National Semiconductor Application Note 2042 Yang Zhang March 17, 2010 LM21305 Overview The device features internal over voltage protection (OVP) and over current protection (OCP) circuits for increased system reliability. A precision enable pin and integrated UVLO allows the turn-on of the device to be tightly controlled and sequenced. Start-up inrush currents are limited by an internal Soft-Start circuit. Fault detection and supply sequencing are possible with the integrated power good circuit. The LM21305 is offered in a 28-pin LLP package with an exposed pad for enhanced thermal performance. The LM21305 Evaluation board comes ready to operate at the following conditions: The LM21305 is a full featured adjustable frequency synchronous buck regulator capable of delivering up to 5A of continuous output current. The device is optimized to work over the input voltage range of 3V to 18V and output voltage range of 0.6V to 5V, making it suitable for wide variety of applications. The LM21305 provides 1% output voltage accuracy and excellent line and fast load transient response for digital loads. The device offers flexible system configuration via programmable switching frequency through an external resistor and ability to synchronize switching frequency. The frequency of this device can be from 300 kHz to 1.5 MHz. The device also provides internal soft-start to limit in-rush current, cycle-by-cycle current limiting, and thermal shutdown. Default Voltage Parameter Range and Options PVIN 12V AVIN =PVIN VOUT 3.3V 0.6V to 5V by changing R5 and/or R6 500 kHz 300 kHz to 1.5 MHz by changing R4 Switching Frequency External supply 5V to 18V IOUT 0 to 5A Size 2 inches x 1.5 inches No. of PCB Layers LM21305 Evaluation Board LM21305 Evaluation Board =PVIN or by separate supply (3V to 18V) selected by JP1 4 Typical Application Circuit © 2010 National Semiconductor Corporation 301194 www.national.com AN-2042 30119406 AN-2042 Evaluation Board Schematic 30119407 www.national.com 2 Component C1 C2,C4,C7 Description Part Number CERAMIC 100 PF 100V ECJ-1VC2A101J Package 603 CERAMIC 0.1 µF 50V UMK107B7104KA-T 603 C3 CERAMIC 1.0 µF 35V X5R GMK107BJ105KA 603 C5 NL NL NL C6 CERAMIC 10000 PF 25V C1608C0G1E103J 603 C8 CERAMIC 1.0 µF 25V X7R C3216X7R1E105K 1206 C9, C10 CERAMIC 10 µF 50V UMK325C7106MM-T 1210 C11, C12 CERAMIC 47 µF X5R GRM32ER61A476KE20L TANT 47 µF 25V T495X476K025ATE150 C14, C15 NL NL NL D1 NL NL NL L1 3.3 µH 9.0A SMD 744314330 SMD LED GREEN CMDA5CG7D1Z 805 C13 LD1 1210 CASE D 10.0 KΩ 0603 1% RC0603FR-710KL 603 R2 249Ω 0603 1% RC0603FR-07249RL 603 R4 100 KΩ 0603 1% RC0603FR-07100KL 603 R5 45.3 KΩ 0603 1% RC0603FR-0745K3L 603 R7 7.15 kΩ 0603 1% RC0603FR-077K15RL 603 R8 1Ω 0603 1% IC BUFF NON-INV RC0603FR-071RL R1, R3, R6 U1 NC7SZ125M5X 603 SOT23-5 Connection Descriptions Terminal Silkscreen Description PVIN Connect the power supply between this terminal and the GND terminal besides it. The device is rated between 3V to 18V. The absolute voltage rating is 22V. GND The GND terminals are meant to provide a close return path to the power and signal terminal besides them. They are all connected together on board. SW SW is connected to the switch node of the power stage. It can be used to monitor the switch node waveform by a scope. VOUT AVIN_EX EN PGOOD VOUT terminal is connected to the output capacitor on the board and should be connected to the load LM21305 Evaluation Board allows using a separate supply voltage to AVIN with JP1 selection and a 2nd supply to AVIN_EX terminal. AVIN around 5V will result in the best efficiency in most of the cases. This terminal connects to the EN pin of the device. The EN is pulled up to AVIN via a 10 kΩ resistor on the board. It also can be externally controlled through this terminal. If driven externally, a voltage typically greater than 1.2V will enable the device. This terminal connects to the power good output of the device. There is a 10 kΩ pull-up resistor from this pin to the 2V5 pin. 3 www.national.com AN-2042 Evaluation Board Bill of Materials AN-2042 Jumper Settings Terminal Silkscreen Description Sets the AVIN of LM21305. Pin 2,3 (upper) connected gives AVIN = PVIN. Pin 1,2 connected gives AVIN = AVIN_EX Default pin 2 and 3 connected JP1 J1 Enables the on board LED LD1. When J1 is ON, LD1 will be ON if PGOOD is high. When J1 is OFF, power used to drive LD1 is saved. Default ON J2 Synchronizing clock input. When J2 is ON, CFQ is connected to ground and switching frequency is controlled by the on board resistor RFQ. When J2 is OFF, switch node waveform will be synchronized to the clock source connected to J2. Default ON J3 Only should be connected when AVIN = 5V. When AVIN is below 5V, especially around 3.3V, connecting J3 can result in better efficiency. Default OFF Caution: if AVIN > 5.5V, connecting J3 could damage the device. Design examples for PVIN = 12V, fs = 500 kHz, IOUT-MAX = 5A, VOUT = 1.2V, 1.8V, 2.5V, 3.3V and 5V: Other Design Examples LM21305 is designed to fit a wide variety of applications. A few examples are given for ease of use. Only the components need to be modified are listed below. VOUT 1.2V 1.8V 2.5V 3.3V 5V C8 10000 pF 25V 10000 pF 25V 4700 pF 25V 4700 pF 25V 4700 pF 25V L1 1.2 µH 2.2 µH 2.2 µH 3.3 µH 9.0A 3.3 µH R5 10.0 kΩ 1% 20 kΩ 1% 31.6 kΩ 1% 45.3 kΩ 1% 73.2 kΩ 1% R7 2.40 kΩ 1% 3.60 kΩ 1% 5.10 kΩ 1% 6.65 kΩ 1% 10.0 kΩ 1% www.national.com 4 AN-2042 Typical Performance Characteristics Efficiency with PVIN = AVIN = 12V VOUT = 3.3V and 5V, fs = 500 kHz Efficiency with PVIN = 12V, AVIN = 5V VOUT = 3.3V and 5V, fs = 500 kHz 30119404 30119405 Soft Start with 1V Pre-Bias Voltage, No Load Soft Start with 5A Load 30119403 30119402 Switching Waveform with 0A Load (DCM Mode) Switching Waveform with 5A Load 30119409 30119408 5 www.national.com AN-2042 switching frequency is also limited if an operation condition is possible to trigger Ton-min and Toff-min. The maximum frequency can be used for given input and output voltage can be found by: Component Selection This section provides a simplified design procedure necessary to select the external components to build a fully functional efficient step-down power supply. As with any DC-DC converter, numerous tradeoffs are possible to optimize the design for efficiency, size, and performance. Unless otherwise indicated all formulas assume units of amps (A) for current, farads (F) for capacitance, henries (H) for inductance, volts (V) for voltages and Hertz (Hz) for frequencies. For more details, please refer to the LM21305 datasheet. The following equation should be used to calculate the resistor R4 value in order to obtain a desired frequency of operation: F[kHz] = 31000 * R−0.9[kΩ] INPUT CAPACITORS PVIN is the supply voltage for the switcher power stage. It is the supply that delivers the output power. The input capacitors on PVIN supplies the large AC switching current drawn by the switching action of the internal MOSFETs. The input current of a buck converter is discontinuous, so the ripple current supplied by the input capacitor is large. The input capacitor must be rated to handle this current. To prevent large voltage transients from occurring, a low ESR input capacitor sized for the maximum RMS current should be used. The maximum RMS current is given by: INDUCTOR A general recommendation for the inductor in the LM21305 application is keeping a peak-to-peak ripple current between 20% and 40% of the maximum DC load current (5 A), 30% is desired. It also should have a high enough current rating and DCR as small as possible. The peak-to-peak current ripple can be calculated by: The current ripple is larger with smaller inductance and/or lower switching frequency. In general, with a fixed Vout, the higher the PVIN, the higher the inductor current ripple. If PVIN is kept constant, the higher the Vout, the higher the inductor current ripple, as long as , otherwise, ripple will decrease with Vout increase. It is recommended to choose L such that: The power dissipated in the input capacitor is given by: PD_CIN = I2RMS_CINRESR_CIN where RESR_CIN is the ESR of the input capacitor. This formula has a maximum at PVIN = 2VOUT, where I RMS ≅ IOUT/2. This simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. Several capacitors may also be paralleled to meet size or height requirements in the design. For low input voltage applications, sufficient bulk input capacitance is needed to minimize transient effects during output load changes. A 0.1 µF or a 1µF ceramic bypass capacitor is also recommended to be placed right between the PVIN and PGND pins. Please refer to the layout recommendation section. The inductor should be rated to handle the maximum load current plus the ripple current. IL(MAX) = ILOAD(MAX) + ΔiL(MAX)/2 An inductor with saturation current higher than the over current protection limit is a safe choice. It is desired to have small inductance in switching power supplies, because it usually means faster transient response, smaller DCR, and smaller size for more compact design. But too small inductance will generate too large inductor current ripple and it could falsely trigger over current protection at the maximum load. It also generates more conduction loss, since the RMS current is higher comparing to smaller ripple with the same DC current. Larger inductor current ripple generates larger output voltage ripple with the same output capacitors as well. With peak current mode control, it is not recommended to have too small inductor current ripple either, so that the peak current comparator has enough signal-to-noise ratio. AVIN FILTER to AVIN. These can be seen on the schematic as components RF and CF. There is a practical limit to the size of the resistor RF as the AVIN pin will draw a short 60mA burst of current during startup, and if RF is too large the resulting voltage drop can trigger the UVLO comparator. For the demo board a 1Ω resistor is used for RF ensuring that UVLO will not be triggered after the part is enabled. A recommended 1μF CF capacitor coupled with the 1Ω resistor provides roughly 16 dB of attenuation at the 1MHz switching frequency. SWITCHING FREQUENCY SELECTION LM21305 supports a wide range of switching frequencies: 300 kHz to 1.5 MHz. The choice of switching frequency is usually a compromise between efficiency and size of the circuit. Lower switching frequency usually means lower switching losses (including gate charge losses, transition IV loss etc.) and would result in a better efficiency most of the time. But higher switching frequency allows using smaller LC filters (more compact design). Smaller L also helps transient response and reduces the conduction loss by smaller DCR. The best switching frequency for efficiency needs to be determined case by case. It is related to the input voltage, the output voltage, the most frequent load level, external component choices, and circuit size requirement. The choice of www.national.com OUTPUT CAPACITOR The device is designed to be used with a wide variety of LC filters. While it is generally desired to use as little output capacitance as possible to keep costs and size down. The output capacitors COUT should be chosen with care since it directly affects the steady state output voltage ripple, loop stability and the voltage over/undershoot during a load transient. The output voltage ripple is composed of two parts. One is caused by the inductor current ripple going through the Equivalent Series Resistance (ESR) of the output capacitors: ΔVOUT-ESR = ΔiLP-P * ESR 6 COMPENSATION CIRCUIT achieve high performance in terms of the transient response, audio susceptibility and output impedance. The LM21305 will typically require only a single resistor Rc and capacitor Cc1 for compensation, but depending on the power stage it could require a second capacitor for a high frequency pole. Since the two components in the ripple are not in phase, the actual peak-to-peak ripple is smaller than the sum of the two peaks: Output capacitance is usually limited by transient performance if the system requires tight voltage regulation with presence of large current steps and fast slew rate. To maintain a small over- or undershoot during transient, small ESR and large capacitance are desired. But these also come with higher cost and size. The control loop should also be fast to reduce the voltage droop. One or more ceramic capacitors are recommended because they have very low ESR and remain capacitive up to high frequencies. The dielectric should be X5R, X7R, or comparable material to maintain proper tolerances. Other types of capacitors also can be used if large capacitance is needed, such as tantalum, poscap and OSCON. Such capacitors have lower 1/(2πESR *C) frequency than ceramic capacitors. The lower RC frequency could affect the control loop if it is close to the crossover frequency. If high switching frequency and high crossover frequency are desired, all ceramic design is more appropriate. 30119421 Compensation Network for LM21305 To select the compensation components, a desired cross over frequency fc should be selected first. It is recommended fc is equal to or lower than fs/8. A simplified procedure is given below for Rc and Cc1, assuming the capacitor ESR zero is at least 3 times higher than fc. The compensation resistor can be found by: Cc1 does not affect the crossover frequency fc, but it sets the compensator zero fzcomp and affects the phase margin of the loop. For a fast design, Cc1 = 10 nF gives adequate performance in most LM21305 applications. Larger Cc1 gives larger phase margin, while lower Cc1 gives higher gain at lower frequency thus faster transient respond. It is recommended to set the compensation zero no higher than fc/3 to ensure enough phase margin, meaning: For more details, please refer to the LM21305 datasheet. 7 www.national.com AN-2042 The other is caused by the inductor current ripple charging and discharging the output capacitors: AN-2042 PCB Layout 30119426 FIGURE 1. Top Layer 30119427 FIGURE 2. Middle Layer 1 www.national.com 8 AN-2042 30119410 FIGURE 3. Middle Layer 2 30119411 FIGURE 4. Bottom Layer 9 www.national.com AN-2042 30119412 FIGURE 5. Top Overlay 30119413 FIGURE 6. Bottom Overlay www.national.com 10 AN-2042 11 www.national.com LM21305 Evaluation Board Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: www.national.com Products Design Support Amplifiers www.national.com/amplifiers WEBENCH® Tools www.national.com/webench Audio www.national.com/audio App Notes www.national.com/appnotes Clock and Timing www.national.com/timing Reference Designs www.national.com/refdesigns Data Converters www.national.com/adc Samples www.national.com/samples Interface www.national.com/interface Eval Boards www.national.com/evalboards LVDS www.national.com/lvds Packaging www.national.com/packaging Power Management www.national.com/power Green Compliance www.national.com/quality/green Switching Regulators www.national.com/switchers Distributors www.national.com/contacts LDOs www.national.com/ldo Quality and Reliability www.national.com/quality LED Lighting www.national.com/led Feedback/Support www.national.com/feedback Voltage References www.national.com/vref Design Made Easy www.national.com/easy www.national.com/powerwise Applications & Markets www.national.com/solutions Mil/Aero www.national.com/milaero PowerWise® Solutions Serial Digital Interface (SDI) www.national.com/sdi Temperature Sensors www.national.com/tempsensors SolarMagic™ www.national.com/solarmagic PLL/VCO www.national.com/wireless www.national.com/training PowerWise® Design University THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. NATIONAL MAKES NO REPRESENTATIONS OR WARRANTIES WITH RESPECT TO THE ACCURACY OR COMPLETENESS OF THE CONTENTS OF THIS PUBLICATION AND RESERVES THE RIGHT TO MAKE CHANGES TO SPECIFICATIONS AND PRODUCT DESCRIPTIONS AT ANY TIME WITHOUT NOTICE. NO LICENSE, WHETHER EXPRESS, IMPLIED, ARISING BY ESTOPPEL OR OTHERWISE, TO ANY INTELLECTUAL PROPERTY RIGHTS IS GRANTED BY THIS DOCUMENT. TESTING AND OTHER QUALITY CONTROLS ARE USED TO THE EXTENT NATIONAL DEEMS NECESSARY TO SUPPORT NATIONAL’S PRODUCT WARRANTY. EXCEPT WHERE MANDATED BY GOVERNMENT REQUIREMENTS, TESTING OF ALL PARAMETERS OF EACH PRODUCT IS NOT NECESSARILY PERFORMED. NATIONAL ASSUMES NO LIABILITY FOR APPLICATIONS ASSISTANCE OR BUYER PRODUCT DESIGN. BUYERS ARE RESPONSIBLE FOR THEIR PRODUCTS AND APPLICATIONS USING NATIONAL COMPONENTS. PRIOR TO USING OR DISTRIBUTING ANY PRODUCTS THAT INCLUDE NATIONAL COMPONENTS, BUYERS SHOULD PROVIDE ADEQUATE DESIGN, TESTING AND OPERATING SAFEGUARDS. EXCEPT AS PROVIDED IN NATIONAL’S TERMS AND CONDITIONS OF SALE FOR SUCH PRODUCTS, NATIONAL ASSUMES NO LIABILITY WHATSOEVER, AND NATIONAL DISCLAIMS ANY EXPRESS OR IMPLIED WARRANTY RELATING TO THE SALE AND/OR USE OF NATIONAL PRODUCTS INCLUDING LIABILITY OR WARRANTIES RELATING TO FITNESS FOR A PARTICULAR PURPOSE, MERCHANTABILITY, OR INFRINGEMENT OF ANY PATENT, COPYRIGHT OR OTHER INTELLECTUAL PROPERTY RIGHT. LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS PRIOR WRITTEN APPROVAL OF THE CHIEF EXECUTIVE OFFICER AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: Life support devices or systems are devices 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. A critical component is any component in 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. National Semiconductor and the National Semiconductor logo are registered trademarks of National Semiconductor Corporation. All other brand or product names may be trademarks or registered trademarks of their respective holders. AN-2042 Copyright© 2010 National Semiconductor Corporation For the most current product information visit us at www.national.com National Semiconductor Americas Technical Support Center Email: [email protected] Tel: 1-800-272-9959 www.national.com National Semiconductor Europe Technical Support Center Email: [email protected] National Semiconductor Asia Pacific Technical Support Center Email: [email protected] National Semiconductor Japan Technical Support Center Email: [email protected]