TI Confidential - NDA Restrictions LM2731 www.ti.com SNVS217F – MAY 2004 – REVISED NOVEMBER 2012 LM2731 0.6/1.6 MHz Boost Converters With 22V Internal FET Switch in SOT-23 Check for Samples: LM2731 FEATURES DESCRIPTION • • The LM2731 switching regulators are current-mode boost converters operating at fixed frequencies of 1.6 MHz (“X” option) and 600 kHz (“Y” option). 1 2 • • • • • • • • 22V DMOS FET Switch 1.6 MHz (“X”), 0.6 MHz (“Y”) Switching Frequency Low RDS(ON) DMOS FET Switch Current up to 1.8A Wide Input Voltage Range (2.7V–14V) Low Shutdown Current (<1 µA) 5-Lead SOT-23 Package Uses Tiny Capacitors and Inductors Cycle-by-Cycle Current Limiting Internally Compensated The use of SOT-23 package, made possible by the minimal power loss of the internal 1.8A switch, and use of small inductors and capacitors result in the industry's highest power density. The 22V internal switch makes these solutions perfect for boosting to voltages up to 20V. These parts have a logic-level shutdown pin that can be used to reduce quiescent current and extend battery life. Protection is provided through cycle-by-cycle current limiting and thermal shutdown. Internal compensation simplifies design and reduces component count. APPLICATIONS • • • • • White LED Current Source PDA’s and Palm-Top Computers Digital Cameras Portable Phones and Games Local Boost Regulator Table 1. Switch Frequency X Y 1.6 MHz 0.6 MHz Typical Application Circuit VIN SHDN R3 51K U1 SW LM2731 ³;´ GND R1/117K FB SHDN GND C1 2.2PF 100 R2 13.3K CF 220pF 12V OUT 500mA (TYP) EFFICIENCY (%) 5 VIN Efficiency vs Load Current D1 MBR0520 L1/10PH C2 4.7PF 5 - 12V Boost ^y_ s Œ•]}v 90 80 70 0 100 200 300 400 500 LOAD CURRENT (mA) VIN SHDN R3 51K U1 GND SW LM2731 ³<´ SHDN GND C1 2.2PF 100 R1/40.5K FB R2 13.3K CF 470pF 5V OUT 700mA (TYP) C2 22PF EFFICIENCY (%) 3.3 VIN Efficiency vs Load Current D1 MBR0520 L1/6.8PH 3.3 -5V Boost ^z_ s Œ•]}v 90 80 70 0 200 400 600 800 LOAD CURRENT (mA) 1 2 Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet. All trademarks are the property of their respective owners. PRODUCTION DATA information is current as of publication date. Products conform to specifications per the terms of the Texas Instruments standard warranty. Production processing does not necessarily include testing of all parameters. Copyright © 2004–2012, Texas Instruments Incorporated TI Confidential - NDA Restrictions LM2731 SNVS217F – MAY 2004 – REVISED NOVEMBER 2012 www.ti.com Efficiency vs Load Current U1 VIN R3 51K SW R1/117K LM2731 ³<´ FB SHDN GND GND EFFICIENCY (%) 3.3 VIN SHDN D1 MBR0520 L1/6.8PH C1 2.2PF CF 270pF R2 13.3K 12V OUT 230mA (TYP) C2 10PF 100 90 80 70 60 50 40 30 20 10 0 3.3 -12V Boost ^z_ s Œ•]}v 50 100 150 200 250 0 LOAD (mA) R3 51K U1 GND SW R1/84K LM2731 ³;´ SHDN GND C1 2.2PF Efficiency vs Load Current 9V OUT 240mA (typ) FB CF 330pF R2 13.3K C2 4.7PF D2 D4 D3 D5 R4 EFFICIENCY (%) VIN SHDN D1 MBR0520 L1/10PH 3.3 VIN R5 100 90 80 70 60 50 40 30 20 10 0 3.3 -9V ^y_ s Œ•]}v 0 50 100 150 200 250 300 LOAD (mA) B1 LI-ION 3.3 - 4.2V L1 / 1.5 PH VIN + - R3 51K FLASH ENABLE SW LM2731"Y" SHDN GND 0 D1 MBR0520 C1 4.7PF FB WHITE LED's C2 4.7PF R2 120 Figure 1. White LED Flash Application Connection Diagram Top View Figure 2. 5-Lead SOT-23 Package See Package Number DBV0005A 2 Submit Documentation Feedback Copyright © 2004–2012, Texas Instruments Incorporated Product Folder Links: LM2731 TI Confidential - NDA Restrictions LM2731 www.ti.com SNVS217F – MAY 2004 – REVISED NOVEMBER 2012 PIN DESCRIPTIONS Pin Name 1 SW 2 GND 3 FB 4 SHDN 5 VIN Function Drain of the internal FET switch. Analog and power ground. Feedback point that connects to external resistive divider. Shutdown control input. Connect to Vin if the feature is not used. Analog and power input. 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. Absolute Maximum Ratings (1) Storage Temperature Range −65°C to +150°C Operating Junction Temperature Range −40°C to +125°C Lead Temp. (Soldering, 5 sec.) 300°C Power Dissipation (2) Internally Limited FB Pin Voltage −0.4V to +6V SW Pin Voltage −0.4V to +22V −0.4V to +14.5V Input Supply Voltage −0.4V to VIN + 0.3V SHDN Pin Voltage θJA (SOT23-5) ESD Rating (3) (1) (2) (3) 265°C/W Human Body Model 2 kV Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Electrical specifications do not apply when operating the device outside of the limits set forth under the operating ratings which specify the intended range of operating conditions. The maximum power dissipation which can be safely dissipated for any application is a function of the maximum junction temperature, TJ(MAX) = 125°C, the junction-to-ambient thermal resistance for the SOT-23 package, θJ-A = 265°C/W, and the ambient temperature, TA. The maximum allowable power dissipation at any ambient temperature for designs using this device can be calculated using the formula: . If power dissipation exceeds the maximum specified above, the internal thermal protection circuitry will protect the device by reducing the output voltage as required to maintain a safe junction temperature. The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. Submit Documentation Feedback Copyright © 2004–2012, Texas Instruments Incorporated Product Folder Links: LM2731 3 TI Confidential - NDA Restrictions LM2731 SNVS217F – MAY 2004 – REVISED NOVEMBER 2012 www.ti.com Electrical Characteristics Limits in standard typeface are for TJ = 25°C, and limits in boldface type apply over the full operating temperature range (−40°C ≤ TJ ≤ +125°C). Unless otherwise specified: VIN = 5V, VSHDN = 5V, IL = 0A. Parameter VIN Test Conditions Input Voltage VOUT (MIN) Minimum Output Voltage Under Load Min (1) Typ (2) 2.7 RL = 43Ω X Option (3) VIN = 2.7V 5.4 7 VIN = 3.3V 8 10 VIN = 2.7V 6 7.5 VIN = 3.3V 8.75 11 VIN = 5V RL = 43Ω Y Option (3) V V 15 VIN = 2.7V 3.75 5 VIN = 3.3V 5 6.5 VIN = 5V RL = 15Ω Y Option (3) Units 14 16 VIN = 5V RL = 15Ω X Option (3) Max (1) 10 VIN = 2.7V 4 VIN = 3.3V 5.5 VIN = 5V 5 7 10 ISW Switch Current Limit See (4) RDS(ON) Switch ON Resistance ISW = 100 mA Vin = 5V 260 400 500 ISW = 100 mA Vin = 3.3V 300 450 550 SHDNTH Shutdown Threshold 1.8 1.4 Device ON 2 1.5 Device OFF ISHDN 0.50 Shutdown Pin Bias Current VSHDN = 0 0 VSHDN = 5V VIN = 3V A 1.230 1.255 V nA IFB Feedback Pin Bias Current VFB = 1.23V 60 500 IQ Quiescent Current VSHDN = 5V, Switching "X" 2 3.0 VSHDN = 5V, Switching "Y" 1.0 2 VSHDN = 5V, Not Switching 400 500 VSHDN = 0 0.024 1 0.02 ΔVFB/ΔVIN FB Voltage Line Regulation 2.7V ≤ VIN ≤ 14V FSW Switching Frequency (5) “X” Option 1 1.6 1.85 “Y” Option 0.40 0.60 0.8 “X” Option 78 86 “Y” Option 88 93 IL (1) (2) (3) (4) (5) 4 Maximum Duty Cycle (5) Switch Leakage Not Switching VSW = 5V µA 2 Feedback Pin Reference Voltage DMAX V 0 VFB 1.205 mΩ mA µA %/V MHz % 1 µA Limits are guaranteed by testing, statistical correlation, or design. Typical values are derived from the mean value of a large quantity of samples tested during characterization and represent the most likely expected value of the parameter at room temperature. L = 10 µH, COUT = 4.7 µF, duty cycle = maximum Switch current limit is dependent on duty cycle (see Typical Performance Characteristics). Guaranteed limits are the same for Vin = 3.3V input. Submit Documentation Feedback Copyright © 2004–2012, Texas Instruments Incorporated Product Folder Links: LM2731 TI Confidential - NDA Restrictions LM2731 www.ti.com SNVS217F – MAY 2004 – REVISED NOVEMBER 2012 Typical Performance Characteristics Unless otherwise specified: VIN = 5V, SHDN pin tied to VIN. Iq Vin (Active) vs Temperature - "Y" 2.2 1.25 2.15 1.2 2.1 1.15 IQ VIN ACTIVE (mA) IQ VIN ACTIVE (mA) Iq Vin (Active) vs Temperature - "X" 2.05 2 1.95 1.1 1.05 1 1.9 0.95 1.85 1.8 -50 -25 0 25 50 75 0.9 -50 100 125 150 -25 50 75 100 125 150 Figure 3. Figure 4. Oscillator Frequency vs Temperature - "X" Oscillator Frequency vs Temperature - "Y" 0.6 1.58 VIN = 5V 1.56 OSCILLATOR FREQUENCY (MHz) OSCILLATOR FREQUENCY (MHz) 25 TEMPERATURE (oC) TEMPERATURE (oC) 1.54 1.52 VIN = 3.3V 1.5 1.48 1.46 1.44 1.42 1.4 -50 VIN = 5V 0.58 0.56 VIN = 3.3V 0.54 0.52 0.5 0.48 -25 0 25 50 75 -50 -25 100 125 150 TEMPERATURE (oC) 0 25 50 75 100 125 150 TEMPERATURE (oC) Figure 5. Figure 6. Max. Duty Cycle vs Temperature - "X" Max. Duty Cycle vs Temperature - "Y" 93 96.8 92.9 96.7 92.8 96.6 MAX DUTY CYCLE (%) MAX DUTY CYCLE (%) 0 92.7 92.6 VIN = 5V 92.5 92.4 VIN = 3.3V 92.3 92.2 96.5 VIN = 3.3V 96.4 96.3 VIN = 5V 96.2 96.1 96 92.1 -50 -25 0 25 50 75 100 125 150 95.9 -50 -25 0 25 50 75 100 125 150 TEMPERATURE (oC) o TEMPERATURE ( C) Figure 7. Figure 8. Submit Documentation Feedback Copyright © 2004–2012, Texas Instruments Incorporated Product Folder Links: LM2731 5 TI Confidential - NDA Restrictions LM2731 SNVS217F – MAY 2004 – REVISED NOVEMBER 2012 www.ti.com Typical Performance Characteristics (continued) Unless otherwise specified: VIN = 5V, SHDN pin tied to VIN. Iq Vin (Idle) vs Temperature Feedback Bias Current vs Temperature 0.09 375 0.08 FEEDBACK BIAS CURRENT (PA) 380 IQ VIN (IDLE) (PA) 370 365 360 355 350 345 340 -50 0 -25 50 25 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0 -50 75 100 125 150 -25 75 100 125 150 Figure 10. Feedback Voltage vs Temperature RDS(ON) vs Temperature 1.231 0.5 1.23 0.45 0.4 1.229 Vin = 3.3V 0.35 1.228 1.227 1.226 1.225 0.3 Vin = 5V 0.25 0.2 0.15 1.224 0.1 1.223 0.05 0 -40 0 -25 25 50 75 100 125 -40 0 -25 TEMPERATURE (oC) 25 50 75 100 125 TEMPERATURE (oC) Figure 11. Figure 12. Current Limit vs Temperature RDS(ON) vs VIN 350 2.6 300 2.5 250 2.4 RDS_ON (m:) CURRENT LIMIT (A) 50 25 Figure 9. 1.222 2.3 2.2 200 150 100 2.1 50 2 0 -40 -25 0 25 50 75 100 125 2.5 TEMPERATURE (oC) 3.5 4.5 5.5 6.5 7.5 8.5 9.5 VIN (V) Figure 13. 6 0 TEMPERATURE (oC) RDS(ON) (:) FEEDBACK VOLTAGE (V) TEMPERATURE (oC) Figure 14. Submit Documentation Feedback Copyright © 2004–2012, Texas Instruments Incorporated Product Folder Links: LM2731 TI Confidential - NDA Restrictions LM2731 www.ti.com SNVS217F – MAY 2004 – REVISED NOVEMBER 2012 Typical Performance Characteristics (continued) Unless otherwise specified: VIN = 5V, SHDN pin tied to VIN. Efficiency vs Load Current - "X" VIN = 2.7V, VOUT = 5V 100 100 90 80 80 EFFICIENCY (%) 90 EFFICIENCY (%) 70 60 50 40 70 60 50 40 30 30 20 20 10 10 0 0 100 50 200 150 250 Efficiency vs Load Current - "X" VIN = 3.3V, VOUT = 5V 0 300 0 100 200 LOAD (mA) 100 300 400 500 600 70 0 LOAD (mA) Figure 15. Figure 16. Efficiency vs Load Current - "X" VIN = 4.2V, VOUT = 5V Efficiency vs Load Current - "X" VIN = 2.7V, VOUT = 12V 80 90 70 60 70 EFFICIENCY (%) EFFICIENCY (%) 80 60 50 40 30 50 40 30 20 20 10 10 0 0 200 400 0 800 1000 1200 1400 600 0 10 20 30 40 50 LOAD (mA) LOAD (mA) Figure 17. Figure 18. Efficiency vs Load Current - "X" VIN = 3.3V, VOUT = 12V Efficiency vs Load Current - "X" VIN = 5V, VOUT = 12V 80 100 70 90 80 EFFICIENCY (%) EFFICIENCY (%) 60 50 40 30 70 60 50 40 30 20 20 10 10 0 0 0 20 40 60 80 100 120 140 160 0 100 200 300 400 500 600 LOAD (mA) LOAD (mA) Figure 19. Figure 20. Submit Documentation Feedback Copyright © 2004–2012, Texas Instruments Incorporated Product Folder Links: LM2731 7 TI Confidential - NDA Restrictions LM2731 SNVS217F – MAY 2004 – REVISED NOVEMBER 2012 www.ti.com Typical Performance Characteristics (continued) Unless otherwise specified: VIN = 5V, SHDN pin tied to VIN. Efficiency vs Load Current - "X" VIN = 5V, VOUT = 18V 100 90 90 80 80 EFFICIENCY (%) EFFICIENCY (%) 100 Efficiency vs Load Current - "Y" VIN = 2.7V, VOUT = 5V 70 60 50 40 70 60 50 40 30 30 20 20 10 10 0 0 50 0 100 150 200 250 300 350 0 100 150 200 250 300 350 400 50 LOAD (mA) LOAD (mA) Figure 22. Efficiency vs Load Current - "Y" VIN = 3.3V, VOUT = 5V Efficiency vs Load Current - "Y" VIN = 4.2V, VOUT = 5V 100 90 90 80 80 EFFICIENCY (%) EFFICIENCY (%) 100 Figure 21. 70 60 50 40 70 60 50 40 30 30 20 20 10 10 0 0 100 200 300 400 500 600 700 800 0 0 200 400 LOAD (mA) Figure 24. Efficiency vs Load Current - "Y" VIN = 2.7V, VOUT = 12V Efficiency vs Load Current - "Y" VIN = 3.3V, VOUT = 12V 100 90 90 80 80 70 70 60 50 40 30 50 40 30 10 10 0 0 20 40 60 80 0 LOAD (mA) 50 100 150 200 250 LOAD (mA) Figure 25. 8 60 20 20 0 800 1000 1200 1400 Figure 23. EFFICIENCY (%) EFFICIENCY (%) 100 600 LOAD (mA) Figure 26. Submit Documentation Feedback Copyright © 2004–2012, Texas Instruments Incorporated Product Folder Links: LM2731 TI Confidential - NDA Restrictions LM2731 www.ti.com SNVS217F – MAY 2004 – REVISED NOVEMBER 2012 Typical Performance Characteristics (continued) Unless otherwise specified: VIN = 5V, SHDN pin tied to VIN. Efficiency vs Load Current - "Y" VIN = 5V, VOUT = 12V 100 90 EFFICIENCY (%) 80 70 60 50 40 30 20 10 0 0 100 200 300 400 500 600 LOAD (mA) Figure 27. BLOCK DIAGRAM Submit Documentation Feedback Copyright © 2004–2012, Texas Instruments Incorporated Product Folder Links: LM2731 9 TI Confidential - NDA Restrictions LM2731 SNVS217F – MAY 2004 – REVISED NOVEMBER 2012 www.ti.com THEORY OF OPERATION The LM2731 is a switching converter IC that operates at a fixed frequency (0.6 or 1.6 MHz) for fast transient response over a wide input voltage range and incorporates pulse-by-pulse current limiting protection. Because this is current mode control, a 33 mΩ sense resistor in series with the switch FET is used to provide a voltage (which is proportional to the FET current) to both the input of the pulse width modulation (PWM) comparator and the current limit amplifier. At the beginning of each cycle, the S-R latch turns on the FET. As the current through the FET increases, a voltage (proportional to this current) is summed with the ramp coming from the ramp generator and then fed into the input of the PWM comparator. When this voltage exceeds the voltage on the other input (coming from the Gm amplifier), the latch resets and turns the FET off. Since the signal coming from the Gm amplifier is derived from the feedback (which samples the voltage at the output), the action of the PWM comparator constantly sets the correct peak current through the FET to keep the output voltage in regulation. Q1 and Q2 along with R3 - R6 form a bandgap voltage reference used by the IC to hold the output in regulation. The currents flowing through Q1 and Q2 will be equal, and the feedback loop will adjust the regulated output to maintain this. Because of this, the regulated output is always maintained at a voltage level equal to the voltage at the FB node "multiplied up" by the ratio of the output resistive divider. The current limit comparator feeds directly into the flip-flop that drives the switch FET. If the FET current reaches the limit threshold, the FET is turned off and the cycle terminated until the next clock pulse. The current limit input terminates the pulse regardless of the status of the output of the PWM comparator. Application Hints SELECTING THE EXTERNAL CAPACITORS The best capacitors for use with the LM2731 are multi-layer ceramic capacitors. They have the lowest ESR (equivalent series resistance) and highest resonance frequency which makes them optimum for use with high frequency switching converters. When selecting a ceramic capacitor, only X5R and X7R dielectric types should be used. Other types such as Z5U and Y5F have such severe loss of capacitance due to effects of temperature variation and applied voltage, they may provide as little as 20% of rated capacitance in many typical applications. Always consult capacitor manufacturer’s data curves before selecting a capacitor. High-quality ceramic capacitors can be obtained from Taiyo-Yuden, AVX, and Murata. SELECTING THE OUTPUT CAPACITOR A single ceramic capacitor of value 4.7 µF to 10 µF will provide sufficient output capacitance for most applications. If larger amounts of capacitance are desired for improved line support and transient response, tantalum capacitors can be used. Aluminum electrolytics with ultra low ESR such as Sanyo Oscon can be used, but are usually prohibitively expensive. Typical AI electrolytic capacitors are not suitable for switching frequencies above 500 kHz due to significant ringing and temperature rise due to self-heating from ripple current. An output capacitor with excessive ESR can also reduce phase margin and cause instability. In general, if electrolytics are used, it is recommended that they be paralleled with ceramic capacitors to reduce ringing, switching losses, and output voltage ripple. SELECTING THE INPUT CAPACITOR An input capacitor is required to serve as an energy reservoir for the current which must flow into the coil each time the switch turns ON. This capacitor must have extremely low ESR, so ceramic is the best choice. We recommend a nominal value of 2.2 µF, but larger values can be used. Since this capacitor reduces the amount of voltage ripple seen at the input pin, it also reduces the amount of EMI passed back along that line to other circuitry. 10 Submit Documentation Feedback Copyright © 2004–2012, Texas Instruments Incorporated Product Folder Links: LM2731 TI Confidential - NDA Restrictions LM2731 www.ti.com SNVS217F – MAY 2004 – REVISED NOVEMBER 2012 FEED-FORWARD COMPENSATION Although internally compensated, the feed-forward capacitor Cf is required for stability (see Figure 29). Adding this capacitor puts a zero in the loop response of the converter. The recommended frequency for the zero fz should be approximately 6 kHz. Cf can be calculated using the formula: Cf = 1 / (2 X π X R1 X fz) (1) SELECTING DIODES The external diode used in the typical application should be a Schottky diode. A 20V diode such as the MBR0520 is recommended. The MBR05XX series of diodes are designed to handle a maximum average current of 0.5A. For applications exceeding 0.5A average but less than 1A, a Microsemi UPS5817 can be used. LAYOUT HINTS High frequency switching regulators require very careful layout of components in order to get stable operation and low noise. All components must be as close as possible to the LM2731 device. It is recommended that a 4layer PCB be used so that internal ground planes are available. As an example, a recommended layout of components is shown: Figure 28. Recommended PCB Component Layout Some additional guidelines to be observed: 1. Keep the path between L1, D1, and C2 extremely short. Parasitic trace inductance in series with D1 and C2 will increase noise and ringing. 2. The feedback components R1, R2 and CF must be kept close to the FB pin of U1 to prevent noise injection on the FB pin trace. 3. If internal ground planes are available (recommended) use vias to connect directly to ground at pin 2 of U1, as well as the negative sides of capacitors C1 and C2. SETTING THE OUTPUT VOLTAGE The output voltage is set using the external resistors R1 and R2 (see Figure 29). A minimum value of 13.3 kΩ is recommended for R2 to establish a divider current of approximately 92 µA. R1 is calculated using the formula: R1 = R2 X (VOUT/1.23 − 1) (2) Submit Documentation Feedback Copyright © 2004–2012, Texas Instruments Incorporated Product Folder Links: LM2731 11 TI Confidential - NDA Restrictions LM2731 SNVS217F – MAY 2004 – REVISED NOVEMBER 2012 www.ti.com SWITCHING FREQUENCY The LM2731 is provided with two switching frequencies: the “X” version is typically 1.6 MHz, while the “Y” version is typically 600 kHz. The best frequency for a specific application must be determined based on the trade-offs involved: Higher switching frequency means the inductors and capacitors can be made smaller and cheaper for a given output voltage and current. The down side is that efficiency is slightly lower because the fixed switching losses occur more frequently and become a larger percentage of total power loss. EMI is typically worse at higher switching frequencies because more EMI energy will be seen in the higher frequency spectrum where most circuits are more sensitive to such interference. Figure 29. Basic Application Circuit DUTY CYCLE The maximum duty cycle of the switching regulator determines the maximum boost ratio of output-to-input voltage that the converter can attain in continuous mode of operation. The duty cycle for a given boost application is defined as: VOUT + VDIODE - VIN Duty Cycle = VOUT + VDIODE - VSW (3) This applies for continuous mode operation. INDUCTANCE VALUE The first question we are usually asked is: “How small can I make the inductor?” (because they are the largest sized component and usually the most costly). The answer is not simple and involves trade-offs in performance. Larger inductors mean less inductor ripple current, which typically means less output voltage ripple (for a given size of output capacitor). Larger inductors also mean more load power can be delivered because the energy stored during each switching cycle is: E = L/2 X (lp)2 (4) Where “lp” is the peak inductor current. An important point to observe is that the LM2731 will limit its switch current based on peak current. This means that since lp(max) is fixed, increasing L will increase the maximum amount of power available to the load. Conversely, using too little inductance may limit the amount of load current which can be drawn from the output. Best performance is usually obtained when the converter is operated in “continuous” mode at the load current range of interest, typically giving better load regulation and less output ripple. Continuous operation is defined as not allowing the inductor current to drop to zero during the cycle. It should be noted that all boost converters shift over to discontinuous operation as the output load is reduced far enough, but a larger inductor stays “continuous” over a wider load current range. To better understand these trade-offs, a typical application circuit (5V to 12V boost with a 10 µH inductor) will be analyzed. We will assume: VIN = 5V, VOUT = 12V, VDIODE = 0.5V, VSW = 0.5V (5) Since the frequency is 1.6 MHz (nominal), the period is approximately 0.625 µs. The duty cycle will be 62.5%, which means the ON time of the switch is 0.390 µs. It should be noted that when the switch is ON, the voltage across the inductor is approximately 4.5V. 12 Submit Documentation Feedback Copyright © 2004–2012, Texas Instruments Incorporated Product Folder Links: LM2731 TI Confidential - NDA Restrictions LM2731 www.ti.com SNVS217F – MAY 2004 – REVISED NOVEMBER 2012 Using the equation: V = L (di/dt) (6) We can then calculate the di/dt rate of the inductor which is found to be 0.45 A/µs during the ON time. Using these facts, we can then show what the inductor current will look like during operation: Figure 30. 10 µH Inductor Current, 5V–12V Boost (LM2731X) During the 0.390 µs ON time, the inductor current ramps up 0.176A and ramps down an equal amount during the OFF time. This is defined as the inductor “ripple current”. It can also be seen that if the load current drops to about 33 mA, the inductor current will begin touching the zero axis which means it will be in discontinuous mode. A similar analysis can be performed on any boost converter, to make sure the ripple current is reasonable and continuous operation will be maintained at the typical load current values. MAXIMUM SWITCH CURRENT The maximum FET switch current available before the current limiter cuts in is dependent on duty cycle of the application. This is illustrated in the graphs below which show typical values of switch current for both the "X" and "Y" versions as a function of effective (actual) duty cycle: 3000 SW CURRENT LIMIT (mA) 2500 VIN = 5V 2000 VIN = 3.3V 1500 1000 VIN = 2.7V 500 VIN = 3V 0 20 30 40 50 60 70 80 90 100 DUTY CYCLE (%) = [1 - EFF*(VIN / VOUT)] Figure 31. Switch Current Limit vs Duty Cycle - "X" 3000 SW CURRENT LIMIT (mA) 2500 VIN = 5V 2000 VIN = 3.3V 1500 VIN = 3V 1000 VIN = 2.7V 500 0 20 30 40 50 60 70 80 90 100 DUTY CYCLE (%) = [1 - EFF*(VIN / VOUT)] Figure 32. Switch Current Limit vs Duty Cycle - "Y" Submit Documentation Feedback Copyright © 2004–2012, Texas Instruments Incorporated Product Folder Links: LM2731 13 TI Confidential - NDA Restrictions LM2731 SNVS217F – MAY 2004 – REVISED NOVEMBER 2012 www.ti.com CALCULATING LOAD CURRENT As shown in the figure which depicts inductor current, the load current is related to the average inductor current by the relation: ILOAD = IIND(AVG) x (1 - DC) (7) Where "DC" is the duty cycle of the application. The switch current can be found by: ISW = IIND(AVG) + ½ (IRIPPLE) (8) Inductor ripple current is dependent on inductance, duty cycle, input voltage and frequency: IRIPPLE = DC x (VIN-VSW) / (f x L) (9) combining all terms, we can develop an expression which allows the maximum available load current to be calculated: ILOAD(max) = (1 - DC) x (ISW(max) - DC (VIN - VSW)) 2fL (10) The equation shown to calculate maximum load current takes into account the losses in the inductor or turn-OFF switching losses of the FET and diode. For actual load current in typical applications, we took bench data for various input and output voltages for both the "X" and "Y" versions of the LM2731 and displayed the maximum load current available for a typical device in graph form: MAX LOAD CURRENT (mA) 1200 1000 800 600 VOUT = 5V 400 VOUT = 10V VOUT = 8V VOUT = 12V 200 VOUT = 18V 0 2 3 4 5 6 7 8 9 10 11 VIN (V) Figure 33. Max. Load Current (typ) vs VIN - "X" MAX LOAD CURRENT (mA) 1200 1000 800 VOUT = 5V 600 VOUT = 8V 400 VOUT = 10V VOUT = 12V 200 0 2 3 4 5 6 7 8 VIN (V) Figure 34. Max. Load Current (typ) vs VIN - "Y" 14 Submit Documentation Feedback Copyright © 2004–2012, Texas Instruments Incorporated Product Folder Links: LM2731 TI Confidential - NDA Restrictions LM2731 www.ti.com SNVS217F – MAY 2004 – REVISED NOVEMBER 2012 DESIGN PARAMETERS VSW AND ISW The value of the FET "ON" voltage (referred to as VSW in the equations) is dependent on load current. A good approximation can be obtained by multiplying the "ON Resistance" of the FET times the average inductor current. FET on resistance increases at VIN values below 5V, since the internal N-FET has less gate voltage in this input voltage range (see Typical Performance Characteristics curves). Above VIN = 5V, the FET gate voltage is internally clamped to 5V. The maximum peak switch current the device can deliver is dependent on duty cycle. For higher duty cycles, see Typical Performance Characteristics curves. THERMAL CONSIDERATIONS At higher duty cycles, the increased ON time of the FET means the maximum output current will be determined by power dissipation within the LM2731 FET switch. The switch power dissipation from ON-state conduction is calculated by: P(SW) = DC x IIND(AVE)2 x RDS(ON) (11) There will be some switching losses as well, so some derating needs to be applied when calculating IC power dissipation. INDUCTOR SUPPLIERS Recommended suppliers of inductors for this product include, but are not limited to Sumida, Coilcraft, Panasonic, TDK and Murata. When selecting an inductor, make certain that the continuous current rating is high enough to avoid saturation at peak currents. A suitable core type must be used to minimize core (switching) losses, and wire power losses must be considered when selecting the current rating. SHUTDOWN PIN OPERATION The device is turned off by pulling the shutdown pin low. If this function is not going to be used, the pin should be tied directly to VIN. If the SHDN function will be needed, a pull-up resistor must be used to VIN (approximately 50k-100kΩ recommended). The SHDN pin must not be left unterminated. Submit Documentation Feedback Copyright © 2004–2012, Texas Instruments Incorporated Product Folder Links: LM2731 15 IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve 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. Buyers should obtain the latest relevant information before placing orders and should verify that such information is current and complete. All semiconductor products (also referred to herein as “components”) are sold subject to TI’s terms and conditions of sale supplied at the time of order acknowledgment. TI warrants performance of its components to the specifications applicable at the time of sale, in accordance with the warranty in TI’s terms and conditions of sale of semiconductor products. Testing and other quality control techniques are used to the extent TI deems necessary to support this warranty. Except where mandated by applicable law, testing of all parameters of each component is not necessarily performed. TI assumes no liability for applications assistance or the design of Buyers’ products. Buyers are responsible for their products and applications using TI components. To minimize the risks associated with Buyers’ products and applications, Buyers should provide adequate design and operating safeguards. TI does not warrant or represent that any license, either express or implied, is granted under any patent right, copyright, mask work right, or other intellectual property right relating to any combination, machine, or process in which TI components or services are used. Information published by TI regarding third-party products or services does not constitute a license to use such products or services or a warranty or endorsement thereof. Use of such information may require a license from a third party under the patents or other intellectual property of the third party, or a license from TI under the patents or other intellectual property of TI. Reproduction of significant portions of TI information in TI data books or data sheets is permissible only if reproduction is without alteration and is accompanied by all associated warranties, conditions, limitations, and notices. TI is not responsible or liable for such altered documentation. Information of third parties may be subject to additional restrictions. Resale of TI components or services with statements different from or beyond the parameters stated by TI for that component or service voids all express and any implied warranties for the associated TI component or service and is an unfair and deceptive business practice. TI is not responsible or liable for any such statements. Buyer acknowledges and agrees that it is solely responsible for compliance with all legal, regulatory and safety-related requirements concerning its products, and any use of TI components in its applications, notwithstanding any applications-related information or support that may be provided by TI. Buyer represents and agrees that it has all the necessary expertise to create and implement safeguards which anticipate dangerous consequences of failures, monitor failures and their consequences, lessen the likelihood of failures that might cause harm and take appropriate remedial actions. Buyer will fully indemnify TI and its representatives against any damages arising out of the use of any TI components in safety-critical applications. In some cases, TI components may be promoted specifically to facilitate safety-related applications. With such components, TI’s goal is to help enable customers to design and create their own end-product solutions that meet applicable functional safety standards and requirements. Nonetheless, such components are subject to these terms. No TI components are authorized for use in FDA Class III (or similar life-critical medical equipment) unless authorized officers of the parties have executed a special agreement specifically governing such use. Only those TI components which TI has specifically designated as military grade or “enhanced plastic” are designed and intended for use in military/aerospace applications or environments. Buyer acknowledges and agrees that any military or aerospace use of TI components which have not been so designated is solely at the Buyer's risk, and that Buyer is solely responsible for compliance with all legal and regulatory requirements in connection with such use. TI has specifically designated certain components as meeting ISO/TS16949 requirements, mainly for automotive use. In any case of use of non-designated products, TI will not be responsible for any failure to meet ISO/TS16949. Products Applications Audio www.ti.com/audio Automotive and Transportation www.ti.com/automotive Amplifiers amplifier.ti.com Communications and Telecom www.ti.com/communications Data Converters dataconverter.ti.com Computers and Peripherals www.ti.com/computers DLP® Products www.dlp.com Consumer Electronics www.ti.com/consumer-apps DSP dsp.ti.com Energy and Lighting www.ti.com/energy Clocks and Timers www.ti.com/clocks Industrial www.ti.com/industrial Interface interface.ti.com Medical www.ti.com/medical Logic logic.ti.com Security www.ti.com/security Power Mgmt power.ti.com Space, Avionics and Defense www.ti.com/space-avionics-defense Microcontrollers microcontroller.ti.com Video and Imaging www.ti.com/video RFID www.ti-rfid.com OMAP Applications Processors www.ti.com/omap TI E2E Community e2e.ti.com Wireless Connectivity www.ti.com/wirelessconnectivity Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265 Copyright © 2013, Texas Instruments Incorporated