LM20146MHEVAL

User's Guide SNVA372A – January 2009 – Revised May 2013 AN-1902 LM20146 Evaluation Board 1 Introduction The LM20146 is a full-featured synchronous buck switching regulator capable of driving up to 6A of load current. The ability to adjust the operating frequency from 250kHz to 750kHz gives the designer flexibility in component selection. The LM20146 is capable of converting an input voltage between 2.95V and 5.5V down to an output voltage as low as 0.8V. Fault protection features include cycle-by-cycle 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 LM20146, and the precision enable pin can be used to easily sequence the LM20146 in sequence critical applications. The LM20146 is available in a 16-pin HTSSOP package with an exposed pad for enhanced thermal performance. The LM20146 evaluation board is designed to balance overall solution size and transient response. The evaluation board measures 1.5” × 1.5” on a four layer PCB with all components placed on the top layer. The power stage and compensation components are 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 by replacing one of the feedback resistors (RFB1 or RFB2). The control loop compensation of the LM20146 evaluation board is designed to provide a stable solution over the entire input voltage range with a fast transient response. The EN pin is tied to VIN with a 10kΩ resistor and can be toggled directly through the enable test point. LM20146 L PGOOD PGOOD VOUT SW RPG VIN RFB1 PVIN CIN AVIN CF FB REN RF EN EN VCC SS/TRK COMP CC2 RC1 PGND AGND CC1 CO1 RFB2 CVCC CSS RT RT Figure 1. Evaluation Board Schematic All trademarks are the property of their respective owners. SNVA372A – January 2009 – Revised May 2013 Submit Documentation Feedback Copyright © 2009–2013, Texas Instruments Incorporated AN-1902 LM20146 Evaluation Board 1 Connection Descriptions 2 www.ti.com Connection Descriptions Table 1. Connection Descriptions PCB Silkscreen VIN This terminal is the input voltage to the device. The device will operate over the input voltage range of 2.95V to 5.5V. The absolute maximum voltage rating for this pin is 6V. GND This terminal is the ground connection to the device. There are two different GND connections on the PCB. One should be used for the input supply and the other for the load. VOUT This terminal connects to the output voltage of the power supply and should be connected to the load. EN This terminal connects to the enable pin of the device. This terminal should be connected to VIN or driven externally. If driven externally, a voltage typically greater than 1.18V will enable the device. The operating voltage for this pin should not exceed 5.5V. The absolute maximum voltage rating on this pin is 6V. 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 0.8V reference. The voltage on this pin should not exceed 5.5V during normal operation. The absolute maximum voltage rating on this pin is 6V. PGOOD This terminal connects to the power good output of the device. There is a 10kΩ pull-up resistor from this pin to the input voltage. The voltage on this pin should not exceed 5.5V during normal operation and has an absolute maximum voltage rating of 6V. SW VOUT TP 3 Description This point allows a scope probe to be connected to observe the switch node voltage. This terminal provides an oscilloscope probe connection directly to VOUT to probe the transient response and output voltage ripple. Component Selection This section provides a walk-through of the design process for the LM20146 evaluation board. Unless otherwise indicated all formulas assume units of Amps (A) for current, Farads (F) for capacitance, Henries (H) for inductance, and Volts (V) for voltage. 3.1 Input Capacitor The required RMS current rating of the input capacitor for a buck regulator can be estimated by the following equation: ICIN(RMS) = IOUT D(1 - D) (1) The variable D refers to the duty cycle and can be approximated by: D= VOUT VIN (2) From this equation, it follows that the maximum ICIN(RMS) requirement will occur at a full 6A load current with the system operating at 50% duty cycle. Under this condition, the maximum ICIN(RMS) is given by: ICIN(RMS) = 6A 0.36 x 0.64 = 2.9A (3) Ceramic capacitors feature a very large IRMS rating in a small footprint, making a ceramic capacitor ideal for this application. A 100µF X5R ceramic capacitor from TDK provides the necessary input capacitance for the evaluation board. 2 AN-1902 LM20146 Evaluation Board SNVA372A – January 2009 – Revised May 2013 Submit Documentation Feedback Copyright © 2009–2013, Texas Instruments Incorporated Component Selection www.ti.com 3.2 AVIN Filter An RC filter should be added to prevent any switching noise on PVIN from interfering with the internal analog circuitry connected to AVIN. This can be seen on the schematic as components RF and CF. There is a practical limit to the value of resistor RF as the AVIN pin will draw a short 60mA burst of current during startup. If RF is too large the resulting voltage drop can trigger the UVLO comparator. For the evaluation board, a 1.0Ω resistor is used for RF ensuring that UVLO will not be triggered after the part is enabled. A 1.0µF capacitor in conjunction with the 1.0Ω resistor is recommended to filter the input to AVIN. 3.3 Inductor As per the datasheet recommendations, the inductor value should initially be chosen to give a peak to peak ripple current equal to roughly 30% of the maximum output current. The peak to peak inductor ripple current can be calculated by the equation: 'IP-P = (VIN - VOUT) x D L x fSW (4) Rearranging this equation and solving for the inductance reveals that for this application (VIN = 5V, VOUT = 1.2V, fSW = 750kHz, and IOUT = 6A) the nominal inductance value is roughly .68 µH. This results in a peak-to-peak ripple current of 1.8A and 1.5A when the converter is operating from 5V and 3.3V respectively. 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 LM20146 evaluation board, a TDK SPM6530T-R68M140 inductor offers a good balance between efficiency (5.39mΩ DCR), size (7.1mm × 6.5mm), and saturation current rating (14A ISAT). If the output voltage of the evaluation board is increased or if the switching frequency is decreased, there is a chance the device may hit current limit at 6A output. To avoid current limit with higher output voltages the value of the inductor should be increased to reduce the ripple current. 3.4 Output Capacitor 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 peakto-peak inductor current ripple (ΔIP-P) the output voltage ripple can be approximated by the equation: 'VOUT = 'IP-P x RESR + 1 8 x fSW x COUT (5) The variable RESR above refers to the Effective Series Resistance of the output capacitor. As can be seen in the above equation, the ripple voltage on the output can be divided in two parts. One part is attributed to the AC ripple current flowing through the ESR of the output capacitor. The other part is due to the AC ripple current charging and discharging the output capacitor. The output capacitor also has an affect on the amount of droop that is seen on the output voltage in response to a load transient event. For the evaluation board, a TDK 100µF ceramic capacitor is selected for the output capacitor to provide good transient and DC performance in a relatively small package. From the technical specifications of this capacitor, the ESR is roughly 3mΩ and the effective in-circuit capacitance is approximately 60µF (reduced from 100µF due to the 1.2V DC bias and worse case tolerance). With these values, the peak to peak voltage ripple on the output when operating from a 5V input can be calculated to be about 10mV. 3.5 CSS A soft-start capacitor can be used to control the startup time of the LM20146. The startup time when using a soft-start capacitor can be estimated by the following equation: tSS = 0.8V x CSS ISS (6) For the LM20146, ISS is nominally 5µA. For the evaluation board, the soft-start time has been designed to be roughly 5 ms, resulting in a CSS capacitor value of 33nF. SNVA372A – January 2009 – Revised May 2013 Submit Documentation Feedback Copyright © 2009–2013, Texas Instruments Incorporated AN-1902 LM20146 Evaluation Board 3 Component Selection 3.6 www.ti.com CVCC The CVCC capacitor is necessary to bypass an internal 2.7V subregulator. This capacitor should be sized equal to or greater than 1µF but less than 10µF. A value of 1µF is sufficient for most applications. 3.7 CC1 The capacitor CC1 is used to set the crossover frequency of the LM20146 control loop. Since this board was optimized to work well over the full input and output voltage range, the value of CC1 was selected to be 1.2nF. Once the operating conditions for the device are known, the transient response can be optimized by reducing the value of CC1 and calculating the value for RC1 as outlined in the next section. 3.8 RC1 Once the value of CC1 is selected, resistor RC1 is used to place a zero in the control loop to cancel the output filter pole. This resistor can be sized according to the equation: CC1 RC1 = COUT x IOUT VOUT + D x fSW D + fSW x L 48750 x VIN 1 1 -1 2 x fSW x L (7) For stability purposes the device should be compensated for the maximum output current expected in the application. 3.9 CC2 A second compensation capacitor CC2 can be used in some designs to provide a high frequency pole, useful for cancelling a possible zero introduced by the ESR of the output capacitor. For the LM20146 evaluation board, the CC2 footprint is unpopulated because the low ESR ceramic capacitor used on the output does not contribute a zero to the control loop before the crossover frequency. If the ceramic capacitor on the evaluation board is replaced with a different capacitor having significant ESR, the required value of the capacitor CC2 can be estimated by the equation: CC2 = COUT x RESR RC1 (8) 3.10 RFB1 and RFB2 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 LM20146 evaluation board is set to 1.2V, giving resistor values of RFB1 = 4.99kΩ and RFB2 = 10.0kΩ. If a different output voltage is required, the value of RFB1 can be adjusted according to the equation: RFB1 = VOUT 0.8 - 1 x RFB2 (9) RFB2 does not need to be changed from its value of 10.0kΩ. 4 AN-1902 LM20146 Evaluation Board SNVA372A – January 2009 – Revised May 2013 Submit Documentation Feedback Copyright © 2009–2013, Texas Instruments Incorporated Bill of Materials www.ti.com 4 Bill of Materials Table 2. Bill of Materials Designator Description Part Number Qty Manufacturer U1 Synchronous Buck Regulator LM20146 1 Texas Instruments CIN 100µF, 1210, X5R, 6.3V C3225X5R0J107M 1 TDK CO1 100µF, 1210, X5R, 6.3V C3225X5R0J107M 1 TDK CO2 OPEN N/A 0 N/A L 0.68µH, 5.39 mΩ, 14A Isat SPM6530T-R68M140 1 TDK RF 1.0Ω, 0603 CRCW06031R0J-e3 1 Vishay-Dale CF 1.0µF, 0603, X5R, 6.3V GRM188R60J105KA01 1 Murata CVCC 1.0µF, 0603, X5R, 6.3V GRM188R60J105KA01 1 Murata RPG 10.0kΩ, 0603 CRCW06031002F-e3 1 Vishay-Dale RC1 8.25 kΩ, 0603 CRCW06038252F-e3 1 Vishay-Dale CC1 1.2nF, 0603, COG, 50V GRM1885C1H122JA01 1 Murata CC2 OPEN N/A 0 N/A CSS 33.0nF, 0603, X7R, 25V VJ0603Y333KXXA 1 Vishay-Vitramon RFB1 4.99kΩ, 0603 CRCW06034991F-e3 1 Vishay-Dale RFB2 10.0kΩ, 0603 CRCW06031002F-e3 1 Vishay-Dale REN 10.0kΩ, 0603 CRCW06031002F-e3 1 Vishay-Dale RT 48.7kΩ, 0603 CRCW06034872F-e3 1 Vishay-Dale VOUT TP Test Point 131503100 1 Tektronix EN, PGOOD, SW, SS/RK Test Points 52F7279 4 Keystone P1,P2,P3,P4 POWER I/O 160-1026-02-01-00 4 Cambion SNVA372A – January 2009 – Revised May 2013 Submit Documentation Feedback Copyright © 2009–2013, Texas Instruments Incorporated AN-1902 LM20146 Evaluation Board 5 Performance Characteristics 5 www.ti.com Performance Characteristics Efficiency vs Load Line Regulation (ILOAD = 6A) VIN = 3.3V VIN = 5V Load Regulation (VIN = 5V) 1.5A to 6A Load Transient Response VOUT (100 mV/DIV) IOUT (2A/DIV) IOUT (1.5A to 6A) TIME (40 és/DIV) Startup Waveform RLOAD = 0.2Ö VEN (5V/Div) VOUT (500 mV/Div) IOUT (2A/Div) PGOOD (2A/Div) TIME (2 ms/DIV) 6 AN-1902 LM20146 Evaluation Board SNVA372A – January 2009 – Revised May 2013 Submit Documentation Feedback Copyright © 2009–2013, Texas Instruments Incorporated PCB Layout www.ti.com 6 PCB Layout Figure 2. Top Layer Figure 3. Mid Layer 1 Figure 4. Mid Layer 2 Figure 5. Bottom Layer SNVA372A – January 2009 – Revised May 2013 Submit Documentation Feedback Copyright © 2009–2013, Texas Instruments Incorporated AN-1902 LM20146 Evaluation Board 7 IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, enhancements, improvements and other changes to its semiconductor products and services per JESD46, latest issue, and to discontinue any product or service per JESD48, latest issue. Buyers should obtain the latest relevant information before placing orders and should verify that such information is current and complete. All semiconductor products (also referred to herein as “components”) are sold subject to TI’s terms and conditions of sale supplied at the time of order acknowledgment. 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