LM21212-1

LM21212-1 NSC | Alldatasheet

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March 21, 2011 Introduction This evaluation board provides a solution to examine the high efficiency LM21212-1 buck switching regulator. The LM21212-1 is capable of driving up to 12A of continuous load current with excellent output voltage accuracy due to its ±1% internal reference. This device also features a clock synchro- nization input that allows the switching frequency to be syn- chronized to an external clock source. The 300 kHz to 1.5 MHz frequency synchronization range enables the user to minimize the power stage component size, while still allowing for high efficiency. The LM21212-1 is capable of down con- verting from an input voltage between 2.95V and 5.5V. Fault protection features include current limit, 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 LM21212-1, and the precision enable pin can be used to easily sequence the LM21212-1 in applications with se- quencing requirements. The LM21212-1 evaluation board has been optimized to work from 2.95V to 5.5V, achieving a balance between overall so- lution size and regulator efficiency. The evaluation board measures just under 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 compo- nents of the LM21212-1 evaluation board have been opti- mized 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 30144801 © 2011 National Semiconductor Corporation 301448 www.national.com LM21212-1 Evaluation BoardAN-2107

Read this entire page prior to attempting to power the evalu- ation 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 12A capability) to VOUT for the positive connection and GND for the negative connection. Step 3: The ENABLE pin should be left open for normal op- eration. 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 12A. 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. Step 7: The frequency of operation can be varied as desired by connecting a 2.0V square-wave positive signal between SYNC and 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 ev- erything 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 LM21212-1 can occur. For these sup- plies, a large capacitor across the terminals of the supply (1000µF) can alleviate this problem. OVER CURRENT PROTECTION The evaluation board is configured with over-current protec- tion. This function is completely contained in the LM21212-1. The peak current is limited to approximately 17A. Connection Descriptions Terminal Silkscreen Description VIN 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. GND 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. VOUT This terminal connects to the output voltage of the power supply and should be connected to the load. ENABLE 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. SS/TRK 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. PGOOD 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. AC INJ This terminal block allows the user to insert an AC injection signal across a 49.9Ω resistor for open- loop gain bode measurements. A jumper shorts out this resistor when it is not needed. SWITCH This terminal allows easy probing of the switch node. Do not apply any external voltage source to this pin. SYNC This terminal connects to the SYNC pin of the device. The LM21212-1 can sychronize the SWITCH pin to a SYNC signal with a frequency between 300kHz and 1.5MHz. If this pin is left open, the switching frequency will default to 1MHz. VIN_SENSE+, VIN_SENSE- VOUT_SENSE+, VOUT_SENSE- These terminals allow a sense connection on the board for accurate VIN and VOUT measurements, respectively. www.national.com 2 AN-2107

The Bill of Materials is shown below, including the manufacturer and part number. ID DESCRIPTION VENDOR PART NUMBER QUANTITY AC INJ Header, TH, 100mil, 2x1, Gold plated, 230 mil above insulator Samtec Inc. TSW-102-07-G-S 1 C1 CAP, CERM, 1 uF, 10V, +/-10%, X7R, 0603 MuRata GRM188R71A105KA61D 1 C3, C4, C5, C6, C7, CAP, CERM, 100 uF, 6.3V, +/-20%, X5R, 1206 MuRata GRM31CR60J107ME39L 6 C9 CAP, CERM, 0.1 uF, 50V, +/-10%, X7R, 0603 TDK C1608X7R1H104K 1 CC1 CAP, CERM, 2700 pF, 50V, MuRata GRM1885C1H272JA01D 1 CC2 CAP, CERM, 82 pF, 50V, +/-5%, C0G/NP0, 0603 MuRata GRM1885C1H820JA01D 1 CC3 CAP, CERM, 820 pF, 50V, +/-5%, C0G/NP0, 0603 MuRata GRM1885C1H821JA01D 1 CSS CAP, CERM, 0.033 uF, 16V, +/-10%, X7R, 0603 MuRata GRM188R71C333KA01D 1 GND_FI, GND_FO, VIN_F, VOUT_F Standard Banana Jack, Uninsulated, 15A Johnson Components 108-0740-001 4 L1 Inductor, Shielded Drum Core, Powdered Iron, 560nH, 27.5A, 0.0018 ohm, SMD Vishay-Dale IHLP4040DZERR56M01 1 R1 RES, 1.0 ohm, 5%, 0.1W, 0603 Vishay-Dale CRCW06031R00JNEA 1 RAC RES, 49.9 ohm, 1%, 0.1W, 0603 Vishay-Dale CRCW060349R9FKEA 1 RC1 RES, 7.32 kohm, 1%, 0.1W, 0603 Vishay-Dale CRCW06037K32FKEA 1 RC2 RES, 165 ohm, 1%, 0.1W, 0603 Vishay-Dale CRCW0603165RFKEA 1 RFB1, RFB2, RPG RES, 10 kohm, 1%, 0.1W, 0603 Vishay-Dale CRCW060310K0JKEA 3 SH-J1 Shunt, 100mil, Gold plated, Black Samtec Inc. SNT-100-BK-G 1 U1 12A Buck DC/DC Converter National Semiconductor LM21212MH-1 1 5 www.national.com AN-2107

This section provides a walk-through of the design process of the LM21212-1 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: The variable D refers to the duty cycle, and can be approxi- mated by: From this equation, it follows that the maximum I CIN(RMS) re- quirement will occur at a full 12A load current with the system operating at 50% duty cycle. Under this condition, the maxi- mum ICIN(RMS) is given by: 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 de- vice 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 ce- ramic 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 27% of the max- imum 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 equa- tion: 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 LM21212-1 evaluation board, a Vishay IHLP4040DZER- R56M01 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 influ- ences 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- ple (ΔIP-P) the output voltage ripple can be approximated by the equation: The variable RESR above refers to the ESR of the output ca- pacitor. 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 capaci- tor. 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 ca- pacitor chemistries, resulting in the lowest output ripple for the given ripple current. Ceramic capacitors (especially high ca- pacitance, small package multi-layer types, or MLCC) lose thier capacitance as the DC voltage is increased. For this configuration, the actual capacitance value was approximat- ed 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 LM21212-1 voltage regulator. The startup time of the regulator when using a soft-start capacitor can be estimated by the following equation: For the LM21212-1, 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 am- plifier 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 pa- rameters needed for the compensation values are given in the table below. Parameter Value VIN 5.0V VOUT 1.2V IOUT 12A fCROSSOVER 80 kHz L 0.56 µH RDCR 1.8 mΩ CO 150 µF RESR 1.0 mΩ ΔVRAMP 0.8V fSW 500 kHz www.national.com 6 AN-2107

where ΔVRAMP is the oscillator peak-to-peak ramp voltage (nominally 0.8 V), f CROSSOVER 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 C O is the effective output capaci- tance at the programmed output voltage. It is recommended that fCROSSOVER not exceed one-fifth of the switching frequen- cy. The output capacitance, CO, depends on capacitor chem- istry 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 recommend- ed 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 com- pensation calculations rather than a larger, as this will result in a lower bandwidth but increased phase margin. First, the value of R FB1 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. Next, the value of CC1 can be calculated by placing a zero at half of the LC double pole frequency. 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 fre- quency as the zero created by the output capacitor ESR. The standard values used for the above calculations are given in the Bill of Materials. 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 LM21212-1 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 10k Ω. Resistor RAC has a value of 49.9Ω and is provided as an in- jection 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 val- ues. 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 de- vice above a programmed V IN, effectively creating a pro- grammable 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 k Ω resistor is recom- mended for R EN1 and the value of R EN2 can be calculated using the equation: where VTO is the desired VIN voltage at which the device will enable. 7 www.national.com AN-2107

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. 30144830 Top Layer (Copper planes outlined in grey) 30144831 Mid Layer1 30144832 Mid Layer2 30144833 Bottom Layer www.national.com 8 AN-2107

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