LMR62421 LMR62421 SIMPLE SWITCHER ® 24Vout, 2.1A Step-Up Voltage Regulator in SOT-23 Literature Number: SNVS734A LMR62421 SIMPLE SWITCHER® 24Vout, 2.1A Step-Up Voltage Regulator in SOT-23 Features Performance Benefits ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Extremely easy to use ■ Tiny overall solution reduces system cost Input voltage range of 2.7V to 5.5V Output voltage up to 24V Switch current up to 2.1A 1.6 MHz switching frequency Low shutdown Iq, 80 nA Cycle-by-cycle current limiting Internally compensated Internal soft-start SOT23-5 (2.92 x 2.84 x 1mm) and LLP-6 (3 x 3 x 0.8 mm) packaging ■ Fully enabled for WEBENCH® Power Designer Applications ■ ■ ■ ■ ■ Boost / SEPIC Conversions from 3.3V, 5V Rails Space Constrained Applications Embedded Systems LCD Displays LED Applications System Performance Efficiency vs Load Current VOUT = 20V Efficiency vs Load Current VOUT = 12V 30167212 30167214 Typical Application 30167201 © 2011 Texas Instruments Incorporated 301672 www.ti.com LMR62421 SIMPLE SWITCHER® 24Vout, 2.1A Step-Up Voltage Regulator in SOT-23 October 31, 2011 LMR62421 Connection Diagrams Top View Top View 30167203 5-Pin SOT23 30167204 6-Pin LLP Ordering Information Order Number Description Package Type Package Drawing LMR62421XMFE LMR62421XMF LMR62421XMFX LMR62421XSDE LMR62421XSD SOT23-5 MF05A 1000 Units on Tape & Reel 3000 Units on Tape & Reel 1.6 MHz 250 Units on Tape & Reel LLP-6 SDE06A LMR62421XSDX www.ti.com Supplied As 250 Units on Tape & Reel 1000 Units on Tape & Reel 4500 Units on Tape & Reel 2 LMR62421 Pin Descriptions - 5-Pin SOT23 Pin Name 1 SW Function 2 GND 3 FB Feedback pin. Connect FB to external resistor divider to set output voltage. 4 EN Shutdown control input. Logic high enables operation. Do not allow this pin to float or be greater than VIN + 0.3V. 5 VIN Supply voltage for power stage, and input supply voltage. Switch node. Connect to the inductor, output diode. Signal and power ground pin. Place the bottom resistor of the feedback network as close as possible to this pin. Pin Descriptions - 6-Pin LLP Pin Name Function 1 PGND Power ground pin. Place PGND and output capacitor GND close together. 2 VIN Supply voltage for power stage, and input supply voltage. 3 EN Shutdown control input. Logic high enables operation. Do not allow this pin to float or be greater than VIN + 0.3V. 4 FB Feedback pin. Connect FB to external resistor divider to set output voltage. 5 AGND 6 SW DAP GND Signal ground pin. Place the bottom resistor of the feedback network as close as possible to this pin & pin 4. Switch node. Connect to the inductor, output diode. Signal & Power ground. Connect to pin 1 & pin 5 on top layer. Place 4-6 vias from DAP to bottom layer GND plane. 3 www.ti.com LMR62421 Storage Temp. Range -65°C to 150°C For soldering specifications: see product folder at www.national.com and www.national.com/ms/MS/MS-SOLDERING.pdf Absolute Maximum Ratings (Note 1) If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and specifications. VIN SW Voltage FB Voltage EN Voltage ESD Susceptibility (Note 6) Junction Temperature (Note 2) -0.5V to 7V -0.5V to 26.5V -0.5V to 3.0V -0.5V to VIN + 0.3V 2kV 150°C Operating Ratings (Note 1) VIN VEN (Note 7) Junction Temperature Range 2.7V to 5.5V 0V to VIN −40°C to +125°C Electrical Characteristics (Note 3), (Note 4) Limits in standard type are for TJ = 25°C only; limits in boldface type apply over the junction temperature range of (TJ = -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. VIN = 5V unless otherwise indicated under the Conditions column. Symbol VFB ΔVFB/VIN Parameter Feedback Voltage Feedback Voltage Line Regulation Conditions Min Typ Max −40°C ≤ to TJ ≤ +125°C (SOT23-5) 1.230 1.255 1.280 0°C ≤ to TJ ≤ +125°C (SOT23-5) 1.236 1.255 1.274 −40°C ≤ to TJ ≤ +125°C (LLP-6) 1.225 1.255 1.285 −0°C ≤ to TJ ≤ +125°C (LLP-6) 1.229 1.255 1.281 VIN = 2.7V to 5.5V 0.06 Units V %/V IFB Feedback Input Bias Current 0.1 1 µA FSW Switching Frequency 1200 1600 2000 kHz DMAX Maximum Duty Cycle 88 96 % DMIN Minimum Duty Cycle 5 % RDS(ON) Switch On Resistance ICL Switch Current Limit SS Soft Start IQ UVLO SOT23-5 170 330 LLP-6 190 350 2.1 3 A 4 Quiescent Current (switching) 7.0 Quiescent Current (shutdown) VEN = 0V Undervoltage Lockout VIN Rising VIN Falling ms 11 80 2.3 1.7 mΩ mA nA 2.65 V 1.9 Shutdown Threshold Voltage (Note 7) Enable Threshold Voltage (Note 7) I-SW Switch Leakage VSW = 24V 1.0 µA I-EN Enable Pin Current Sink/Source 100 nA VEN_TH www.ti.com 4 0.4 1.8 V Parameter Conditions Min Typ LLP-6 80 SOT23-5 118 LLP-6 18 SOT23-5 60 θJA Junction to Ambient 0 LFPM Air Flow (Note 5) θJC Junction to Case TSD Thermal Shutdown Temperature (Note 2) 160 Thermal Shutdown Hysteresis 10 Max Units °C/W °C/W °C Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is intended to be functional, but specific performance is not guaranteed. For guaranteed specifications and the test conditions, see Electrical Characteristics. Note 2: Thermal shutdown will occur if the junction temperature exceeds the maximum junction temperature of the device. Note 3: Min and Max limits are 100% production tested at 25°C. Limits over the operating temperature range are guaranteed through correlation using Statistical Quality Control (SQC) methods. Limits are used to calculate National’s Average Outgoing Quality Level (AOQL). Note 4: Typical numbers are at 25°C and represent the most likely parametric norm. Note 5: Applies for packages soldered directly onto a 3” x 3” PC board with 2oz. copper on 4 layers in still air. Note 6: The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. Note 7: Do not allow this pin to float or be greater than VIN +0.3V. 5 www.ti.com LMR62421 Symbol LMR62421 Typical Performance Characteristics Current Limit vs Temperature FB Pin Voltage vs Temperature 30167206 30167207 Oscillator Frequency vs Temperature Typical Maximum Output Current vs VIN 30167208 30167210 RDSON vs Temperature Efficiency vs Load Current, Vo = 20V 30167212 30167211 www.ti.com 6 LMR62421 Efficiency vs Load Current, Vo = 12V Output Voltage Load Regulation 30167214 30167216 Output Voltage Line Regulation 30167217 7 www.ti.com LMR62421 Simplified Internal Block Diagram 30167218 FIGURE 1. Simplified Block Diagram schematic (Figure 2), and its associated waveforms (Figure 3). The LMR62421 supplies a regulated output voltage by switching the internal NMOS 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 NMOS control switch. During this ontime, the SW pin voltage (VSW) decreases to approximately GND, 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 sensed 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 diode D1, which forces the SW pin to swing to the output voltage plus the forward voltage (VD) of the diode. The regulator loop adjusts the duty cycle (D) to maintain a constant output voltage . General Description The LMR62421 is an easy-to-use, space-efficient 2.1A lowside switch regulator ideal for Boost and SEPIC DC-DC regulation. It provides all the active functions to provide local DC/ DC conversion with fast-transient response and accurate regulation in the smallest PCB area. Switching frequency is internally set to 1.6 MHz, allowing the use of extremely small surface mount inductor and chip capacitors while providing efficiencies near 90%. Current-mode control and internal compensation provide ease-of-use, minimal component count, and high-performance regulation over a wide range of operating conditions. External shutdown features an ultra-low standby current of 80 nA ideal for portable applications. Tiny SOT23-5 and LLP-6 packages provide space-savings. Additional features include internal soft-start, circuitry to reduce inrush current, pulse-by-pulse current limit, and thermal shutdown. Application Information THEORY OF OPERATION The following operating description of the LMR62421 will refer to the Simplified Block Diagram (Figure 1) the simplified www.ti.com 8 LMR62421 30167219 FIGURE 2. Simplified Schematic 30167220 FIGURE 3. Typical Waveforms 9 www.ti.com LMR62421 CURRENT LIMIT The LMR62421 uses cycle-by-cycle current limiting to protect the internal NMOS switch. It is important to note that this current limit will not protect the output from excessive current during an output short circuit. The input supply is connected to the output by the series connection of an inductor and a diode. If a short circuit is placed on the output, excessive current can damage both the inductor and diode. 30167224 Design Guide FIGURE 4. Inductor Current ENABLE PIN / SHUTDOWN MODE The LMR62421 has a shutdown mode that is controlled by the Enable pin (EN). When a logic low voltage is applied to EN, the part is in shutdown mode and its quiescent current drops to typically 80 nA. Switch leakage adds up to another 1 µA from the input supply. The voltage at this pin should never exceed VIN + 0.3V. THERMAL SHUTDOWN Thermal shutdown limits total power dissipation by turning off the output switch when the IC junction temperature exceeds 160°C. After thermal shutdown occurs, the output switch doesn’t turn on until the junction temperature drops to approximately 150°C. A good design practice is to design the inductor to produce 10% to 30% ripple of maximum load. From the previous equations, the inductor value is then obtained. 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 to its nominal value of 1.255V in approximately 4.0ms. This forces the regulator output to ramp up in a more linear and controlled fashion, which helps reduce inrush current. Where: 1/TS = FSW = switching frequency One must also ensure that the minimum current limit (2.1A) is not exceeded, so the peak current in the inductor must be calculated. The peak current (ILPK ) in the inductor is calculated by: INDUCTOR SELECTION The Duty Cycle (D) can be approximated quickly using the ratio of output voltage (VO) to input voltage (VIN): or ILpk = IIN + ΔIL ILpk = IOUT / D' + ΔIL When selecting an inductor, make sure that it is capable of supporting the peak input 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 input current. For example, if the designed maximum input current is 1.5A and the peak current is 1.75A, then the inductor should be specified with a saturation current limit of >1.75A. There is no need to specify the saturation or peak current of the inductor at the 3A typical switch current limit. Because of the operating frequency of the LMR62421, 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 (DCR) will provide better operating efficiency. For recommended inductors see Example Circuits. Therefore: Power losses due to the diode (D1) forward voltage drop, the voltage drop across the internal NMOS switch, the voltage drop across the inductor resistance (RDCR) and switching losses must be included to calculate a more accurate duty cycle (See Calculating Efficiency and Junction Temperature for a detailed explanation). A more accurate formula for calculating the conversion ratio is: INPUT CAPACITOR 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 10 µF to 44 µF depending on the application. The capacitor manufacturer specifically states the input voltage rating. Make sure to check any recommended deratings and also verify if there is any Where η equals the efficiency of the LMR62421 application. The inductor value determines the input ripple current. Lower inductor values decrease the size of the inductor, but increase the input ripple current. An increase in the inductor value will decrease the input ripple current. www.ti.com 10 LMR62421 significant change in capacitance at the operating input voltage and the operating temperature. The ESL of an input capacitor is usually determined by the effective cross sectional area of the current path. At the operating frequencies of the LMR62421, certain 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. Multilayer ceramic capacitors (MLCC) are good choices for both input and output capacitors and have very low ESL. For MLCCs it is recommended to use X7R or X5R dielectrics. Consult capacitor manufacturer datasheet to see how rated capacitance varies over operating conditions. OUTPUT CAPACITOR The LMR62421 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. 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 impedance will therefore determine the maximum voltage perturbation. The output ripple of the converter is a function of the capacitor’s reactance and its equivalent series resistance (ESR): 30167229 FIGURE 5. Setting Vout A good value for R1 is 10kΩ. COMPENSATION The LMR62421 uses constant frequency peak current mode control. This mode of control allows for a simple external compensation scheme that can be optimized for each application. A complicated mathematical analysis can be completed to fully explain the LMR62421’s internal & external compensation, but for simplicity, a graphical approach with simple equations will be used. Below is a Gain & Phase plot of a LMR62421 that produces a 12V output from a 5V input voltage. The Bode plot shows the total loop Gain & Phase without external compensation. 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 LMR62421, 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 at 4.7 µF of output capacitance. Like the input capacitor, recommended multilayer ceramic capacitors are X7R or X5R. Again, verify actual capacitance at the desired operating voltage and temperature. SETTING THE OUTPUT VOLTAGE The output voltage is set using the following equation where R1 is connected between the FB pin and GND, and R2 is connected between VOUT and the FB pin. 30167231 FIGURE 6. LMR62421 Without External Compensation One can see that the Crossover frequency is fine, but the phase margin at 0dB is very low (22°). A zero can be placed just above the crossover frequency so that the phase margin will be bumped up to a minimum of 45°. Below is the same application with a zero added at 8 kHz. 11 www.ti.com LMR62421 30167229 FIGURE 8. Setting External Pole-Zero 30167232 FIGURE 7. LMR62421 With External Compensation The simplest method to determine the compensation component value is as follows. Set the output voltage with the following equation. There is an associated pole with the zero that was created in the above equation. It is always higher in frequency than the zero. A right-half plane zero (RHPZ) is inherent to all boost converters. One must remember that the gain associated with a right-half plane zero increases at 20dB per decade, but the phase decreases by 45° per decade. For most applications there is little concern with the RHPZ due to the fact that the frequency at which it shows up is well beyond crossover, and has little to no effect on loop stability. One must be concerned with this condition for large inductor values and high output currents. Where R1 is the bottom resistor and R2 is the resistor tied to the output voltage. The next step is to calculate the value of C3. The internal compensation has been designed so that when a zero is added between 5 kHz & 10 kHz the converter will have good transient response with plenty of phase margin for all input & output voltage combinations. Lower output voltages will have the zero set closer to 10 kHz, and higher output voltages will usually have the zero set closer to 5 kHz. It is always recommended to obtain a Gain/Phase plot for your actual application. One could refer to the Typical applications section to obtain examples of working applications and the associated component values. Pole @ origin due to internal gm amplifier: There are miscellaneous poles and zeros associated with parasitics internal to the LMR62421, external components, and the PCB. They are located well over the crossover frequency, and for simplicity are not discussed. FP-ORIGIN PCB Layout Considerations Pole due to output load and capacitor: When planning layout there are a few things to consider when trying to achieve a clean, regulated output. The most important consideration when completing a Boost Converter layout is the close coupling of the GND connections of the COUT capacitor and the LMR62421 PGND pin. The GND ends should be close to one another and be connected to the GND plane with at least two through-holes. There should be a continuous ground plane on the bottom layer of a two-layer board. 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 AGND of R1 placed as close as possible to the GND (pin 5 for the LLP) 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 This equation only determines the frequency of the pole for perfect current mode control (CMC). I.e, it doesn’t take into account the additional internal artificial ramp that is added to the current signal for stability reasons. By adding artificial ramp, you begin to move away from CMC to voltage mode control (VMC). The artifact is that the pole due to the output load and output capacitor will actually be slightly higher in frequency than calculated. In this example it is calculated at 650 Hz, but in reality it is around 1 kHz. The zero created with capacitor C3 & resistor R2: www.ti.com 12 LMR62421 Substituting IL1 into IL2 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 LMR62421 demo board as an example of a good layout. The average inductor current of L2 is the average output load. SEPIC Converter The LMR62421 can easily be converted into a SEPIC converter. A SEPIC converter has the ability to regulate an output voltage that is either larger or smaller in magnitude than the input voltage. Other converters have this ability as well (CUK and Buck-Boost), but usually create an output voltage that is opposite in polarity to the input voltage. This topology is a perfect fit for Lithium Ion battery applications where the input voltage for a single cell Li-Ion battery will vary between 3V & 4.5V and the output voltage is somewhere in between. Most of the analysis of the LMR62421 Boost Converter is applicable to the LMR62421 SEPIC Converter. SEPIC Design Guide: SEPIC Conversion ratio without loss elements: 30167263 FIGURE 9. Inductor Volt-Sec Balance Waveform Applying Charge balance on C1: Therefore: Since there are no DC voltages across either inductor, and capacitor C6 is connected to Vin through L1 at one end, or to ground through L2 on the other end, we can say that Small ripple approximation: In a well-designed SEPIC converter, the output voltage, and input voltage ripple, the inductor ripple and is small in comparison to the DC magnitude. Therefore it is a safe approximation to assume a DC value for these components. The main objective of the Steady State Analysis is to determine the steady state duty-cycle, voltage and current stresses on all components, and proper values for all components. In a steady-state converter, the net volt-seconds across an inductor after one cycle will equal zero. Also, the charge into a capacitor will equal the charge out of a capacitor in one cycle. Therefore: VC1 = VIN Therefore: This verifies the original conversion ratio equation. It is important to remember that the internal switch current is equal to IL1 and IL2. During the D interval. Design the converter so that the minimum guaranteed peak switch current limit (2.1A) is not exceeded. 13 www.ti.com LMR62421 30167280 FIGURE 10. SEPIC CONVERTER Schematic Steady State Analysis with Loss Elements 30167266 Using inductor volt-second balance & capacitor charge balance, the following equations are derived: Therefore: One can see that all variables are known except for the duty cycle (D). A quadratic equation is needed to solve for D. A www.ti.com 14 LMR62421 less accurate method of determining the duty cycle is to assume efficiency, and calculate the duty cycle. 30167290 Efficiencies for Typical SEPIC Application 30167272 SEPIC Converter PCB Layout FIGURE 11. SEPIC PCB Layout The layout guidelines described for the LMR62421 BoostConverter are applicable to the SEPIC Converter. Below is a proper PCB layout for a SEPIC Converter. 15 www.ti.com LMR62421 LLP Package The LMR62421 packaged in the 6–pin LLP: 30167273 FIGURE 12. Internal LLP Connection For certain high power applications, the PCB land may be modified to a "dog bone" shape (see Figure 19). Increasing the size of ground plane, and adding thermal vias can reduce the RθJA for the application. 30167274 FIGURE 13. PCB Dog Bone Layout www.ti.com 16 LMR62421 LMR62421 Design Example 1 30167275 Vin = 3V - 5V, Vout = 12V @ 500 mA LMR62421 Design Example 2 30167278 Vin = 3V, Vout = 5V @ 500 mA 17 www.ti.com LMR62421 LMR62421 Design Example 3 30167279 Vin = 3.3V, Vout = 20V @ 100 mA LMR62421 SEPIC Design Example 4 30167281 Vin = 2.7V - 5V, Vout = 3.3V @ 500mA www.ti.com 18 LMR62421 Physical Dimensions inches (millimeters) unless otherwise noted 6-Lead LLP Package NS Package Number SDE06A 5-Lead SOT23-5 Package NS Package Number MF05A 19 www.ti.com LMR62421 SIMPLE SWITCHER® 24Vout, 2.1A Step-Up Voltage Regulator in SOT-23 Notes TI/NATIONAL INTERIM IMPORTANT NOTICE Texas Instruments has purchased National Semiconductor. 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