Glossary ................................................................ 32 32 32 32 32 12 Mechanical, Packaging, and Orderable Information ........................................................... 33 4 Revision History Changes from Revision E (April 2012) to Revision F * 2 Page Added Pin Configuration and Functions section, Handling Rating table, Feature Description section, Device Functional Modes, Application and Implementation section, Power Supply Recommendations section, Layout section, Device and Documentation Support section, and Mechanical, Packaging, and Orderable Information section ................................................................................................................................................................................... 1 Submit Documentation Feedback Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 LM3481, LM3481-Q1 www.ti.com SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 5 Pin Configuration and Functions 10-Pin VSSOP Package Top View ISEN UVLO COMP FB AGND 1 10 2 9 3 8 LM3481 4 7 5 6 VIN VCC DR PGND FA/SYNC/SD Pin Functions PIN I/O DESCRIPTION NO. NAME 1 ISEN A Current sense input pin. Voltage generated across an external sense resistor is fed into this pin. 2 UVLO A Under voltage lockout pin. A resistor divider from VIN to ground is connected to the UVLO pin. The ratio of these resistances determine the input voltage which allows switching and the hysteresis to disable switching. 3 COMP A Compensation pin. A resistor and capacitor combination connected to this pin provides compensation for the control loop. 4 FB A Feedback pin. Inverting input of the error amplifier. 5 AGND G Analog ground pin. Internal bias circuitry reference. Should be connected to PGND at a single point. 6 FA/SYNC/SD I/A 7 PGND G Power ground pin. External power circuitry reference. Should be connected to AGND at a single point. 8 DR O Drive pin of the IC. The gate of the external MOSFET should be connected to this pin. 9 VCC O Driver supply voltage pin. A bypass capacitor must be connected from this pin to PGND. See Driver Supply Capacitor Selection section. Do not bias externally. 10 VIN P Power supply input pin. Frequency adjust, synchronization, and shutdown pin. A resistor connected from this pin to ground sets the oscillator frequency. An external clock signal at this pin will synchronize the controller to the frequency of the clock. A high level on this pin for 30 s will turn the device off and the device will then draw 5 A from the supply typically. Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 Submit Documentation Feedback 3 LM3481, LM3481-Q1 SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 www.ti.com 6 Specifications 6.1 Absolute Maximum Ratings over operating free-air temperature range (unless otherwise noted) (1) MIN MAX UNIT VIN Pin Voltage -0.4 50 V FB Pin Voltage -0.4 6 V FA/SYNC/SD Pin Voltage -0.4 6 V COMP Pin Voltage -0.4 6 V UVLO Pin Voltage -0.4 6 V VCC Pin Voltage -0.4 6.5 V DR Pin Voltage -0.4 6 V ISEN Pin Voltage -400 600 mV 1 A Peak Driver Output Current Power Dissipation Lead Temperature (only applies to operating conditions) Peak Body Temperature (1) (2) Internally Limited Junction Temperature 150 C DGS Package 220 C 260 C (2) Absolute Maximum Ratings are limits beyond which damage to the device may occur. Recommended Operating Ratings indicates conditions for which the device is intended to be functional, but does not ensure specific performance limits. For ensured specifications and test conditions, see the Electrical Characteristics. The ensured specifications apply only for the test conditions. Part is MSL1-260C qualified 6.2 Handling Ratings: LM3481 Tstg V(ESD) (1) (2) MIN MAX UNIT -65 150 C Human body model (HBM), per ANSI/ESDA/JEDEC JS-001, all pins (1) -2000 +2000 Charged device model (CDM), per JEDEC specification JESD22-C101, all pins (2) -750 +750 Storage temperature range Electrostatic discharge V JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process. JEDEC document JEP157 states that 250-V CDM allows safe manufacturing with a standard ESD control process. 6.3 Handling Ratings: LM3481-Q1 Tstg MIN MAX UNIT -65 150 C -2000 +2000 Corner pins (1, 5, 6, and 10) -750 +750 Other pins -750 +750 Storage temperature range Human body model (HBM), per AEC Q100-002 (1) V(ESD) (1) Electrostatic discharge Charged device model (CDM), per AEC Q100-011 V AEC Q100-002 indicates HBM stressing is done in accordance with the ANSI/ESDA/JEDEC JS-001 specification. 6.4 Recommended Operating Ratings MIN MAX UNIT Supply Voltage 2.97 48 V Junction Temperature Range -40 125 C Switching Frequency Range 100 1 4 Submit Documentation Feedback kHz/MHz Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 LM3481, LM3481-Q1 www.ti.com SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 6.5 Thermal Information LM3481 THERMAL METRIC (1) VSSOP UNIT 10 PINS RJA Junction-to-ambient thermal resistance 157.3 RJC(top) Junction-to-case (top) thermal resistance 51.1 RJB Junction-to-board thermal resistance 76.9 JT Junction-to-top characterization parameter 5.0 JB Junction-to-board characterization parameter 75.6 RJC(bot) Junction-to-case (bottom) thermal resistance - (1) C/W For more information about traditional and new thermal metrics, see the IC Package Thermal Metrics application report, SPRA953. 6.6 Electrical Characteristics VIN= 12 V, RFA= 40 k, TJ = 25C, unless otherwise indicated. PARAMETER TEST CONDITIONS MIN VCOMP = 1.4 V, 2.97 VIN 48 V TYP MAX UNIT 1.275 VFB Feedback Voltage VCOMP = 1.4 V, 2.97 VIN 48 V, -40C TJ 125C VLINE Feedback Voltage Line Regulation 2.97 VIN 48 V 0.003 %/V VLOAD Output Voltage Load Regulation IEAO Source/Sink 0.5 %/A VUVLOSEN Undervoltage Lockout Reference Voltage IUVLO UVLO Source Current VUVLOSD UVLO Shutdown Voltage ICOMP COMP pin Current Sink 1.256 VUVLO Ramping Down VUVLO Ramping Down, -40C TJ 125C 1.345 1.517 5 3 0.55 VFB = 0V VCOMP V 1.430 Enabled Enabled, -40C TJ 125C 1.294 VFB = 1.275V RFA = 40 k 6 0.7 0.82 V A V 640 A 1 V 475 fnom Nominal Switching Frequency Vsync-HI Threshold for Synchronization on FA/SYNC/SD pin Synchronization Voltage Rising 1.4 V Vsync-LOW Threshold for Synchronization on FA/SYNC/SD pin Synchronization Voltage Falling 0.7 V RDS1 (ON) Driver Switch On Resistance (top) IDR = 0.2A, VIN= 5 V 4 RDS2 (ON) Driver Switch On Resistance (bottom) IDR = 0.2A 2 RFA = 40 k, -40C TJ 125C VDR (max) Maximum Drive Voltage Swing (1) Dmax Maximum Duty Cycle tmin (on) Minimum On Time ISUPPLY (1) (2) 406 550 VIN < 6V VIN VIN 6V 6 RFA=40 k 81% 250 See Supply Current (switching) See (2), -40C TJ 125C V 85% worst case over temperature (2) kHz 363 ns 571 ns 3.7 5.0 mA The drive pin voltage, VDR, is equal to the input voltage when input voltage is less than 6 V. VDR is equal to 6 V when the input voltage is greater than or equal to 6 V. For this test, the FA/SYNC/SD Pin is pulled to ground using a 40-k resistor. Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 Submit Documentation Feedback 5 LM3481, LM3481-Q1 SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 www.ti.com Electrical Characteristics (continued) VIN= 12 V, RFA= 40 k, TJ = 25C, unless otherwise indicated. PARAMETER TEST CONDITIONS VFA/SYNC/SD = 3 V IQ Quiescent Current in Shutdown Mode MIN (3) , VIN = 12 V TYP 15 VFA/SYNC/SD = 3 V (3), VIN = 5 V Current Sense Threshold Voltage VSC Over Load Current Limit Sense Voltage VSL Internal Compensation Ramp Voltage VOVP Output Over-voltage Protection (with respect to feedback voltage) (4) VCOMP = 1.4 V VOVP(HYS) Output Over-Voltage Protection Hysteresis VCOMP = 1.4 V 10 Gm Error Amplifier Transconductance -40C TJ 125C 100 -40C TJ 125C 157 VCOMP = 1.4 V, -40C TJ 125C 26 70 VCOMP = 1.4 V 216 mV 690 mV mV mho 60 VCOMP = 1.4 V, IEAO = 100 A (Source/Sink), -40C TJ 125C V/V 35 Source, VCOMP = 1.4 V, VFB = 1.1 V, -40C TJ 125C 66 640 475 Sink, VCOMP = 1.4 V, VFB = 1.4 V 837 A 65 31 Upper Limit: VFB = 0 V, COMP Pin Floating Error Amplifier Output Voltage Swing 106 450 VCOMP = 1.4 V, -40C TJ 125C Sink, VCOMP = 1.4 V, VFB = 1.4 V, -40C TJ 125C VEAO 135 28 mV mV 85 VCOMP = 1.4 V, IEAO = 100 A (Source/Sink) Error Amplifier Output Current (Source/ Sink) 275 90 Source, VCOMP = 1.4 V, VFB = 1.1 V IEAO 190 220 VCOMP = 1.4 V, -40C TJ 125C Error Amplifier Voltage Gain A 5 160 VSENSE UNIT 9 VFA/SYNC/SD = 3 V (3), VIN = 12 V, -40C TJ 125C VFA/SYNC/SD = 3 V (3), VIN = 5 V, -40C TJ 125C AVOL MAX Upper Limit: VFB = 0 V, COMP Pin Floating, -40C TJ 125C 100 A 2.70 V 2.45 Lower Limit: VFB = 1.4 V 2.93 0.60 Lower Limit: VFB = 1.4 V, -40C TJ 125C 0.32 8.7 0.90 tSS Internal Soft-Start Delay VFB = 1.2 V, COMP Pin Floating tr Drive Pin Rise Time Cgs = 3000 pf, VDR = 0 V to 3 V 25 ns tf Drive Pin Fall Time Cgs = 3000 pf, VDR = 3 V to 0 V 25 ns Output = High (Shutdown) VSD Shutdown signal threshold (5)FA/SYNC/SD pin (5) 6 21.3 ms 1.31 Output = High (Shutdown), -40C TJ 125C 1.40 Output = Low (Enable) V 0.68 Output = Low (Enable), -40C TJ 125C (3) (4) 15 V 0.40 V For this test, the FA/SYNC/SD Pin is pulled to 3 V using a 40-k resistor. The overvoltage protection is specified with respect to the feedback voltage. This is because the overvoltage protection tracks the feedback voltage. The overvoltage threshold can be calculated by adding the feedback voltage (VFB) to the overvoltage protection specification. The FA/SYNC/SD pin should be pulled to VIN through a resistor to turn the regulator off. The voltage on the FA/SYNC/SD pin must be above the max limit for the Output = High longer than 30 s to keep the regulator off and must be below the minimum limit for Output = Low to keep the regulator on. Submit Documentation Feedback Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 LM3481, LM3481-Q1 www.ti.com SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 Electrical Characteristics (continued) VIN= 12 V, RFA= 40 k, TJ = 25C, unless otherwise indicated. PARAMETER TEST CONDITIONS ISD Shutdown Pin Current FA/SYNC/SD pin TSD Thermal Shutdown Tsh Thermal Shutdown Hysteresis MIN TYP VSD = 5 V -1 VSD = 0 V 20 Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 MAX UNIT A 165 C 10 C Submit Documentation Feedback 7 LM3481, LM3481-Q1 SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 www.ti.com 6.7 Typical Characteristics Unless otherwise specified, VIN = 12 V, TJ = 25C. 8 Figure 3. Comp Pin Voltage vs. Load Current Figure 4. Switching Frequency vs. RFA Figure 5. Efficiency vs. Load Current (3.3 VIN and 12 VOUT) Figure 6. Efficiency vs. Load Current (5 VIN and 12 VOUT) Figure 7. Efficiency vs. Load Current (9 VIN and 12 VOUT) Figure 8. Frequency vs. Temperature Submit Documentation Feedback Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 LM3481, LM3481-Q1 www.ti.com SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 Typical Characteristics (continued) Unless otherwise specified, VIN = 12 V, TJ = 25C. Figure 9. COMP Pin Source Current vs. Temperature Figure 10. ISupplyvs. Input Voltage (Nonswitching) Figure 11. ISupply vs. Input Voltage (Switching) Figure 12. Shutdown Threshold Hysteresis vs. Temperature Figure 13. Drive Voltage vs. Input Voltage Figure 14. Short Circuit Protection vs. VIN Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 Submit Documentation Feedback 9 LM3481, LM3481-Q1 SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 www.ti.com Typical Characteristics (continued) Unless otherwise specified, VIN = 12 V, TJ = 25C. Figure 15. Current Sense Threshold vs. Input Voltage Figure 16. Compensation Ramp Amplitude vs. Input Voltage Figure 17. Minimum On-Time vs. Temperature 10 Submit Documentation Feedback Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 LM3481, LM3481-Q1 www.ti.com SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 7 Detailed Description 7.1 Overview The LM3481 device uses a fixed frequency, Pulse Width Modulated (PWM), current mode control architecture. In a typical application circuit, the peak current through the external MOSFET is sensed through an external sense resistor. The voltage across this resistor is fed into the ISEN pin. This voltage is then level shifted and fed into the positive input of the PWM comparator. The output voltage is also sensed through an external feedback resistor divider network and fed into the error amplifier (EA) negative input (feedback pin, FB). The output of the error amplifier (COMP pin) is added to the slope compensation ramp and fed into the negative input of the PWM comparator. At the start of any switching cycle, the oscillator sets the RS latch using the SET/Blank-out and switch logic blocks. This forces a high signal on the DR pin (gate of the external MOSFET) and the external MOSFET turns on. When the voltage on the positive input of the PWM comparator exceeds the negative input, the RS latch is reset and the external MOSFET turns off. The voltage sensed across the sense resistor generally contains spurious noise spikes, as shown in Figure 18. These spikes can force the PWM comparator to reset the RS latch prematurely. To prevent these spikes from resetting the latch, a blank-out circuit inside the IC prevents the PWM comparator from resetting the latch for a short duration after the latch is set. This duration, called the blank-out time, is typically 250 ns and is specified as tmin (on) in the Electrical Characteristics section. Under extremely light load or no-load conditions, the energy delivered to the output capacitor when the external MOSFET is on during the blank-out time is more than what is delivered to the load. An overvoltage comparator inside the LM3481 prevents the output voltage from rising under these conditions by sensing the feedback (FB pin) voltage and resetting the RS latch. The latch remains in a reset state until the output decays to the nominal value. Thus the operating frequency decreases at light loads, resulting in excellent efficiency. Blank-Out prevents false reset PWM Comparator resets the RS latch VSL _ + PWM Comparator Oscillator Sets the RS Latch Tmin (on) Blank-Out time Figure 18. Basic Operation of the PWM Comparator Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 Submit Documentation Feedback 11 LM3481, LM3481-Q1 SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 www.ti.com 7.2 Functional Block Diagram Shutdown Detect VIN SYNC/Fixed Frequency detect FA/SYNC/SD Oscillator Set/Blankout Slope Compensation 6V Soft-start UVLO Bias Voltages UVLO Thermal Shutdown 40 eA 1.275V Reference Ramp Adjust - COMP + 6 V-I Converter EA Vfb + Vovp Overvoltage Comparator Switch Logic + + FB I-V Converter PWM Comparator R - S Q + VCC 220 mV ISEN Short-circuit Comparator One Shot + Switch Driver DR Level Shifter AGND PGND Figure 19. LM3481 Simplified Functional Block Diagram 7.3 Feature Description 7.3.1 Overvoltage Protection The LM3481 has overvoltage protection (OVP) for the output voltage. OVP is sensed at the feedback pin (FB). If at anytime the voltage at the feedback pin rises to VFB + VOVP, OVP is triggered. See the Electrical Characteristics section for limits on VFB and VOVP. OVP will cause the drive pin (DR) to go low, forcing the power MOSFET off. With the MOSFET off, the output voltage will drop. The LM3481 will begin switching again when the feedback voltage reaches VFB + (VOVP VOVP(HYS)). See the Electrical Characteristics section for limits on VOVP(HYS). The Error Amplifier is operationnal during OVP events. 7.3.2 Bias Voltage The internal bias of the LM3481 comes from either the internal bias voltage generator as shown in the block diagram or directly from the voltage at the VIN pin. At input voltages lower than 6 V the internal IC bias is the input voltage and at voltages above 6 V the internal bias voltage generator of the LM3481 provides the bias. The voltage for the gate driver is output on the VCC pin for compensation by an external capacitor (0.47F to 4.7F depending on the FET requirements). Biasing the VCC pin by an external voltage source should not be attempted. 12 Submit Documentation Feedback Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 LM3481, LM3481-Q1 www.ti.com SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 Feature Description (continued) 7.3.3 Slope Compensation Ramp The LM3481 uses a current mode control scheme. The main advantages of current mode control are inherent cycle-by-cycle current limit for the switch and simpler control loop characteristics. It is easy to parallel power stages using current mode control because current sharing is automatic. However there is a natural instability that will occur for duty cycles, D, greater than 50% if additional slope compensation is not addressed as described below. The current mode control scheme samples the inductor current, IL, and compares the sampled signal, Vsamp, to a internally generated control signal, Vc. The current sense resistor, RSEN, as shown in Figure 23, converts the sampled inductor current, IL, to the voltage signal, Vsamp, that is proportional to IL such that: Vsamp = IL x RSEN (1) The rising and falling slopes, M1 and -M2 respectively, of Vsamp are also proportional to the inductor current rising and falling slopes, Mon and -Moff respectively. Where Mon is the inductor slope during the switch on-time and -Moff is the inductor slope during the switch off-time and are related to M1 and -M2 by: M1 = Mon x RSEN -M2 = -Moff x RSEN (2) (3) For the boost topology: Mon = VIN / L -Moff = (VIN - VOUT) / L M1 = [VIN / L] x RSEN -M2 = [(VIN - VOUT) / L] x RSEN M2 = [(VOUT - VIN) / L] x RSEN (4) (5) (6) (7) (8) Current mode control has an inherent instability for duty cycles greater than 50%, as shown in Figure 20, where the control signal slope, MC, equals zero. In Figure 20, a small increase in the load current causes the sampled signal to increase by Vsamp0. The effect of this load change, Vsamp1, at the end of the first switching cycle is : 'Vsamp1 = - M2 D 'Vsamp0 'Vsamp0 = M1 1-D (9) From Equation 9, when D > 0.5, Vsamp1 will be greater than Vsamp0. In other words, the disturbance is divergent. So a very small perturbation in the load will cause the disturbance to increase. To ensure that the perturbed signal converges we must maintain: - M2 < M1 1 (10) Control Signal MC = 0 Perturbed Signal 'Vsamp0 -M2 _ M1 'Vsamp1 Steady State Signal Vsamp + PWM Comparator (1-D)TS DTS Figure 20. Subharmonic Oscillation for D>0.5 Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 Submit Documentation Feedback 13 LM3481, LM3481-Q1 SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 www.ti.com Feature Description (continued) Control Signal Compensation Ramp -MC VSL Perturbed Signal -M2 M1 'Vsamp0 _ Vsamp + PWM Comparator 'Vsamp1 Steady State Signal Vsamp DTS Control Signal (1-D)TS Figure 21. Compensation Ramp Avoids Subharmonic Oscillation To prevent the subharmonic oscillations, a compensation ramp is added to the control signal, as shown in Figure 21. With the compensation ramp, Vsamp1 and the convergence criteria are expressed by, 'Vsamp1 = - - M2 - MC 'Vsamp0 M 1 + MC (11) M 2 - MC <1 M1 + MC (12) The compensation ramp has been added internally in the LM3481. The slope of this compensation ramp has been selected to satisfy most applications, and it's value depends on the switching frequency. This slope can be calculated using the formula: MC = VSL x fS (13) In Equation 13, VSL is the amplitude of the internal compensation ramp and fS is the controller's switching frequency. Limits for VSL have been specified in the Electrical Characteristics section. To provide the user additional flexibility, a patented scheme has been implemented inside the IC to increase the slope of the compensation ramp externally, if the need arises. Adding a single external resistor, RSL(as shown in Figure 23) increases the amplitude of the compensation ramp as shown in Figure 22. Control Signal Compensation Ramp with RSL Control Signal Compensation Ramp without RSL 'VSL -MC VSL Figure 22. Additional Slope Compensation Added Using External Resistor RSL Where, VSL = K x RSL (14) K = 40 A typically and changes slightly as the switching frequency changes. Figure 24 shows the effect the current K has on VSLand different values of RSL as the switching frequency changes. A more general equation for the slope compensation ramp, MC, is shown below to include VSL caused by the resistor RSL. MC = (VSL + VSL) x fS 14 Submit Documentation Feedback (15) Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 LM3481, LM3481-Q1 www.ti.com SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 Feature Description (continued) It is good design practice to only add as much slope compensation as needed to avoid subharmonic oscillation. Additional slope compensation minimizes the influence of the sensed current in the control loop. With very large slope compensation the control loop characteristics are similar to a voltage mode regulator which compares the error voltage to a saw tooth waveform rather than the inductor current. VIN L1 D1 VOUT + DR COUT Q1 LM3481 ISEN RSL RSEN CSEN Figure 23. Increasing the Slope of the Compensation Ramp Figure 24. VSL vs RSL 7.3.4 Frequency Adjust, Synchronization, and Shutdown The switching frequency of the LM3481 can be adjusted between 100 kHz and 1 MHz using a single external resistor. This resistor must be connected between the FA/SYNC/SD pin and ground, as shown in Figure 25. Refer to the Typical Characteristics to determine the value of the resistor required for a desired switching frequency. Equation 16 can also be used to estimate the frequency adjust resistor. Where fS is in kHz and RFA in k. RFA = 22 x 103 - 5.74 fS (16) Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 Submit Documentation Feedback 15 LM3481, LM3481-Q1 SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 www.ti.com Feature Description (continued) The LM3481 can be synchronized to an external clock. The external clock must be connected between the FA/SYNC/SD pin and ground, as shown in Figure 26. The frequency adjust resistor may remain connected while synchronizing a signal, therefore if there is a loss of signal, the switching frequency will be set by the frequency adjust resistor. It is also necessary to have the width of the synchronization pulse wider than the duty cycle of the converter and to have the synchronization pulse width 300 ns. The FA/SYNC/SD pin also functions as a shutdown pin. If a high signal (refer to the Electrical Characteristics section for definition of high signal) appears on the FA/SYNC/SD pin, the LM3481 stops switching and goes into a low current mode. The total supply current of the IC reduces to 5 A, typically, under these conditions. Figure 27 and Figure 28 show an implementation of a shutdown function when operating in frequency adjust mode and synchronization mode, respectively. In frequency adjust mode, connecting the FA/SYNC/SD pin to ground forces the clock to run at a certain frequency. Pulling this pin high shuts down the IC. In frequency adjust or synchronization mode, a high signal for more than 30 s shuts down the IC. RFA FA/SYNC/SD LM3481 Figure 25. Frequency Adjust RFA FA/SYNC/SD LM3481 Freq. clock 100 kHz to 1 MHz Figure 26. Frequency Synchronization RFA 10 k: >1.3V FA/SYNC/SD LM3481 MOSFET State On-Normal Operation OFF- Shutdown Figure 27. Shutdown Operation in Frequency Adjust Mode 16 Submit Documentation Feedback Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 LM3481, LM3481-Q1 www.ti.com SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 Feature Description (continued) 30 Ps 40 k: FA/SYNC/SD LM3481 DR Figure 28. Shutdown Operation in Synchronization Mode 7.3.5 Undervoltage Lockout (UVLO) Pin The UVLO pin provides user programmable enable and shutdown thresholds. The UVLO pin is compared to an internal reference of 1.43 V (typical), and a resistor divider programs the enable threshold, VEN. When the IC is enabled, a 5-A current is sourced out of the UVLO pin, which effectively causes a hysteresis, and the UVLO shutdown threshold, VSH, is now lower than the enable threshold. Setting these thresholds requires two resistors connected from the VIN pin to the UVLO pin and from the UVLO pin to GND (see Figure 29). Select the desired enable, VEN, and UVLO shutdown, VSH, threshold voltages and use the Equation 17 and Equation 18 to determine the resistance values: 1.43V VSH 1.43V x 1 + IUVLO (c) VEN 1.43V (c) R8 = (17) VEN -1 R7 = R8 x (c) 1.43V (c) (18) VIN IUVLO R7 2 + UVLO R8 VUVLOSEN + - Figure 29. UVLO Pin Resistor Divider If the system is designed to work over wide input voltages, the voltage at the UVLO pin could exceed the voltage limit for the UVLO pin. In this case a zener diode can be connected between the UVLO pin and ground to prevent the UVLO voltage from rising above the maximum value. If the UVLO pin function is not desired, select R8 and R7 of equal magnitude greater than 100 k. This will allow VIN to be in control of the UVLO thresholds. The UVLO pin may also be used to implement the enable/disable function. If a signal pulls the UVLO pin below the 1.43 V (typical) threshold, the converter will be disabled. 7.3.6 Short-Circuit Protection When the voltage across the sense resistor (measured on the ISEN Pin) exceeds 220 mV, short-circuit current limit gets activated. A comparator inside the LM3481 reduces the switching frequency by a factor of 8 and maintains this condition until the short is removed. Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 Submit Documentation Feedback 17 LM3481, LM3481-Q1 SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 www.ti.com 7.4 Device Functional Modes The device is set to run as soon as the input voltage crosses above the UVLO set point and at a frequency set according to the FA/SYNC/SD pin pull-down resistor or to run at a frequency set by the waveform applied to the FA/SYNC/SD pin. If the FA/SYNC/SD pin is pulled high, the LM3481 enters shut-down mode. 18 Submit Documentation Feedback Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 LM3481, LM3481-Q1 www.ti.com SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 8 Application and Implementation NOTE Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI's customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality. 8.1 Application Information The LM3481 may be operated in either continuous or discontinuous conduction mode. The following applications are designed for continuous conduction operation. This mode of operation has higher efficiency and lower EMI characteristics than the discontinuous mode. 8.2 Typical Applications 8.2.1 Boost Converter R7 121 k: VIN = 5V R8 121 k: C8 390 pF ISEN VIN UVLO L1 6.8 PH 1 PF VCC C9 COMP RC 22.6 k: CC 82 nF FB RF2 20 k: AGND LM3481 D1 VOUT = 12V IOUT = 1.8A + COUT1, COUT2 150 PF PGND FA/SYNC/SD RFA 90.9 k: RF1 169 k: Q1 DR + CIN1, CIN2 150 PF CSEN 1 nF RSEN 20 m: Figure 30. Typical High Efficiency Step-Up (Boost) Converter using LM3481 The most common topology for the LM3481 is the boost or step-up topology. The boost converter converts a low input voltage into a higher output voltage. The basic configuration for a boost regulator is shown in Figure 31. In continuous conduction mode (when the inductor current never reaches zero at steady state), the boost regulator operates in two cycles. In the first cycle of operation, MOSFET Q is turned on and energy is stored in the inductor. During this cycle, diode D1 is reverse biased and load current is supplied by the output capacitor, COUT. In the second cycle, MOSFET Q is off and the diode is forward biased. The energy stored in the inductor is transferred to the load and output capacitor. The ratio of these two cycles determines the output voltage. The output voltage is defined as: (19) (ignoring the voltage drop across the MOSFET and the diode), or VOUT + VD1 - VQ = VIN - VQ 1-D (20) where D is the duty cycle of the switch, VD1 is the forward voltage drop of the diode, and VQ is the drop across the MOSFET when it is on. The following sections describe selection of components for a boost converter. Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 Submit Documentation Feedback 19 LM3481, LM3481-Q1 SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 www.ti.com Typical Applications (continued) L VIN + PWM D1 VOUT + Q L COUT L D1 + VOUT RLOAD + VOUT VIN + - - COUT + + + VIN COUT - A. First Cycle of Operation B. Second Cycle of Operation RLOAD - Figure 31. Simplified Boost Converter Diagram 8.2.1.1 Design Requirements To properly size the components for the application, the designer needs the following parameters: Input voltage range, output voltage, output current range and required switching frequency. These four main parameters will affect the choices of component available to achieve a proper system behavior. 8.2.1.2 Detailed Design Procedure 8.2.1.2.1 Custom Design with WEBENCH Tools Click here to create a custom design using the LM3481 device with the WEBENCH(R) Power Designer. 1. Start by entering your VIN, VOUT and IOUT requirements. 2. Optimize your design for key parameters like efficiency, footprint and cost using the optimizer dial and compare this design with other possible solutions from Texas Instruments. 3. WEBENCH Power Designer provides you with a customized schematic along with a list of materials with real time pricing and component availability. 4. In most cases, you will also be able to: - Run electrical simulations to see important waveforms and circuit performance, - Run thermal simulations to understand the thermal performance of your board, - Export your customized schematic and layout into popular CAD formats, - Print PDF reports for the design, and share your design with colleagues. 5. Get more information about WEBENCH tools at www.ti.com/webench. 8.2.1.2.2 Power Inductor Selection The inductor is one of the two energy storage elements in a boost converter. Figure 32 shows how the inductor current varies during a switching cycle. The current through an inductor is quantified as: (21) 20 Submit Documentation Feedback Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 LM3481, LM3481-Q1 www.ti.com SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 Typical Applications (continued) IL (A) VIN VIN - VOUT L L 'i L IL_AVG t (s) D*Ts Ts (a) ID (A) VIN - VOUT L ID_AVG = IOUT_AVG t (s) D*Ts Ts (b) ISW (A) VIN L ISW_AVG t (s) D*Ts Ts (c) Figure 32. (a) Inductor Current (b) Diode Current (c) Switch Current If VL(t) is constant, diL(t)/dt must be constant. Hence, for a given input voltage and output voltage, the current in the inductor changes at a constant rate. The important quantities in determining a proper inductance value are IL (the average inductor current) and iL (the inductor current ripple difference between the peak inductor current and the average inductor current). If iL is larger than IL, the inductor current drops to zero for a portion of the cycle and the converter operates in discontinuous conduction mode. If iL is smaller than IL, the inductor current stays above zero and the converter operates in continuous conduction mode. All the analysis in this data sheet assumes operation in continuous conduction mode. To operate in continuous conduction mode, the following conditions must be met: IL > iL (22) (23) (24) Choose the minimum IOUT to determine the minimum L. A common choice is to set (2 x iL) to 30% of IL. Choosing an appropriate core size for the inductor involves calculating the average and peak currents expected through the inductor. In a boost converter, IL = IOUT 1-D (25) (26) IL_peak = IL(max) + iL(max) 'iL = DVIN 2LfS (27) Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 Submit Documentation Feedback 21 LM3481, LM3481-Q1 SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 www.ti.com Typical Applications (continued) A core size with ratings higher than these values should be chosen. If the core is not properly rated, saturation will dramatically reduce overall efficiency. The LM3481 can be set to switch at very high frequencies. When the switching frequency is high, the converter can operate with very small inductor values. With a small inductor value, the peak inductor current can be extremely higher than the output currents, especially under light load conditions. The LM3481 senses the peak current through the switch. The peak current through the switch is the same as the peak current calculated above. 8.2.1.2.3 Programming the Output Voltage and Output Current The output voltage can be programmed using a resistor divider between the output and the feedback pins, as shown in Figure 33. The resistors are selected such that the voltage at the feedback pin is 1.275 V. RF1 and RF2 can be selected using the equation, VOUT = 1.275 (1+ RF1 ) RF2 (28) A 100-pF capacitor may be connected between the feedback and ground pins to reduce noise. The maximum amount of current that can be delivered at the output can be controlled by the sense resistor, RSEN. Current limit occurs when the voltage that is generated across the sense resistor equals the current sense threshold voltage, VSENSE. Limits for VSENSE have been specified in the Electrical Characteristics section. This can be expressed as: Isw(peak) x RSEN = VSENSE- D x VSL (29) The peak current through the switch is equal to the peak inductor current. Isw(peak) = IL(max) + iL (30) Therefore for a boost converter, Isw(peak) = (D x VIN) IOUT(max) + (1-D) (2 x fS x L) (31) Combining the two equations yields an expression for RSEN, RSEN = VSENSE - (D x VSL) IOUT(max) (1-D) + (D x VIN) (2 x fS x L) (32) Evaluate RSEN at the maximum and minimum VIN values and choose the smallest RSEN calculated. VIN L D1 DR VOUT Q + LM3481 COUT ISEN FB RF1 RSEN RF2 Figure 33. Adjusting the Output Voltage 22 Submit Documentation Feedback Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 LM3481, LM3481-Q1 www.ti.com SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 Typical Applications (continued) 8.2.1.2.4 Current Limit With Additional Slope Compensation If an external slope compensation resistor is used (see Figure 23) the internal control signal will be modified and this will have an effect on the current limit. If RSL is used, then this will add to the existing slope compensation. The command voltage, VCS, will then be given by: VCS = VSENSE - D x (VSL + VSL) (33) Where VSENSE is a defined parameter in the Electrical Characteristics section and VSL is the additional slope compensation generated as discussed in the Slope Compensation Ramp section. This changes the equation for RSEN to: RSEN = VSENSE - D x (VSL+'VSL) IOUT(max) (1-D) + (D x VIN) (2 x fS x L) (34) Note that because VSL = RSL x K as defined earlier, RSLcan be used to provide an additional method for setting the current limit. In some designs RSL can also be used to help filter noise to keep the ISEN pin quiet. 8.2.1.2.5 Power Diode Selection Observation of the boost converter circuit shows that the average current through the diode is the average load current, and the peak current through the diode is the peak current through the inductor. The diode should be rated to handle more than the inductor peak current. The peak diode current can be calculated using the formula: ID(Peak) = [IOUT/ (1-D)] + iL (35) In Equation 35, IOUT is the output current and iL has been defined in Figure 32. The peak reverse voltage for a boost converter is equal to the regulator output voltage. The diode must be capable of handling this peak reverse voltage. To improve efficiency, a low forward drop Schottky diode is recommended. 8.2.1.2.6 Power MOSFET Selection The drive pin, DR, of the LM3481 must be connected to the gate of an external MOSFET. In a boost topology, the drain of the external N-Channel MOSFET is connected to the inductor and the source is connected to the ground. The drive pin voltage, VDR, depends on the input voltage (see Typical Characteristics). In most applications, a logic level MOSFET can be used. For very low input voltages, a sub-logic level MOSFET should be used. The selected MOSFET directly controls the efficiency. The critical parameters for selection of a MOSFET are: * Minimum threshold voltage, VTH(MIN) * On-resistance, RDS(ON) * Total gate charge, Qg * Reverse transfer capacitance, CRSS * Maximum drain to source voltage, VDS(MAX) The off-state voltage of the MOSFET is approximately equal to the output voltage. VDS(MAX) of the MOSFET must be greater than the output voltage. The power losses in the MOSFET can be categorized into conduction losses and ac switching or transition losses. RDS(ON) is needed to estimate the conduction losses. The conduction loss, PCOND, is the I2R loss across the MOSFET. The maximum conduction loss is given by: PCOND(MAX) = IOUT(max) 2 DMAXRDS(ON) 1 - DMAX (36) where DMAX is the maximum duty cycle. DMAX = 1- VIN(MIN) VOUT (37) At high switching frequencies the switching losses may be the largest portion of the total losses. Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 Submit Documentation Feedback 23 LM3481, LM3481-Q1 SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 www.ti.com Typical Applications (continued) The switching losses are very difficult to calculate due to changing parasitics of a given MOSFET in operation. Often, the individual MOSFET datasheet does not give enough information to yield a useful result. Equation 38 and Equation 39 give a rough idea how the switching losses are calculated: ILmax x Vout x fSW x (tLH + tHL) 2 RGate Qgs x tLH = Qgd + VDR - Vgsth 2 PSW = (38) (39) 8.2.1.2.7 Input Capacitor Selection Due to the presence of an inductor at the input of a boost converter, the input current waveform is continuous and triangular, as shown in Figure 32. The inductor ensures that the input capacitor sees fairly low ripple currents. However, as the input capacitor gets smaller, the input ripple goes up. The rms current in the input capacitor is given by: ICIN(RMS) = 'iL / 3 = (VOUT - VIN)VIN 12 VOUTLfS (40) The input capacitor should be capable of handling the rms current. Although the input capacitor is not as critical in a boost application, low values can cause impedance interactions. Therefore a good quality capacitor should be chosen in the range of 100 F to 200 F. If a value lower than 100 F is used, then problems with impedance interactions or switching noise can affect the LM3481. To improve performance, especially with VIN below 8 V, it is recommended to use a 20 resistor at the input to provide a RC filter. This resistor is placed in series with the VIN pin with only a bypass capacitor attached to the VIN pin directly (see Figure 34). A 0.1-F or 1-F ceramic capacitor is necessary in this configuration. The bulk input capacitor and inductor will connect on the other side of the resistor with the input power supply. RIN VIN VIN LM3481 CBYPASS CIN Figure 34. Reducing IC Input Noise 8.2.1.2.8 Output Capacitor Selection The output capacitor in a boost converter provides all the output current when the inductor is charging. As a result it sees very large ripple currents. The output capacitor should be capable of handling the maximum rms current. The rms current in the output capacitor is: (41) Where 'iL = DVIN 2LfS (42) and D, the duty cycle is equal to (VOUT - VIN)/VOUT. The ESR and ESL of the output capacitor directly control the output ripple. Use capacitors with low ESR and ESL at the output for high efficiency and low ripple voltage. Surface mount tantalums, surface mount polymer electrolytic and polymer tantalum, Sanyo- OSCON, or multi-layer ceramic capacitors are recommended at the output. 8.2.1.2.9 Driver Supply Capacitor Selection A good quality ceramic bypass capacitor must be connected from the VCC pin to the PGND pin for proper operation. This capacitor supplies the transient current required by the internal MOSFET driver, as well as filtering the internal supply voltage for the controller. A value of between 0.47 F and 4.7 F is recommended. 24 Submit Documentation Feedback Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 LM3481, LM3481-Q1 www.ti.com SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 Typical Applications (continued) 8.2.1.2.10 Compensation For detailed explanation on how to select the right compensation components to attach to the compensation pin for a boost topology please see AN-1286 Compensation for the LM3478 Boost Controller (SNVA067). When calculating the Error Amplifier DC gain, AEA, ROUT = 152 k for the LM3481. 8.2.1.3 Application Curve Figure 35. Start-Up Pattern for a 5-Vin, 12-Vout Boost Converter Using LM3481 Boost Evaluation Module (C1: Inductor Current, C2: Vin, C3:Vout) Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 Submit Documentation Feedback 25 LM3481, LM3481-Q1 SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 www.ti.com Typical Applications (continued) 8.2.2 Typical SEPIC Converter R7 10 k: VIN = 3.0V to 24V D2 5.1V VIN ISEN 0.47 F RC 4.7 k: CC 0.1 PF UVLO COMP FB RF2 20 k: + CIN 15 PF, 35V x2 L1 10 PH 1 PF, ceramic LM3481 CS Q1 IRF7807 DR PGND AGND MBRS130LT3 VCC D1 L2 10 PH VOUT = 5V, 1A + COUT 100 PF, 10V FA/SYNC/SD RFA 40 k: RF1 60 k: CSEN 1 nF RSEN 0.05: Figure 36. Typical SEPIC Converter using LM3481 Because the LM3481 controls a low-side N-Channel MOSFET, it can also be used in SEPIC (Single Ended Primary Inductance Converter) applications. An example of SEPIC using the LM3481 is shown in Figure 36. As shown in Figure 36, the output voltage can be higher or lower than the input voltage. The SEPIC uses two inductors to step-up or step-down the input voltage. The inductors L1 and L2 can be two discrete inductors or two windings of a coupled transformer because equal voltages are applied across the inductor throughout the switching cycle. Using two discrete inductors allows use of catalog magnetics, as opposed to a custom transformer. The input ripple can be reduced along with size by using the coupled windings of transformer for L1 and L2. Due to the presence of the inductor L1 at the input, the SEPIC inherits all the benefits of a boost converter. One main advantage of SEPIC over a boost converter is the inherent input to output isolation. The capacitor CS isolates the input from the output and provides protection against shorted or malfunctioning load. Hence, the SEPIC is useful for replacing boost circuits when true shutdown is required. This means that the output voltage falls to 0V when the switch is turned off. In a boost converter, the output can only fall to the input voltage minus a diode drop. The duty cycle of a SEPIC is given by: (43) In Equation 43, VQ is the on-state voltage of the MOSFET, Q1, and VDIODE is the forward voltage drop of the diode. 8.2.2.1 Design Requirements To properly size the components for the application, the designer needs the following parameters: Input voltage range, output voltage, output current range and required switching frequency. These four main parameters will affect the choices of component available to achieve a proper system behavior. 8.2.2.2 Detailed Design Procedure 8.2.2.2.1 Power MOSFET Selection As in a boost converter, the parameters governing the selection of the MOSFET are the minimum threshold voltage, VTH(MIN), the on-resistance, RDS(ON), the total gate charge, Qg, the reverse transfer capacitance, CRSS, and the maximum drain to source voltage, VDS(MAX). The peak switch voltage in a SEPIC is given by: VSW(PEAK) = VIN + VOUT + VDIODE (44) The selected MOSFET should satisfy the condition: VDS(MAX) > VSW(PEAK) 26 Submit Documentation Feedback (45) Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 LM3481, LM3481-Q1 www.ti.com SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 Typical Applications (continued) The peak switch current is given by: ISWPEAK = IL1(AVG) + IOUT + 'IL1 + 'IL2 (46) 2 Where IL1 and IL2 are the peak-to-peak inductor ripple currents of inductors L1 and L2 respectively. The rms current through the switch is given by: (47) 8.2.2.2.2 Power Diode Selection The Power diode must be selected to handle the peak current and the peak reverse voltage. In a SEPIC, the diode peak current is the same as the switch peak current. The off-state voltage or peak reverse voltage of the diode is VIN + VOUT. Similar to the boost converter, the average diode current is equal to the output current. Schottky diodes are recommended. 8.2.2.2.3 Selection of Inductors L1 and L2 Proper selection of the inductors L1 and L2 to maintain constant current mode requires calculations of the following parameters. Average current in the inductors: (48) (49) IL2AVE = IOUT Peak-to-peak ripple current, to calculate core loss if necessary: (50) (51) Maintaining the condition IL > IL/2 to ensure continuous conduction mode yields the following minimum values for L1 and L2: L1 > L2 > (VIN - VQ)(1-D) 2IOUTfS (52) (VIN - VQ)D 2IOUTfS (53) Peak current in the inductor, to ensure the inductor does not saturate: (54) (55) IL1PK must be lower than the maximum current rating set by the current sense resistor. The value of L1 can be increased above the minimum recommended value to reduce input ripple and output ripple. However, once IL1 is less than 20% of IL1AVE, the benefit to output ripple is minimal. By increasing the value of L2 above the minimum recommendation, IL2 can be reduced, which in turn will reduce the output ripple voltage: 'VOUT = ( IOUT 1-D + 'IL2 2 ) ESR (56) where ESR is the effective series resistance of the output capacitor. Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 Submit Documentation Feedback 27 LM3481, LM3481-Q1 SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 www.ti.com Typical Applications (continued) If L1 and L2 are wound on the same core, then L1 = L2 = L. All the equations above will hold true if the inductance is replaced by 2L. A good choice for transformer with equal turns is Coiltronics CTX series Octopack. 8.2.2.2.4 Sense Resistor Selection The peak current through the switch, ISWPEAK, can be adjusted using the current sense resistor, RSEN, to provide a certain output current. Resistor RSEN can be selected using the formula: RSEN = VSENSE - D x (VSL+'VSL) ISWPEAK (57) 8.2.2.2.5 SEPIC Capacitor Selection The selection of SEPIC capacitor, CS, depends on the rms current. The rms current of the SEPIC capacitor is given by: (58) The SEPIC capacitor must be rated for a large ACrms current relative to the output power. This property makes the SEPIC much better suited to lower power applications where the rms current through the capacitor is small (relative to capacitor technology). The voltage rating of the SEPIC capacitor must be greater than the maximum input voltage. Tantalum capacitors are the best choice for SMT, having high rms current ratings relative to size. Ceramic capacitors could be used, but the low C values will tend to cause larger changes in voltage across the capacitor due to the large currents, and high C value ceramics are expensive. Electrolytics work well for through hole applications where the size required to meet the rms current rating can be accommodated. There is an energy balance between CS and L1, which can be used to determine the value of the capacitor. The basic energy balance equation is: 1 1 2 C 'V 2 = (L1)'IL1 2 S S 2 (59) Where (60) is the ripple voltage across the SEPIC capacitor, and 'IL1 = (VIN - VQ) D (L1)fS (61) is the ripple current through the inductor L1. The energy balance equation can be solved to provide a minimum value for CS: CS t L1 IOUT2 (VIN - VQ)2 (62) 8.2.2.2.6 Input Capacitor Selection Similar to a boost converter, the SEPIC has an inductor at the input. Hence, the input current waveform is continuous and triangular. The inductor ensures that the input capacitor sees fairly low ripple currents. However, as the input capacitor gets smaller, the input ripple goes up. The rms current in the input capacitor is given by: D ICIN(RMS) = 'IL1 / 12 = 2 3 VIN - VQ (L1)fS (63) The input capacitor should be capable of handling the rms current. Although the input capacitor is not as critical in a SEPIC application, low values can cause impedance interactions. Therefore a good quality capacitor should be chosen in the range of 100 F to 200 F. If a value lower than 100 F is used, then problems with impedance interactions or switching noise can affect the LM3481. To improve performance, especially with VIN below 8 V, it is recommended to use a 20 resistor at the input to provide a RC filter. This resistor is placed in series with the VIN pin with only a bypass capacitor attached to the VIN pin directly (see Figure 34). A 0.1 F or 1 F ceramic capacitor is necessary in this configuration. The bulk input capacitor and inductor will connect on the other side of the resistor with the input power supply. 28 Submit Documentation Feedback Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 LM3481, LM3481-Q1 www.ti.com SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 Typical Applications (continued) 8.2.2.2.7 Output Capacitor Selection The output capacitor of the SEPIC sees very large ripple currents similar to the output capacitor of a boost converter. The rms current through the output capacitor is given by: IRMS = 2 2 ISWPEAK2 - ISWPEAK ('IL1 + 'IL2) + ('IL1 + 'IL2) (1-D) - IOUT 3 (64) The ESR and ESL of the output capacitor directly control the output ripple. Use capacitors with low ESR and ESL at the output for high efficiency and low ripple voltage. Surface mount tantalums, surface mount polymer electrolytic and polymer tantalum, Sanyo- OSCON, or multi-layer ceramic capacitors are recommended at the output for low ripple. 8.2.2.3 Application Curve Figure 37. Start-Up Pattern for a 5-Vin, 12-Vout SEPIC Converter on LM3481 SEPIC Evaluation Module (C2: Vin, C3:Vout) Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 Submit Documentation Feedback 29 LM3481, LM3481-Q1 SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 www.ti.com 9 Power Supply Recommendations The LM3481 is designed to operate from various DC power supply including a car battery. If so, VIN input should be protected from reversal voltage and voltage dump over 48 Volts. The impedance of the input supply rail should be low enough that the input current transient does not cause drop below VIN UVLO level. If the input supply is connected by using long wires, additional bulk capacitance may be required in addition to normal input capacitor. 10 Layout 10.1 Layout Guidelines Good board layout is critical for switching controllers such as the LM3481. First the ground plane area must be sufficient for thermal dissipation purposes and second, appropriate guidelines must be followed to reduce the effects of switching noise. Switch mode converters are very fast switching devices. In such devices, the rapid increase of input current combined with the parasitic trace inductance generates unwanted Ldi/dt noise spikes. The magnitude of this noise tends to increase as the output current increases. This parasitic spike noise may turn into electromagnetic interference (EMI), and can also cause problems in device performance. Therefore, care must be taken in layout to minimize the effect of this switching noise. The current sensing circuit in current mode devices can be easily effected by switching noise. This noise can cause duty cycle jitter which leads to increased spectral noise. Although the LM3481 has 250 ns blanking time at the beginning of every cycle to ignore this noise, some noise may remain after the blanking time. The most important layout rule is to keep the AC current loops as small as possible. Figure 38 shows the current flow of a boost converter. The top schematic shows a dotted line which represents the current flow during onstate and the middle schematic shows the current flow during off-state. The bottom schematic shows the currents we refer to as AC currents. These currents are the most critical currents because current is changing in very short time periods. The dotted lined traces of the bottom schematic are the once to make as short as possible. Figure 38. Current Flow in a Boost Application The PGND and AGND pins have to be connected to the same ground very close to the IC. To avoid ground loop currents attach all the grounds of the system only at one point. A ceramic input capacitor should be connected as close as possible to the Vin pin and grounded close to the GND pin. For a layout example please see AN-2094 LM3481 SEPIC Evaluation Board (SNVA461). For more information about layout in switch mode power supplies refer to AN-1229 SIMPLE SWITCHER(R) PCB Layout Guidelines (SNVA054). 30 Submit Documentation Feedback Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 LM3481, LM3481-Q1 www.ti.com SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 10.2 Layout Example COMP UVLO FB LM3481 Isense FA Rsense OUT+ GND IN+ Figure 39. Typical Layout for a Boost Converter Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 Submit Documentation Feedback 31 LM3481, LM3481-Q1 SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 www.ti.com 11 Device and Documentation Support 11.1 Documentation Support 11.1.1 Custom Design with WEBENCH Tools Click here to create a custom design using the LM3481 device with the WEBENCH(R) Power Designer. 1. Start by entering your VIN, VOUT and IOUT requirements. 2. Optimize your design for key parameters like efficiency, footprint and cost using the optimizer dial and compare this design with other possible solutions from Texas Instruments. 3. WEBENCH Power Designer provides you with a customized schematic along with a list of materials with real time pricing and component availability. 4. In most cases, you will also be able to: - Run electrical simulations to see important waveforms and circuit performance, - Run thermal simulations to understand the thermal performance of your board, - Export your customized schematic and layout into popular CAD formats, - Print PDF reports for the design, and share your design with colleagues. 5. Get more information about WEBENCH tools at www.ti.com/webench. 11.1.2 Receiving Notification of Documentation Updates To receive notification of documentation updates, navigate to the device product folder on ti.com. In the upper right corner, click on Alert me to register and receive a weekly digest of any product information that has changed. For change details, review the revision history included in any revised document. 11.1.3 Related Documentation * AN-1286 Compensation for the LM3478 Boost Controller (SNVA067) * AN-2094 LM3481 SEPIC Evaluation Board (SNVA461) * AN-1229 SIMPLE SWITCHER(R) PCB Layout Guidelines (SNVA054) * SNVU111 330mW AC or DC Tiny Flyback Converter Power Supply (SNVU111) 11.2 Related Links The table below lists quick access links. Categories include technical documents, support and community resources, tools and software, and quick access to sample or buy. Table 1. Related Links PARTS PRODUCT FOLDER SAMPLE & BUY TECHNICAL DOCUMENTS TOOLS & SOFTWARE SUPPORT & COMMUNITY LM3481 Click here Click here Click here Click here Click here LM3481-Q1 Click here Click here Click here Click here Click here 11.3 Trademarks WEBENCH is a registered trademark of Texas Instruments. 11.4 Electrostatic Discharge Caution These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam during storage or handling to prevent electrostatic damage to the MOS gates. 11.5 Glossary SLYZ022 -- TI Glossary. This glossary lists and explains terms, acronyms, and definitions. 32 Submit Documentation Feedback Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 LM3481, LM3481-Q1 www.ti.com SNVS346F - NOVEMBER 2007 - REVISED NOVEMBER 2014 12 Mechanical, Packaging, and Orderable Information The following pages include mechanical, packaging, and orderable information. This information is the most current data available for the designated devices. This data is subject to change without notice and revision of this document. For browser-based versions of this data sheet, refer to the left-hand navigation. Copyright (c) 2007-2014, Texas Instruments Incorporated Product Folder Links: LM3481 LM3481-Q1 Submit Documentation Feedback 33 PACKAGE OPTION ADDENDUM www.ti.com 21-Sep-2016 PACKAGING INFORMATION Orderable Device Status (1) Package Type Package Pins Package Drawing Qty Eco Plan Lead/Ball Finish MSL Peak Temp (2) (6) (3) Op Temp (C) Device Marking (4/5) LM3481MM/NOPB ACTIVE VSSOP DGS 10 1000 Green (RoHS CU NIPDAUAG | CU SN Level-1-260C-UNLIM & no Sb/Br) -40 to 125 SJPB LM3481MMX/NOPB ACTIVE VSSOP DGS 10 3500 Green (RoHS CU NIPDAUAG | CU SN Level-1-260C-UNLIM & no Sb/Br) -40 to 125 SJPB LM3481QMM/NOPB ACTIVE VSSOP DGS 10 1000 Green (RoHS & no Sb/Br) CU SN Level-1-260C-UNLIM -40 to 125 SUAB LM3481QMMX/NOPB ACTIVE VSSOP DGS 10 3500 Green (RoHS & no Sb/Br) CU SN Level-1-260C-UNLIM -40 to 125 SUAB (1) The marketing status values are defined as follows: ACTIVE: Product device recommended for new designs. LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect. NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design. PREVIEW: Device has been announced but is not in production. Samples may or may not be available. OBSOLETE: TI has discontinued the production of the device. (2) Eco Plan - The planned eco-friendly classification: Pb-Free (RoHS), Pb-Free (RoHS Exempt), or Green (RoHS & no Sb/Br) - please check http://www.ti.com/productcontent for the latest availability information and additional product content details. TBD: The Pb-Free/Green conversion plan has not been defined. Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements for all 6 substances, including the requirement that lead not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes. Pb-Free (RoHS Exempt): This component has a RoHS exemption for either 1) lead-based flip-chip solder bumps used between the die and package, or 2) lead-based die adhesive used between the die and leadframe. The component is otherwise considered Pb-Free (RoHS compatible) as defined above. Green (RoHS & no Sb/Br): TI defines "Green" to mean Pb-Free (RoHS compatible), and free of Bromine (Br) and Antimony (Sb) based flame retardants (Br or Sb do not exceed 0.1% by weight in homogeneous material) (3) MSL, Peak Temp. - The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder temperature. (4) There may be additional marking, which relates to the logo, the lot trace code information, or the environmental category on the device. (5) Multiple Device Markings will be inside parentheses. Only one Device Marking contained in parentheses and separated by a "~" will appear on a device. If a line is indented then it is a continuation of the previous line and the two combined represent the entire Device Marking for that device. (6) Lead/Ball Finish - Orderable Devices may have multiple material finish options. Finish options are separated by a vertical ruled line. Lead/Ball Finish values may wrap to two lines if the finish value exceeds the maximum column width. Addendum-Page 1 Samples PACKAGE OPTION ADDENDUM www.ti.com 21-Sep-2016 Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is provided. TI bases its knowledge and belief on information provided by third parties, and makes no representation or warranty as to the accuracy of such information. Efforts are underway to better integrate information from third parties. TI has taken and continues to take reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on incoming materials and chemicals. TI and TI suppliers consider certain information to be proprietary, and thus CAS numbers and other limited information may not be available for release. In no event shall TI's liability arising out of such information exceed the total purchase price of the TI part(s) at issue in this document sold by TI to Customer on an annual basis. OTHER QUALIFIED VERSIONS OF LM3481, LM3481-Q1 : * Catalog: LM3481 * Automotive: LM3481-Q1 NOTE: Qualified Version Definitions: * Catalog - TI's standard catalog product * Automotive - Q100 devices qualified for high-reliability automotive applications targeting zero defects Addendum-Page 2 PACKAGE MATERIALS INFORMATION www.ti.com 8-Aug-2017 TAPE AND REEL INFORMATION *All dimensions are nominal Device Package Package Pins Type Drawing SPQ Reel Reel A0 Diameter Width (mm) (mm) W1 (mm) B0 (mm) K0 (mm) P1 (mm) W Pin1 (mm) Quadrant LM3481MM/NOPB VSSOP DGS 10 1000 178.0 12.4 5.3 3.4 1.4 8.0 12.0 Q1 LM3481MMX/NOPB VSSOP DGS 10 3500 330.0 12.4 5.3 3.4 1.4 8.0 12.0 Q1 LM3481QMM/NOPB VSSOP DGS 10 1000 178.0 12.4 5.3 3.4 1.4 8.0 12.0 Q1 LM3481QMMX/NOPB VSSOP DGS 10 3500 330.0 12.4 5.3 3.4 1.4 8.0 12.0 Q1 Pack Materials-Page 1 PACKAGE MATERIALS INFORMATION www.ti.com 8-Aug-2017 *All dimensions are nominal Device Package Type Package Drawing Pins SPQ Length (mm) Width (mm) Height (mm) LM3481MM/NOPB VSSOP DGS 10 1000 210.0 185.0 35.0 LM3481MMX/NOPB VSSOP DGS 10 3500 367.0 367.0 35.0 LM3481QMM/NOPB VSSOP DGS 10 1000 210.0 185.0 35.0 LM3481QMMX/NOPB VSSOP DGS 10 3500 367.0 367.0 35.0 Pack Materials-Page 2 IMPORTANT NOTICE Texas Instruments Incorporated (TI) reserves the right to make corrections, enhancements, improvements and other changes to its semiconductor products and services per JESD46, latest issue, and to discontinue any product or service per JESD48, latest issue. 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