杰鸿顺电子 TEX:61306581 August 2006 LM2831 High Frequency 1.5A Load - Step-Down DC-DC Regulator General Description Features The LM2831 regulator is a monolithic, high frequency, PWM step-down DC/DC converter in a 5 pin SOT23 and a 6 Pin LLP package. It provides all the active functions to provide local DC/DC conversion with fast transient response and accurate regulation in the smallest possible PCB area. With a minimum of external components, the LM2831 is easy to use. The ability to drive 1.5A loads with an internal 130 mΩ PMOS switch using state-of-the-art 0.5 µm BiCMOS technology results in the best power density available. The worldclass control circuitry allows on-times as low as 30ns, thus supporting exceptionally high frequency conversion over the entire 3V to 5.5V input operating range down to the minimum output voltage of 0.6V. Switching frequency is internally set to 550 kHz, 1.6 MHz, or 3.0 MHz, allowing the use of extremely small surface mount inductors and chip capacitors. Even though the operating frequency is high, efficiencies up to 93% are easy to achieve. External shutdown is included, featuring an ultra-low stand-by current of 30 nA. The LM2831 utilizes current-mode control and internal compensation to provide high-performance regulation over a wide range of operating conditions. Additional features include internal soft-start circuitry to reduce inrush current, pulse-by-pulse current limit, thermal shutdown, and output over-voltage protection. n n n n n QQ 鸿 07 :5 顺电 55 30 -6 93 子 13 06 58 2 n n n n n n Space Saving SOT23-5 Package Input voltage range of 3.0V to 5.5V Output voltage range of 0.6V to 4.5V 1.5A output current High Switching Frequencies 1.6MHz (LM2831X) 0.55MHz (LM2831Y) 3.0MHz (LM2831Z) 130mΩ PMOS switch 0.6V, 2% Internal Voltage Reference Internal soft-start Current mode, PWM operation Thermal Shutdown Over voltage protection Applications Local 5V to Vcore Step-Down Converters Core Power in HDDs Set-Top Boxes USB Powered Devices DSL Modems 杰 n n n n n Typical Application Circuit LM2831 High Frequency 1.5A Load - Step-Down DC-DC Regulator 0755-61306582 20174864 20174881 © 2006 National Semiconductor Corporation QQ:53093 DS201748 www.national.com 百度一下：杰鸿顺电子 QQ:85811868 LM2831 0755-61306582 杰鸿顺电子 TEX:61306581 Connection Diagrams 20174803 20174801 5-Pin SOT-23 6-Pin LLP Ordering Information Order Number Frequency Option LM2831XMF LM2831XMFX LM2831XSD LM2831YMF LM2831YSD LM2831ZMF LM2831ZSD Top Mark SOT23-5 MF05A SKYB LLP-6 SDE06A L193B SOT23-5 MF05A SKZB LLP-6 SDE06A L194B SOT23-5 MF05A SLAB LLP-6 SDE06A L195B 0.55MHz LM2831YSDX LM2831ZMFX NSC Package Drawing 1.6MHz LM2831XSDX LM2831YMFX Package Type 3MHz LM2831ZSDX Supplied As 1000 units Tape and Reel 3000 units Tape and Reel 1000 units Tape and Reel 4500 units Tape and Reel 1000 units Tape and Reel 3000 units Tape and Reel 1000 units Tape and Reel 4500 units Tape and Reel 1000 units Tape and Reel 3000 units Tape and Reel 1000 units Tape and Reel 4500 units Tape and Reel NOPB versions available as well www.national.com QQ:53093 2 百度一下：杰鸿顺电子 QQ:85811868 杰鸿顺电子 TEX:61306581 LM2831 0755-61306582 Pin Descriptions 5-Pin SOT23 Pin Name 1 SW 2 GND Function Output switch. Connect to the inductor and catch diode. Signal and power ground pin. Place the bottom resistor of the feedback network as close as possible to this pin. 3 FB Feedback pin. Connect to external resistor divider to set output voltage. 4 EN Enable control input. Logic high enables operation. Do not allow this pin to float or be greater than VIN + 0.3V. 5 VIN Input supply voltage. Pin Descriptions 6-Pin LLP Pin Name 1 FB Function 2 GND 3 SW 4 VIND Power Input supply. 5 VINA Control circuitry supply voltage. Connect VINA to VIND on PC board. 6 EN DAP Die Attach Pad Feedback pin. Connect to external resistor divider to set output voltage. Signal and power ground pin. Place the bottom resistor of the feedback network as close as possible to this pin. Output switch. Connect to the inductor and catch diode. Enable control input. Logic high enables operation. Do not allow this pin to float or be greater than VINA + 0.3V. Connect to system ground for low thermal impedance, but it cannot be used as a primary GND connection. 3 QQ:53093 百度一下：杰鸿顺电子 www.national.com QQ:85811868 LM2831 0755-61306582 杰鸿顺电子 TEX:61306581 Absolute Maximum Ratings (Note 1) Storage Temperature If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Soldering Information VIN -0.5V to 7V FB Voltage -0.5V to 3V EN Voltage -0.5V to 7V SW Voltage -0.5V to 7V ESD Susceptibility Infrared or Convection Reflow (15 sec) 220˚C Operating Ratings VIN 3V to 5.5V Junction Temperature 2kV Junction Temperature (Note 2) −65˚C to +150˚C −40˚C to +125˚C 150˚C Electrical Characteristics VIN = 5V unless otherwise indicated under the Conditions column. Limits in standard type are for TJ = 25˚C only; limits in boldface type apply over the junction temperature (TJ) range of -40˚C to +125˚C. Minimum and Maximum limits are guaranteed through test, design, or statistical correlation. Typical values represent the most likely parametric norm at TJ = 25˚C, and are provided for reference purposes only. Symbol VFB ∆VFB/VIN IB UVLO Parameter Conditions Min Typ Max 0.588 0.600 0.612 Feedback Voltage Feedback Voltage Line Regulation VIN = 3V to 5V 0.02 0.1 100 nA VIN Rising 2.73 2.90 V Feedback Input Bias Current Undervoltage Lockout DMAX DMIN RDS(ON) ICL VEN_TH Maximum Duty Cycle Minimum Duty Cycle Switch On Resistance Switch Current Limit 1.85 LM2831-X 1.2 1.6 LM2831-Y 0.4 0.55 0.7 LM2831-Z 2.25 3.0 3.75 LM2831-X 86 94 LM2831-Y 90 96 LM2831-Z 82 90 0.43 LM2831-X 5 LM2831-Y 2 LM2831-Z 7 LLP-6 Package 150 SOT23-5 Package 130 VIN = 3.3V 1.8 Switch Leakage Enable Pin Current V 1.95 MHz % % 195 2.5 mΩ A 0.4 Enable Threshold Voltage IEN www.national.com 2.3 Shutdown Threshold Voltage ISW IQ QQ:53093 Switching Frequency V %/V VIN Falling UVLO Hysteresis FSW Units LLP-6 and SOT23-5 Package 1.8 V 100 nA Sink/Source 100 nA LM2831X VFB = 0.55 3.3 5 Quiescent Current (switching) LM2831Y VFB = 0.55 2.8 4.5 LM2831Z VFB = 0.55 4.3 6.5 Quiescent Current (shutdown) All Options VEN = 0V 30 mA nA 4 百度一下：杰鸿顺电子 QQ:85811868 杰鸿顺电子 TEX:61306581 Electrical Characteristics VIN = 5V unless otherwise indicated under the Conditions column. Limits in standard type are for TJ = 25˚C only; limits in boldface type apply over the junction temperature (TJ) range of -40˚C to +125˚C. Minimum and Maximum limits are guaranteed through test, design, or statistical correlation. Typical values represent the most likely parametric norm at TJ = 25˚C, and are provided for reference purposes only. (Continued) Symbol Parameter θJA Junction to Ambient 0 LFPM Air Flow (Note 3) θJC Junction to Case (Note 3) TSD Thermal Shutdown Temperature Conditions Min Typ LLP-6 Package 80 SOT23-5 Package 118 LLP-6 Package 18 SOT23-5 Package 80 165 Max LM2831 0755-61306582 Units ˚C/W ˚C/W ˚C Note 1: Absolute maximum ratings indicate limits beyond which damage to the device may occur. Operating Range indicates conditions for which the device is intended to be functional, but does not guarantee specfic performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics. Note 2: Thermal shutdown will occur if the junction temperature exceeds the maximum junction temperature of the device. Note 3: Applies for packages soldered directly onto a 3” x 3” PC board with 2oz. copper on 4 layers in still air. 5 QQ:53093 百度一下：杰鸿顺电子 www.national.com QQ:85811868 LM2831 0755-61306582 杰鸿顺电子 TEX:61306581 Typical Performance Characteristics All curves taken at VIN = 5.0V with configuration in typical application circuit shown in Application Information section of this datasheet. TJ = 25˚C, unless otherwise specified. η vs Load "X" Vin = 5V, Vo = 1.8V & 3.3V η vs Load - "Y" Vin = 5V, Vo = 3.3V & 1.8V 20174839 20174886 η vs Load "Z" Vin = 5V, Vo = 3.3V & 1.8V η vs Load "X, Y and Z" Vin = 5V, Vo = 3.3V & 1.8V 20174842 20174885 Load Regulation Vin = 3.3V, Vo = 1.8V (All Options) Load Regulation Vin = 5V, Vo = 1.8V (All Options) 20174845 20174844 www.national.com QQ:53093 6 百度一下：杰鸿顺电子 QQ:85811868 杰鸿顺电子 TEX:61306581 Typical Performance Characteristics All curves taken at VIN = 5.0V with configuration in typical application circuit shown in Application Information section of this datasheet. TJ = 25˚C, unless otherwise specified. (Continued) Load Regulation Vin = 5V, Vo = 3.3V (All Options) LM2831 0755-61306582 Oscillator Frequency vs Temperature - "X" 20174846 20174824 Oscillator Frequency vs Temperature - "Y" Oscillator Frequency vs Temperature - "Z" 20174836 20174825 Current Limit vs Temperature Vin = 3.3V RDSON vs Temperature (LLP-6 Package) 20174883 20174823 7 QQ:53093 百度一下：杰鸿顺电子 www.national.com QQ:85811868 LM2831 0755-61306582 杰鸿顺电子 TEX:61306581 Typical Performance Characteristics All curves taken at VIN = 5.0V with configuration in typical application circuit shown in Application Information section of this datasheet. TJ = 25˚C, unless otherwise specified. (Continued) RDSON vs Temperature (SOT23-5 Package) LM2831X IQ (Quiescent Current) 20174884 20174828 LM2831Y IQ (Quiescent Current) LM2831Z IQ (Quiescent Current) 20174829 20174837 www.national.com QQ:53093 8 百度一下：杰鸿顺电子 QQ:85811868 杰鸿顺电子 TEX:61306581 Typical Performance Characteristics All curves taken at VIN = 5.0V with configuration in typical application circuit shown in Application Information section of this datasheet. TJ = 25˚C, unless otherwise specified. (Continued) Line Regulation Vo = 1.8V, Io = 500mA VFB vs Temperature 20174827 20174853 Gain vs Frequency (Vin = 5V, Vo = 1.2V @ 1A) Phase Plot vs Frequency (Vin = 5V, Vo = 1.2V @ 1A) 20174856 20174857 9 QQ:53093 LM2831 0755-61306582 百度一下：杰鸿顺电子 www.national.com QQ:85811868 LM2831 0755-61306582 杰鸿顺电子 TEX:61306581 Simplified Block Diagram 20174804 FIGURE 1. www.national.com QQ:53093 10 百度一下：杰鸿顺电子 QQ:85811868 杰鸿顺电子 TEX:61306581 LM2831 0755-61306582 Applications Information THEORY OF OPERATION OUTPUT OVERVOLTAGE PROTECTION The LM2831 is a constant frequency PWM buck regulator IC that delivers a 1.5A load current. The regulator has a preset switching frequency of 550kHz, 1.6MHz, or 3.0MHz. This high frequency allows the LM2831 to operate with small surface mount capacitors and inductors, resulting in a DC/DC converter that requires a minimum amount of board space. The LM2831 is internally compensated, so it is simple to use and requires few external components. The LM2831 uses current-mode control to regulate the output voltage. The following operating description of the LM2831 will refer to the Simplified Block Diagram (Figure 1) and to the waveforms in Figure 2. The LM2831 supplies a regulated output voltage by switching the internal PMOS control switch at constant frequency and variable duty cycle. A switching cycle begins at the falling edge of the reset pulse generated by the internal oscillator. When this pulse goes low, the output control logic turns on the internal PMOS control switch. During this on-time, the SW pin voltage (VSW) swings up to approximately VIN, and the inductor current (IL) increases with a linear slope. IL is measured by the current sense amplifier, which generates an output proportional to the switch current. The sense signal is summed with the regulator’s corrective ramp and compared to the error amplifier’s output, which is proportional to the difference between the feedback voltage and VREF. When the PWM comparator output goes high, the output switch turns off until the next switching cycle begins. During the switch off-time, inductor current discharges through the Schottky catch diode, which forces the SW pin to swing below ground by the forward voltage (VD) of the Schottky catch diode. The regulator loop adjusts the duty cycle (D) to maintain a constant output voltage. The over-voltage comparator compares the FB pin voltage to a voltage that is 15% higher than the internal reference VREF. Once the FB pin voltage goes 15% above the internal reference, the internal PMOS control switch is turned off, which allows the output voltage to decrease toward regulation. UNDERVOLTAGE LOCKOUT Under-voltage lockout (UVLO) prevents the LM2831 from operating until the input voltage exceeds 2.73V (typ). The UVLO threshold has approximately 430 mV of hysteresis, so the part will operate until VIN drops below 2.3V (typ). Hysteresis prevents the part from turning off during power up if VIN is non-monotonic. CURRENT LIMIT The LM2831 uses cycle-by-cycle current limiting to protect the output switch. During each switching cycle, a current limit comparator detects if the output switch current exceeds 2.5A (typ), and turns off the switch until the next switching cycle begins. THERMAL SHUTDOWN Thermal shutdown limits total power dissipation by turning off the output switch when the IC junction temperature exceeds 165˚C. After thermal shutdown occurs, the output switch doesn’t turn on until the junction temperature drops to approximately 150˚C. Design Guide INDUCTOR SELECTION The Duty Cycle (D) can be approximated quickly using the ratio of output voltage (VO) to input voltage (VIN): The catch diode (D1) forward voltage drop and the voltage drop across the internal PMOS must be included to calculate a more accurate duty cycle. Calculate D by using the following formula: VSW can be approximated by: VSW = IOUT x RDSON 20174866 The diode forward drop (VD) can range from 0.3V to 0.7V depending on the quality of the diode. The lower the VD, the higher the operating efficiency of the converter. The inductor value determines the output ripple current. Lower inductor values decrease the size of the inductor, but increase the output ripple current. An increase in the inductor value will decrease the output ripple current. One must ensure that the minimum current limit (1.8A) is not exceeded, so the peak current in the inductor must be calculated. The peak current (ILPK) in the inductor is calculated by: ILPK = IOUT + ∆iL FIGURE 2. Typical Waveforms SOFT-START This function forces VOUT to increase at a controlled rate during start up. During soft-start, the error amplifier’s reference voltage ramps from 0V to its nominal value of 0.6V in approximately 600 µs. This forces the regulator output to ramp up in a controlled fashion, which helps reduce inrush current. 11 QQ:53093 百度一下：杰鸿顺电子 www.national.com QQ:85811868 LM2831 0755-61306582 杰鸿顺电子 Design Guide TEX:61306581 INPUT CAPACITOR (Continued) An input capacitor is necessary to ensure that VIN does not drop excessively during switching transients. The primary specifications of the input capacitor are capacitance, voltage, RMS current rating, and ESL (Equivalent Series Inductance). The recommended input capacitance is 22 µF.The input voltage rating is specifically stated by the capacitor manufacturer. Make sure to check any recommended deratings and also verify if there is any significant change in capacitance at the operating input voltage and the operating temperature. The input capacitor maximum RMS input current rating (IRMS-IN) must be greater than: 20174805 FIGURE 3. Inductor Current Neglecting inductor ripple simplifies the above equation to: In general, ∆iL = 0.1 x (IOUT) → 0.2 x (IOUT) If ∆iL = 20% of 1.50A, the peak current in the inductor will be 1.8A. The minimum guaranteed current limit over all operating conditions is 1.8A. One can either reduce ∆iL, or make the engineering judgment that zero margin will be safe enough. The typical current limit is 2.5A. The LM2831 operates at frequencies allowing the use of ceramic output capacitors without compromising transient response. Ceramic capacitors allow higher inductor ripple without significantly increasing output ripple. See the output capacitor section for more details on calculating output voltage ripple. Now that the ripple current is determined, the inductance is calculated by: It can be shown from the above equation that maximum RMS capacitor current occurs when D = 0.5. Always calculate the RMS at the point where the duty cycle D is closest to 0.5. The ESL of an input capacitor is usually determined by the effective cross sectional area of the current path. A large leaded capacitor will have high ESL and a 0805 ceramic chip capacitor will have very low ESL. At the operating frequencies of the LM2831, leaded capacitors may have an ESL so large that the resulting impedance (2πfL) will be higher than that required to provide stable operation. As a result, surface mount capacitors are strongly recommended. Sanyo POSCAP, Tantalum or Niobium, Panasonic SP, and multilayer ceramic capacitors (MLCC) are all good choices for both input and output capacitors and have very low ESL. For MLCCs it is recommended to use X7R or X5R type capacitors due to their tolerance and temperature characteristics. Consult capacitor manufacturer datasheets to see how rated capacitance varies over operating conditions. Where OUTPUT CAPACITOR The output capacitor is selected based upon the desired output ripple and transient response. The initial current of a load transient is provided mainly by the output capacitor. The output ripple of the converter is: When selecting an inductor, make sure that it is capable of supporting the peak output current without saturating. Inductor saturation will result in a sudden reduction in inductance and prevent the regulator from operating correctly. Because of the speed of the internal current limit, the peak current of the inductor need only be specified for the required maximum output current. For example, if the designed maximum output current is 1.0A and the peak current is 1.25A, then the inductor should be specified with a saturation current limit of > 1.25A. There is no need to specify the saturation or peak current of the inductor at the 2.5A typical switch current limit. The difference in inductor size is a factor of 5. Because of the operating frequency of the LM2831, ferrite based inductors are preferred to minimize core losses. This presents little restriction since the variety of ferrite-based inductors is huge. Lastly, inductors with lower series resistance (RDCR) will provide better operating efficiency. For recommended inductors see Example Circuits. www.national.com QQ:53093 When using MLCCs, the ESR is typically so low that the capacitive ripple may dominate. When this occurs, the output ripple will be approximately sinusoidal and 90˚ phase shifted from the switching action. Given the availability and quality of MLCCs and the expected output voltage of designs using the LM2831, there is really no need to review any other capacitor technologies. Another benefit of ceramic capacitors is their ability to bypass high frequency noise. A certain amount of switching edge noise will couple through parasitic capacitances in the inductor to the output. A ceramic capacitor will bypass this noise while a tantalum will not. Since the output capacitor is one of the two external components that control the stability of the regulator control loop, most applications will require a minimum of 22 µF of output capacitance. Capacitance often, but not always, can be increased 12 百度一下：杰鸿顺电子 QQ:85811868 杰鸿顺电子 Design Guide TEX:61306581 PCB LAYOUT CONSIDERATIONS When planning layout there are a few things to consider when trying to achieve a clean, regulated output. The most important consideration is the close coupling of the GND connections of the input capacitor and the catch diode D1. These ground ends should be close to one another and be connected to the GND plane with at least two through-holes. Place these components as close to the IC as possible. Next in importance is the location of the GND connection of the output capacitor, which should be near the GND connections of CIN and D1. There should be a continuous ground plane on the bottom layer of a two-layer board except under the switching node island. The FB pin is a high impedance node and care should be taken to make the FB trace short to avoid noise pickup and inaccurate regulation. The feedback resistors should be placed as close as possible to the IC, with the GND of R1 placed as close as possible to the GND of the IC. The VOUT trace to R2 should be routed away from the inductor and any other traces that are switching. High AC currents flow through the VIN, SW and VOUT traces, so they should be as short and wide as possible. However, making the traces wide increases radiated noise, so the designer must make this trade-off. Radiated noise can be decreased by choosing a shielded inductor. The remaining components should also be placed as close as possible to the IC. Please see Application Note AN-1229 for further considerations and the LM2831 demo board as an example of a four-layer layout. (Continued) significantly with little detriment to the regulator stability. Like the input capacitor, recommended multilayer ceramic capacitors are X7R or X5R types. CATCH DIODE The catch diode (D1) conducts during the switch off-time. A Schottky diode is recommended for its fast switching times and low forward voltage drop. The catch diode should be chosen so that its current rating is greater than: ID1 = IOUT x (1-D) The reverse breakdown rating of the diode must be at least the maximum input voltage plus appropriate margin. To improve efficiency, choose a Schottky diode with a low forward voltage drop. OUTPUT VOLTAGE The output voltage is set using the following equation where R2 is connected between the FB pin and GND, and R1 is connected between VO and the FB pin. A good value for R2 is 10k. When designing a unity gain converter (Vo = 0.6V), R1 should be between 0Ω and 100Ω, and R2 should be equal or greater than 10kΩ. LM2831 0755-61306582 VREF = 0.60V 13 QQ:53093 百度一下：杰鸿顺电子 www.national.com QQ:85811868 LM2831 0755-61306582 杰鸿顺电子 TEX:61306581 PCOND = IOUT2 x RDSON x D Calculating Efficiency, and Junction Temperature Switching losses are also associated with the internal PFET. They occur during the switch on and off transition periods, where voltages and currents overlap resulting in power loss. The simplest means to determine this loss is to empirically measuring the rise and fall times (10% to 90%) of the switch at the switch node. The complete LM2831 DC/DC converter efficiency can be calculated in the following manner. Switching Power Loss is calculated as follows: PSWR = 1/2(VIN x IOUT x FSW x TRISE) Or PSWF = 1/2(VIN x IOUT x FSW x TFALL) PSW = PSWR + PSWF Another loss is the power required for operation of the internal circuitry: PQ = IQ x VIN Calculations for determining the most significant power losses are shown below. Other losses totaling less than 2% are not discussed. IQ is the quiescent operating current, and is typically around 2.5mA for the 0.55MHz frequency option. Typical Application power losses are: Power loss (PLOSS) is the sum of two basic types of losses in the converter: switching and conduction. Conduction losses usually dominate at higher output loads, whereas switching losses remain relatively fixed and dominate at lower output loads. The first step in determining the losses is to calculate the duty cycle (D): Power Loss Tabulation VIN VSW is the voltage drop across the internal PFET when it is on, and is equal to: VSW = IOUT x RDSON VD is the forward voltage drop across the Schottky catch diode. It can be obtained from the diode manufactures Electrical Characteristics section. If the voltage drop across the inductor (VDCR) is accounted for, the equation becomes: 5.0V VOUT 3.3V IOUT 1.25A POUT 4.125W PDIODE 188mW VD 0.45V FSW 550kHz IQ 2.5mA PQ 12.5mW 7mW TRISE 4nS PSWR TFALL 4nS PSWF 7mW RDS(ON) 150mΩ PCOND 156mW INDDCR 70mΩ PIND 110mW D 0.667 PLOSS 481mW η 88% PINTERNAL 183mW ΣPCOND + PSW + PDIODE + PIND + PQ = PLOSS ΣPCOND + PSWF + PSWR + PQ = PINTERNAL PINTERNAL = 183mW Thermal Definitions TJ = Chip junction temperature TA = Ambient temperature RθJC = Thermal resistance from chip junction to device case RθJA = Thermal resistance from chip junction to ambient air Heat in the LM2831 due to internal power dissipation is removed through conduction and/or convection. Conduction: Heat transfer occurs through cross sectional areas of material. Depending on the material, the transfer of heat can be considered to have poor to good thermal conductivity properties (insulator vs. conductor). Heat Transfer goes as: Silicon → package → lead frame → PCB Convection: Heat transfer is by means of airflow. This could be from a fan or natural convection. Natural convection occurs when air currents rise from the hot device to cooler air. Thermal impedance is defined as: The conduction losses in the free-wheeling Schottky diode are calculated as follows: PDIODE = VD x IOUT x (1-D) Often this is the single most significant power loss in the circuit. Care should be taken to choose a Schottky diode that has a low forward voltage drop. Another significant external power loss is the conduction loss in the output inductor. The equation can be simplified to: PIND = IOUT2 x RDCR The LM2831 conduction loss is mainly associated with the internal PFET: If the inductor ripple current is fairly small, the conduction losses can be simplified to: www.national.com QQ:53093 14 百度一下：杰鸿顺电子 QQ:85811868 Thermal Definitions 杰鸿顺电子 TEX:61306581 ambient temperature in the given working application until the circuit enters thermal shutdown. If the SW-pin is monitored, it will be obvious when the internal PFET stops switching, indicating a junction temperature of 165˚C. Knowing the internal power dissipation from the above methods, the junction temperature, and the ambient temperature RθJA can be determined. (Continued) Thermal impedance from the silicon junction to the ambient air is defined as: LM2831 0755-61306582 Once this is determined, the maximum ambient temperature allowed for a desired junction temperature can be found. An example of calculating RθJA for an application using the National Semiconductor LM2831 LLP demonstration board is shown below. The PCB size, weight of copper used to route traces and ground plane, and number of layers within the PCB can greatly effect RθJA. The type and number of thermal vias can also make a large difference in the thermal impedance. Thermal vias are necessary in most applications. They conduct heat from the surface of the PCB to the ground plane. Four to six thermal vias should be placed under the exposed pad to the ground plane if the LLP package is used. Thermal impedance also depends on the thermal properties of the application operating conditions (Vin, Vo, Io etc), and the surrounding circuitry. Silicon Junction Temperature Determination Method 1: To accurately measure the silicon temperature for a given application, two methods can be used. The first method requires the user to know the thermal impedance of the silicon junction to top case temperature. Some clarification needs to be made before we go any further. RθJC is the thermal impedance from all six sides of an IC package to silicon junction. RΦJC is the thermal impedance from top case to the silicon junction. In this data sheet we will use RΦJC so that it allows the user to measure top case temperature with a small thermocouple attached to the top case. RΦJC is approximately 30˚C/Watt for the 6-pin LLP package with the exposed pad. Knowing the internal dissipation from the efficiency calculation given previously, and the case temperature, which can be empirically measured on the bench we have: The four layer PCB is constructed using FR4 with 1⁄2 oz copper traces. The copper ground plane is on the bottom layer. The ground plane is accessed by two vias. The board measures 3.0cm x 3.0cm. It was placed in an oven with no forced airflow. The ambient temperature was raised to 144˚C, and at that temperature, the device went into thermal shutdown. From the previous example: PINTERNAL = 189mW If the junction temperature was to be kept below 125˚C, then the ambient temperature could not go above 109˚C Tj - (RθJA x PLOSS) = TA 125˚C - (111˚C/W x 189mW) = 104˚C LLP Package Therefore: 20174868 Tj = (RΦJC x PLOSS) + TC From the previous example: Tj = (RΦJC x PINTERNAL) + TC Tj = 30˚C/W x 0.189W + TC The second method can give a very accurate silicon junction temperature. The first step is to determine RθJA of the application. The LM2831 has over-temperature protection circuitry. When the silicon temperature reaches 165˚C, the device stops switching. The protection circuitry has a hysteresis of about 15˚C. Once the silicon temperature has decreased to approximately 150˚C, the device will start to switch again. Knowing this, the RθJA for any application can be characterized during the early stages of the design one may calculate the RθJA by placing the PCB circuit into a thermal chamber. Raise the FIGURE 4. Internal LLP Connection For certain high power applications, the PCB land may be modified to a "dog bone" shape (see Figure 6). By increasing the size of ground plane, and adding thermal vias, the RθJA for the application can be reduced. 15 QQ:53093 百度一下：杰鸿顺电子 www.national.com QQ:85811868 LM2831 0755-61306582 杰鸿顺电子 LLP Package TEX:61306581 (Continued) 20174806 FIGURE 5. 6-Lead LLP PCB Dog Bone Layout www.national.com QQ:53093 16 百度一下：杰鸿顺电子 QQ:85811868 杰鸿顺电子 TEX:61306581 LM2831 0755-61306582 LM2831X Design Example 1 20174807 FIGURE 6. LM2831X (1.6MHz): Vin = 5V, Vo = 1.2V @ 1.5A Bill of Materials Part ID Part Value Manufacturer U1 1.5A Buck Regulator NSC LM2831X C1, Input Cap 22µF, 6.3V, X5R TDK C3216X5ROJ226M C2, Output Cap 2x22µF, 6.3V, X5R TDK C3216X5ROJ226M D1, Catch Diode 0.3Vf Schottky 1.5A, 30VR TOSHIBA CRS08 L1 3.3µH, 2.2A TDK VLCF5020T-3R3N2R0-1 R2 15.0kΩ, 1% Vishay CRCW08051502F R1 15.0kΩ, 1% Vishay CRCW08051502F R3 100kΩ, 1% Vishay CRCW08051003F 17 QQ:53093 Part Number 百度一下：杰鸿顺电子 www.national.com QQ:85811868 LM2831 0755-61306582 杰鸿顺电子 TEX:61306581 LM2831X Design Example 2 20174860 FIGURE 7. LM2831X (1.6MHz): Vin = 5V, Vo = 0.6V @ 1.5A Bill of Materials Part ID Part Value Manufacturer U1 1.5A Buck Regulator NSC LM2831X C1, Input Cap 22µF, 6.3V, X5R TDK C3216X5ROJ226M C2, Output Cap 2x22µF, 6.3V, X5R TDK C3216X5ROJ226M D1, Catch Diode 0.3Vf Schottky 1.5A, 30VR TOSHIBA CRS08 L1 3.3µH, 2.2A TDK VLCF5020T- 3R3N2R0-1 Vishay CRCW08051000F Vishay CRCW08051003F www.national.com QQ:53093 Part Number R2 10.0kΩ, 1% R1 0Ω R3 100kΩ, 1% 18 百度一下：杰鸿顺电子 QQ:85811868 杰鸿顺电子 TEX:61306581 LM2831 0755-61306582 LM2831X Design Example 3 20174808 FIGURE 8. LM2831X (1.6MHz): Vin = 5V, Vo = 3.3V @ 1.5A Bill of Materials Part ID Part Value Manufacturer U1 1.5A Buck Regulator NSC LM2831X C1, Input Cap 22µF, 6.3V, X5R TDK C3216X5ROJ226M C2, Output Cap 2x22µF, 6.3V, X5R TDK C3216X5ROJ226M D1, Catch Diode 0.3Vf Schottky 1.5A, 30VR TOSHIBA CRS08 L1 2.7µH 2.3A TDK VLCF5020T-2R7N2R2-1 R2 10.0kΩ, 1% Vishay CRCW08051002F R1 45.3kΩ, 1% Vishay CRCW08054532F R3 100kΩ, 1% Vishay CRCW08051003F 19 QQ:53093 Part Number 百度一下：杰鸿顺电子 www.national.com QQ:85811868 LM2831 0755-61306582 杰鸿顺电子 TEX:61306581 LM2831Y Design Example 4 20174808 FIGURE 9. LM2831Y (550kHz): Vin = 5V, Vout = 3.3V @ 1.5A Bill of Materials Part ID Part Value Manufacturer U1 1.5A Buck Regulator NSC LM2831Y C1, Input Cap 22µF, 6.3V, X5R TDK C3216X5ROJ226M C2, Output Cap 2x22µF, 6.3V, X5R TDK C3216X5ROJ226M D1, Catch Diode 0.3Vf Schottky 1.5A, 30VR TOSHIBA CRS08 L1 4.7µH 2.1A TDK SLF7045T-4R7M2R0-PF R1 10.0kΩ, 1% Vishay CRCW08051002F R2 10.0kΩ, 1% Vishay CRCW08051002F www.national.com QQ:53093 Part Number 20 百度一下：杰鸿顺电子 QQ:85811868 杰鸿顺电子 TEX:61306581 LM2831 0755-61306582 LM2831Y Design Example 5 20174807 FIGURE 10. LM2831Y (550kHz): Vin = 5V, Vout = 1.2V @ 1.5A Bill of Materials Part ID Part Value Manufacturer U1 1.5A Buck Regulator NSC LM2831Y C1, Input Cap 22µF, 6.3V, X5R TDK C3216X5ROJ226M C2, Output Cap 2x22µF, 6.3V, X5R TDK C3216X5ROJ226M D1, Catch Diode 0.3Vf Schottky 1.5A, 30VR TOSHIBA CRS08 L1 6.8µH 1.8A TDK SLF7045T-6R8M1R7 R1 10.0kΩ, 1% Vishay CRCW08051002F R2 10.0kΩ, 1% Vishay CRCW08051002F 21 QQ:53093 Part Number 百度一下：杰鸿顺电子 www.national.com QQ:85811868 LM2831 0755-61306582 杰鸿顺电子 TEX:61306581 LM2831Z Design Example 6 20174808 FIGURE 11. LM2831Z (3MHz): Vin = 5V, Vo = 3.3V @ 1.5A Bill of Materials Part ID Part Value Manufacturer U1 1.5A Buck Regulator NSC LM2831Z C1, Input Cap 22µF, 6.3V, X5R TDK C3216X5ROJ226M C2, Output Cap 2x22µF, 6.3V, X5R TDK C3216X5ROJ226M D1, Catch Diode 0.3Vf Schottky 1.5A, 30VR TOSHIBA CRS08 L1 1.6µH 2.0A TDK VLCF4018T-1R6N1R7-2 R2 10.0kΩ, 1% Vishay CRCW08051002F R1 45.3kΩ, 1% Vishay CRCW08054532F R3 100kΩ, 1% Vishay CRCW08051003F www.national.com QQ:53093 Part Number 22 百度一下：杰鸿顺电子 QQ:85811868 杰鸿顺电子 TEX:61306581 LM2831 0755-61306582 LM2831Z Design Example 7 20174807 FIGURE 12. LM2831Z (3MHz): Vin = 5V, Vo = 1.2V @ 1.5A Bill of Materials Part ID Part Value Manufacturer U1 1.5A Buck Regulator NSC LM2831Z C1, Input Cap 22µF, 6.3V, X5R TDK C3216X5ROJ226M C2, Output Cap 2x22µF, 6.3V, X5R TDK C3216X5ROJ226M D1, Catch Diode 0.3Vf Schottky 1.5A, 30VR TOSHIBA CRS08 L1 1.6µH, 2.0A TDK VLCF4018T- 1R6N1R7-2 R2 10.0kΩ, 1% Vishay CRCW08051002F R1 10.0kΩ, 1% Vishay CRCW08051002F R3 100kΩ, 1% Vishay CRCW08051003F 23 QQ:53093 Part Number 百度一下：杰鸿顺电子 www.national.com QQ:85811868 LM2831 0755-61306582 杰鸿顺电子 TEX:61306581 LM2831X Dual Converters with Delayed Enabled Design Example 8 20174862 FIGURE 13. LM2831X (1.6MHz): Vin = 5V, Vo = 1.2V @ 1.5A & 3.3V @1.5A Bill of Materials Part ID Part Value Manufacturer U1, U2 1.5A Buck Regulator NSC LM2831X U3 Power on Reset NSC LP3470M5X-3.08 C1, C3 Input Cap 22µF, 6.3V, X5R TDK C3216X5ROJ226M C2, C4 Output Cap 2x22µF, 6.3V, X5R TDK C3216X5ROJ226M C7 Trr delay capacitor TDK D1, D2 Catch Diode 0.3Vf Schottky 1.5A, 30VR TOSHIBA CRS08 VLCF5020T-3R3N2R0-1 L1, L2 3.3µH, 2.2A TDK R2, R4, R5 10.0kΩ, 1% Vishay CRCW08051002F R1, R6 45.3kΩ, 1% Vishay CRCW08054532F R3 100kΩ, 1% Vishay CRCW08051003F www.national.com QQ:53093 Part Number 24 百度一下：杰鸿顺电子 QQ:85811868 杰鸿顺电子 TEX:61306581 LM2831X Buck Converter & Voltage Double Circuit with LDO Follower Design Example 9 LM2831 0755-61306582 20174863 FIGURE 14. LM2831X (1.6MHz): Vin = 5V, Vo = 3.3V @ 1.5A & LP2986-5.0 @ 150mA Bill of Materials Part ID Part Value Manufacturer Part Number U1 1.5A Buck Regulator NSC LM2831X U2 200mA LDO NSC LP2986-5.0 C1, Input Cap 22µF, 6.3V, X5R TDK C3216X5ROJ226M C2, Output Cap 22µF, 6.3V, X5R TDK C3216X5ROJ226M C1608X5R0J225M C3 – C6 2.2µF, 6.3V, X5R TDK D1, Catch Diode 0.3Vf Schottky 1.5A, 30VR TOSHIBA CRS08 D2 0.4Vf Schottky 20VR, 500mA ON Semi MBR0520 L2 10µH, 800mA CoilCraft ME3220-103 L1 3.3µH, 2.2A TDK VLCF5020T-3R3N2R0-1 R2 45.3kΩ, 1% Vishay CRCW08054532F R1 10.0kΩ, 1% Vishay CRCW08051002F 25 QQ:53093 百度一下：杰鸿顺电子 www.national.com QQ:85811868 LM2831 0755-61306582 杰鸿顺电子 Physical Dimensions TEX:61306581 inches (millimeters) unless otherwise noted 5-Lead SOT-23 Package NS Package Number MF05A 6-Lead LLP Package NS Package Number SDE06A www.national.com QQ:53093 26 百度一下：杰鸿顺电子 QQ:85811868 杰鸿顺电子 TEX:61306581 Notes National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications. For the most current product information visit us at www.national.com. LM2831 High Frequency 1.5A Load - Step-Down DC-DC Regulator 0755-61306582 LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. 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