LM21215

LM21215 Evaluation Board LM21215 Evaluation Board National Semiconductor Application Note 2130 Michael Hartshorne March 29, 2011 Introduction This evaluation board provides a solution to examine the high efficiency LM21215 buck switching regulator. The LM21215 is capable of driving up to 15A of continuous load current with excellent output voltage accuracy due to its ±1% internal reference. The LM21215 is capable of down converting from an input voltage between 2.95V and 5.5V at a fixed switching frequency of 500 kHz. This device also features an adjustable current limit that allows the user to set the internal current limit protection value. Other fault protection features include output power good and output over-voltage protection. The dual function soft-start/tracking pin can be used to control the startup response of the LM21215, and the precision enable pin can be used to easily sequence the LM21215 in applications with sequencing requirements. The LM21215 evaluation board has been optimized to work from 2.95V to 5.5V, achieving a balance between overall solution size and regulator efficiency. The evaluation board measures just over 2” x 2” on a four layer PCB, and exhibits a junction-to-ambient thermal impedance (θJA) of 24°C/W with no air flow. The power stage and compensation components of the LM21215 evaluation board have been optimized for an input voltage of 5V, but for testing purposes, the input can be varied across the entire operating range. The output voltage of the evaluation board is nominally 1.2V, but this voltage can be easily changed to any voltage between 0.6V and VIN by modifying the feedback resistor network. Evaluation Board Schematic 30154001 AN-2130 © 2011 National Semiconductor Corporation 301540 www.national.com AN-2130 Powering and Loading Considerations Read this entire page prior to attempting to power the evaluation board. QUICK SETUP PROCEDURE Step 1: Set the input source current limit to 10A. Turn off the input source. Connect the positive output of the input source to VIN and the negative output to the corresponding GND. Step 2: Connect the load (with 15A capability) to VOUT for the positive connection and GND for the negative connection. Step 3: The ENABLE pin should be left open for normal operation. Step 4: Set the input source voltage to 5V. The load voltage should be in regulation with a nominal 1.2V output. Step 5: Slowly increase the load while monitoring the load voltage at VOUT. It should remain in regulation with a nominal 1.2V output as the load is increased up to 15A. Step 6: Slowly sweep the input source voltage from 2.95V to 5.5V. The load voltage should remain in regulation with a nominal 1.2V output. If desired, the output of the device can be disabled by connecting the ENABLE pin to GND. POWERING UP It is suggested that the load power be kept low during the first power up. Once the device is powered up, immediately check for 1.2V at the output. A quick efficiency check is the best way to confirm that everything is operating properly. If something is amiss you can be reasonably sure that it will affect the efficiency adversely. Few parameters can be incorrect in a switching power supply without creating losses and potentially damaging heat. Some voltage supplies can exhibit severe voltage overshoot during high current transients. If a supply overshoots above 6.0V, damage to the LM21215 can occur. For these supplies, a large capacitor across the terminals of the supply (1000µF) can alleviate this problem. OVER CURRENT PROTECTION The LM21215 features a resistor-programmable current limit that allows the user to set the current limit to any desired value between 4A and 20A by changing the resistor RILIM. This evaluation board allows the maximum possible current limit for the LM21215 by populating RILIM with a 0Ω resistor. If the user desires a lower current limit, the current limit should be set above the peak inductor current under maximum load and ripple conditions to avoid a current limit event in normal operation. See PROGRAMMABLE CURRENT LIMIT: RILIM for details on how to determine the RILIM value. Connection Descriptions Terminal Silkscreen VIN GND VOUT ENABLE Description This terminal is the input voltage to the device. The evaluation board will operate over the input voltage range of 2.95V to 5.5V. These terminals are the ground connections to the device. The input power ground should be connected next to the input VIN connection, and the output power ground next to the VOUT connection. This terminal connects to the output voltage of the power supply and should be connected to the load. This terminal connects to the enable pin of the device. This terminal can be left floating or driven externally. If left floating, a 2µA current source will pull the pin high, thereby enabling the device. If driven externally, a voltage typically less than 1.2V will disable the device. This terminal provides access to the SS/TRK pin of the device. Connections to this terminal are not needed for most applications. The feedback pin of the device will track the voltage on the SS/TRK pin if it is driven with an external voltage source that is below the 0.6V reference. This terminal connects to the power good output of the device. This pin is pulled up through a 10 kΩ pull-up resistor to VIN. This terminal block allows the user to insert an AC injection signal across a 49.9Ω resistor for openloop gain bode measurements. A jumper shorts out this resistor when it is not needed. This terminal allows easy probing of the switch node. Do not apply any external voltage source to this pin. These terminals allow a sense connection on the board for accurate VIN and VOUT measurements, respectively. SS/TRK PGOOD AC INJ SWITCH VIN_SENSE+, VIN_SENSEVOUT_SENSE+, VOUT_SENSE- www.national.com 2 AN-2130 Performance Characteristics EFFICIENCY PLOTS Figure 1 shows the conversion efficiency versus output current for a 5V input voltage for output voltages of 1.2V and 3.3V. The default output voltage is 1.2V. OUTPUT RIPPLE WAVEFORM Figure 3 shows the output voltage ripple. This measurement was taken with the scope probe tip placed on the output capacitor C9 VOUT connection and the scope probe ground "barrel" wired to the GND connection of C9. The scope bandwidth is set to 20 MHz. 100 98 96 EFFICIENCY (%) 94 92 90 88 86 84 82 80 0 VIN = 3.3V VIN = 5.0V 30154004 3 6 9 12 OUTPUT CURRENT (A) 15 30154035 FIGURE 3. OUTPUT RIPPLE (2 µs/DIV) PRIMARY SWITCHNODE WAVEFORM Figure 4 shows the typical SW pin voltage with the output current set to 10A. FIGURE 1. Efficiency STARTUP WAVEFORM A soft-start sequence occurs when applying power to the LM21215 evaluation board. Figure 2 shows the output voltage during a typical start-up sequence. 30154005 FIGURE 4. OUTPUT RIPPLE (400 ns/DIV) 30154003 FIGURE 2. STARTUP (2 ms/DIV) 3 www.national.com AN-2130 OUTPUT TRANSIENT RESPONSE Figure 5 shows the VOUT deviation for a 3A to 12A output current transient condition. OPEN LOOP BODE RESPONSE Figure 7 shows the open loop bode response generated by inserting a stimulus signal across RAC and using a network analyzer to plot the gain and phase. 80 60 GAIN (dB) 40 20 0 -20 30154034 160 120 80 40 0 -40 -80 1M 30154042 GAIN PHASE MARGIN 1k 10k 100k FREQUENCY (Hz) 100 FIGURE 5. VOUT TRANSIENT RESPONSE (50 µs/DIV) OUTPUT CURRENT LIMIT Figure 6 shows the VOUT output response to an output current limit condition. FIGURE 7. OPEN LOOP BODE RESPONSE 30154041 FIGURE 6. CURRENT LIMIT (10 µs/DIV) www.national.com 4 PHASE MARGIN (°) AN-2130 Bill of Materials The Bill of Materials is shown below, including the manufacturer and part number. ID AC INJ C1 DESCRIPTION Header, TH, 100mil, 2x1, Gold plated, 230 mil above insulator VENDOR Samtec Inc. PART NUMBER TSW-102-07-G-S GRM188R71A105KA61D GRM31CR60J107ME39L C1608X7R1H104K C1608C0G1H182J GRM1885C1H680JA01D GRM1885C1H821JA01D GRM188R71C333KA01D 108-0740-001 IHLP4040DZERR56M01 1 1 6 1 1 1 1 1 4 1 QUANTITY CAP, CERM, 1 uF, 10V, +/-10%, MuRata X7R, 0603 MuRata TDK MuRata C3, C4, C5, C6, C7, CAP, CERM, 100 uF, 6.3V, C8 +/-20%, X5R, 1206 C9 CC1 CC2 CC3 CSS CAP, CERM, 0.1 uF, 50V, +/-10%, X7R, 0603 CAP, CERM, 1800 pF, 50V, +/-5%, C0G/NP0, 0603 CAP, CERM, 68 pF, 50V, +/-5%, MuRata C0G/NP0, 0603 CAP, CERM, 820 pF, 50V, +/-5%, MuRata C0G/NP0, 0603 CAP, CERM, 0.033 uF, 16V, +/-10%, X7R, 0603 MuRata Johnson Components Vishay-Dale GND_FI, GND_FO, Standard Banana Jack, VIN_F, VOUT_F Uninsulated, 15A L1 Inductor, Shielded Drum Core, Powdered Iron, 560nH, 27.5A, 0.0018 ohm, SMD RES, 1.0 ohm, 5%, 0.1W, 0603 R1 RAC RC1 RC2 RFB1, RFB2, RPG RILIM SH-J1 U1 Vishay-Dale CRCW06031R00JNEA CRCW060349R9FKEA CRCW06039K31FKEA CRCW0603165RFKEA CRCW060310K0FKEA CRCW06030000Z0EA SNT-100-BK-G LM21215MH 1 1 1 1 3 1 1 1 RES, 49.9 ohm, 1%, 0.1W, 0603 Vishay-Dale RES, 9.31 kohm, 1%, 0.1W, 0603 Vishay-Dale RES, 165 ohm, 1%, 0.1W, 0603 RES, 10 kohm, 1%, 0.1W, 0603 RES, 0 ohm, 1%, 0.1W, 0603 15A Buck DC/DC Converter Vishay-Dale Vishay-Dale Vishay-Dale National Semiconductor Shunt, 100mil, Gold plated, Black Samtec Inc. 5 www.national.com AN-2130 Component Selection This section provides a walk-through of the design process of the LM21215 evaluation board. Unless otherwise indicated all equations assume units of amps (A) for current, farads (F) for capacitance, henries (H) for inductance, and volts (V) for voltages. INPUT CAPACITORS: C1, C2, C3 The required RMS current rating of the input capacitor for a buck regulator can be estimated by the following equation: ple (ΔIP-P) the output voltage ripple can be approximated by the equation: The variable D refers to the duty cycle, and can be approximated by: From this equation, it follows that the maximum ICIN(RMS) requirement will occur at a full 15A load current with the system operating at 50% duty cycle. Under this condition, the maximum ICIN(RMS) is given by: The variable RESR above refers to the ESR of the output capacitor. As can be seen in the above equation, the ripple voltage on the output can be divided into two parts, one of which is attributed to the AC ripple current flowing through the ESR of the output capacitor and another due to the AC ripple current actually charging and discharging the output capacitor. The output capacitor also has an effect on the amount of droop that is seen on the output voltage in response to a load transient event. For the evaluation board, three 100µF ceramic capacitors were selected to provide good transient and DC performance. Ceramic capacitors give the lowest RESR of any standard capacitor chemistries, resulting in the lowest output ripple for the given ripple current. Ceramic capacitors (especially high capacitance, small package multi-layer types, or MLCC) lose thier capacitance as the DC voltage is increased. For this configuration, the actual capacitance value was approximated to be 50 µF per capacitor, or 150 µF total. This is lower than measured capacitance values for 1.2V, but will allow the user to change the output voltage up to 3.3V and maintain stability. SOFT-START CAPACITOR: CSS A soft-start capacitor can be used to control the startup time of the LM21215 voltage regulator. The startup time of the regulator when using a soft-start capacitor can be estimated by the following equation: Ceramic capacitors feature a very large IRMS rating in a small footprint, making a ceramic capacitor ideal for this application. The input capacitors also keep the input stable during load transient conditions. If the input capacitance is too low, the input can drop below the UVLO threshold and cause the device to disable the output. This may result in repetitive dropout and re-enable oscillation, or "motorboating". To give the user the ability to operate with a low VIN voltage, three 100 µF ceramic capacitors were used on the input. INDUCTOR: L1 The value of the inductor was selected to allow the device to achieve a 5V to 1.2V conversion at 500kHz to provide a peak to peak ripple current of 3.2A, which is about 21% of the maximum output current. To have an optimized design, generally the peak to peak inductor ripple current should be kept to within 20% to 40% of the rated output current for a given input voltage, output voltage and operating frequency. The peak to peak inductor ripple current can be calculated by the equation: For the LM21215, ISS is nominally 5 µA. For the evaluation board, the soft-start time has been designed to be roughly 10 ms, resulting in a CSS capacitor value of 33 nF. COMPENSATION COMPONENTS: CC1, CC2, CC3, RC1, RC2 These components are used in conjunction with the error amplifier to create a type 3 voltage-mode compensation network. The analysis of type 3 compensation is outside the scope of this document, but an example of the step-by-step procedure to generate comensation component values is given. The parameters needed for the compensation values are given in the table below. Parameter VIN Value 5.0V 1.2V 15A 100 kHz 0.56 µH 1.8 mΩ 150 µF 1.0 mΩ 0.8V 500 kHz Once an inductance value is calculated, an actual inductor needs to be selected based on a trade-off between physical size, efficiency, and current carrying capability. For the LM21215 evaluation board, a Vishay IHLP4040DZERR56M01 inductor offers a good balance between efficiency (1.8 mΩ DCR) and size. OUTPUT CAPACITOR: C3, C4, C5, C9 The value of the output capacitor in a buck regulator influences the voltage ripple that will be present on the output voltage as well as the large signal output voltage response to a load transient. Given the peak-to-peak inductor current rip- VOUT IOUT fCROSSOVER L RDCR CO RESR ΔVRAMP fSW 6 www.national.com AN-2130 where ΔVRAMP is the oscillator peak-to-peak ramp voltage (nominally 0.8 V), fCROSSOVER is the frequency at which the open-loop gain is a magnitude of 1, RDCR is the effective DC resistance of the inductor, RESR is the effective resistance of the output capacitor, and CO is the effective output capacitance at the programmed output voltage. It is recommended that fCROSSOVER not exceed one-fifth of the switching frequency. The output capacitance, CO, depends on capacitor chemistry and bias voltage. For Multi-Layer Ceramic Capacitors (MLCC), the total capacitance will degrade as the DC bias voltage is increased. Measuring the actual capacitance value for the output capacitors at the output voltage is recommended to accurately calculate the compensation network. Note that it is more conservative, from a stability standpoint, to err on the side of a smaller output capacitance value in the compensation calculations rather than a larger, as this will result in a lower bandwidth but increased phase margin. First, the value of RFB1 should be chosen. A typical value is 10kΩ. From this, the value of RC1 can be calculated to set the mid-band gain so that the desired crossover frequency is achieved. FEEDBACK RESISTORS: RFB1, RFB2, and RAC The resistors labeled RFB1 and RFB2 create a voltage divider from VOUT to the feedback pin that is used to set the output of the voltage regulator. Nominally, the output of the LM21215 evaluation board is set to 1.2V, giving resistor values of RFB1= RFB2 = 10kΩ. If a different output voltage is required, the value of RFB2 can be adjusted according to the equation: RFB1 does not need to be changed from its value of 10kΩ. Resistor RAC has a value of 49.9Ω and is provided as an injection point for loop stability measurements, as well as, a way to further tweak the output voltage accuracy to account for resistor tolerance values differing from ideal calculated values. The jumper is used to short out RAC when not needed. PROGRAMMABLE UVLO: REN1 and REN2 The resistors labeled REN1 and REN2 create a voltage divider from VIN to the enable pin that can be used to enable the device above a programmed VIN, effectively creating a programmable UVLO voltage above the device's internal UVLO (nominally 2.7V). To allow evaluation of the device down to 2.95V, these components are not installed. To change the turn-on threshold of the device a 10 kΩ resistor is recommended for REN1 and the value of REN2 can be calculated using the equation: Next, the value of CC1 can be calculated by placing a zero at half of the LC double pole frequency. where VTO is the desired VIN voltage at which the device will enable. PROGRAMMABLE CURRENT LIMIT: RILIM The resistor RILIM will set the internal current limit on the LM21215. A 0Ω resistor is used on this board, allowing the maximum current capability of the device. If a user desires a lower current limit, it should be programmed such that the peak inductor current (IL) does not trigger the current limit in normal operation. This requires setting the resistor RILIM to the appropriate value to allow the maximum ripple current plus the DC output current through the high-side FET during normal operation. The maximum ripple current can be described as: Now the value of CC2 can be calculated to place a pole at half of the switching frequency. RC2 can then be calculated to set the second zero at the LC double pole frequency. Last, CC3 can be calculated to place a pole at the same frequency as the zero created by the output capacitor ESR. The standard values used for the above calculations are given in the Bill of Materials. where VINMAX, VINMIN, VOUTMAX, VOUTMIN, LMIN and FSWMIN are the respective maximum and minimum conditions of the system as defined by the component tolerance and device variation. From this, the maximum allowable current through the high-side FET (IHSMAX) of device can be described as: 7 www.national.com AN-2130 where IOUTMAX is the maximum defined DC output current, up to 15 A. Once the IHSMAX value has been determined, a nominal value of the RILIMresistor can be calculated as follows: where the RILIMvalue is the nominal resistance necessary for the given IHSMAX value. A conservative design should also take into account the device variation over VIN and temperature, which can be seen in the Electrical Characteristics table for the ICLR parameter and the typical performance characteristics in the LM21215 data sheet. These variations can cause the IHSMAX value to increase, depending on the range of the input voltage and junction temperature. www.national.com 8 AN-2130 PCB Layout The PCB was manufactured with 2oz. copper outer layers, and 1oz. copper inner layers. Twenty 8 mil. diameter vias placed underneath the device, along with addional vias placed throughout the ground plane around the device, help improve the thermal dissipation of the board. 30154030 30154032 Top Layer (Copper planes outlined in grey) Mid Layer2 30154031 Mid Layer1 30154033 Bottom Layer 9 www.national.com LM21215 Evaluation Board For more National Semiconductor product information and proven design tools, visit the following Web sites at: www.national.com Products Amplifiers Audio Clock and Timing Data Converters Interface LVDS Power Management Switching Regulators LDOs LED Lighting Voltage References PowerWise® Solutions Temperature Sensors PLL/VCO www.national.com/amplifiers www.national.com/audio www.national.com/timing www.national.com/adc www.national.com/interface www.national.com/lvds www.national.com/power www.national.com/switchers www.national.com/ldo www.national.com/led www.national.com/vref www.national.com/powerwise WEBENCH® Tools App Notes Reference Designs Samples Eval Boards Packaging Green Compliance Distributors Quality and Reliability Feedback/Support Design Made Easy Design Support www.national.com/webench www.national.com/appnotes www.national.com/refdesigns www.national.com/samples www.national.com/evalboards www.national.com/packaging www.national.com/quality/green www.national.com/contacts www.national.com/quality www.national.com/feedback www.national.com/easy www.national.com/solutions www.national.com/milaero www.national.com/solarmagic www.national.com/training Applications & Markets Mil/Aero PowerWise® Design University Serial Digital Interface (SDI) www.national.com/sdi www.national.com/wireless www.national.com/tempsensors SolarMagic™ THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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