LM20143EVAL

User's Guide SNVA277B – October 2007 – Revised May 2013 AN-1691 LM20143 Evaluation Board 1 Introduction The LM20143 is a full featured buck switching regulator capable of driving up to 3A of load current. The switching frequency of the LM20143 can be varied from 500kHz to 1.5Mhz with an external resistor allowing the size of the power stage components to be reduced while still providing efficient operation. The LM20143 is capable of converting an input voltage between 2.95 V and 5.5 V down to an output voltage as low as 0.8 V. 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 LM20143, and the precision enable pin can be used to easily sequence the LM20143 in applications with sequencing requirements. The LM20143 is available in a 16-pin HTSSOP package with an exposed pad for enhanced thermal performance. The LM20143 evaluation board has been designed to balance overall solution size with the efficiency of the regulator. The evaluation board measures just under 1.3” x 1.1” on a two layer PCB, with all components placed on the top layer. The power stage and compensation components of the LM20143 evaluation board have been optimized for an input voltage of 5 V and a switching frequency of 1.5 MHz, but for testing purposes, the input can be varied across the entire operating range. The output voltage of the evaluation board is nominally 1.2 V, but this voltage can be easily changed by replacing one of the feedback resistors (RFB1 or RFB2). The control loop compensation of the LM20143 evaluation board has been designed to provide a stable solution over the entire input and output voltage range with a reasonable transient response. The EN pin must be above 1.18 V (typ) on the board to initiate switching. If the EN function is not necessary, the EN pin should be externally tied to VIN. LM20143 PGOOD L PGOOD VIN RPG RFB1 PVIN CIN CBYP VOUT SW RF EN EN AVIN COUT RFB2 VCC SS/TRK CVCC CSS CF COMP RC1 CC2 FB RT PGND AGND CC1 RT Figure 1. Evaluation Board Schematic All trademarks are the property of their respective owners. SNVA277B – October 2007 – Revised May 2013 Submit Documentation Feedback Copyright © 2007–2013, Texas Instruments Incorporated AN-1691 LM20143 Evaluation Board 1 Bill of Materials (BOM) 2 3 2 www.ti.com Bill of Materials (BOM) Designator Description Part Number Qty Manufacturer U1 Synchronous Buck Regulator LM20143 1 Texas Instruments CIN 47 µF, 1210, X5R, 6.3 V GRM32ER60J476ME20 1 Murata CBYP 1 µF, 0603, X5R, 6.3 V GRM188R60J105KA01 1 Murata COUT 47 µF, 1210, X5R, 6.3 V GRM32ER60J476ME20 1 Murata L 1.2 µH, 17 mΩ DO1813H-122ML 1 Coilcraft RF 1Ω, 0603 CRCW06031R0J-e3 1 Vishay-Dale CF 100 nF, 0603, X7R, 16 V GRM188R71C104KA01 1 Murata CVCC 1 µF, 0603, X5R, 6.3 V GRM188R60J105KA01 1 Murata RPG 10 kΩ, 0603 CRCW06031002F-e3 1 Vishay-Dale RC1 1 kΩ, 0603 CRCW06031001F-e3 1 Vishay-Dale CC1 4.7 nF, 0603, X7R, 25 V VJ0603Y472KXXA 1 Vishay-Vitramon CC2 OPEN OPEN 0 N/A CSS 33 nF, 0603, X7R, 25 V VJ0603Y333KXXA 1 Vishay-Vitramon RFB1 4.99 kΩ, 0603 CRCW06034991F-e3 1 Vishay-Dale RFB2 10 kΩ, 0603 CRCW06031002F-e3 1 Vishay-Dale RT 49.9 kΩ, 0603 CRCW06034992F-e3 1 Vishay-Dale Test Points Test Points 160-1026-02-01-00 7 Cambion Connection Descriptions Terminal Silkscreen Description VIN This terminal is the input voltage to the device. The device will operation over the input voltage range of 2.95 V to 5.5 V. The absolute maximum voltage rating for this pin is 6 V. 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 1.18 V will enable the device. The operating voltage for this pin should not exceed 5.5 V. The absolute maximum voltage rating on this pin is 6 V. SS/TRACK 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 0.8 V reference. The voltage on this pin should not exceed 5.5 V during normal operation. The absolute maximum voltage rating on this pin is 6 V. PGOOD This terminal connects to the power good output of the device. There is a 10 kΩ pull-up resistor from this pin to the input voltage. The voltage on this pin should not exceed 5.5 V during normal operation and has an absolute maximum voltage rating of 6 V. AN-1691 LM20143 Evaluation Board SNVA277B – October 2007 – Revised May 2013 Submit Documentation Feedback Copyright © 2007–2013, Texas Instruments Incorporated Performance Characteristics www.ti.com 4 Performance Characteristics Efficiency vs Load Line Regulation (ILOAD = 3A) Load Regulation (VIN = 5 V) 0.5A to 3A Load Transient Response (200 µs/DIV) Startup Waveform SNVA277B – October 2007 – Revised May 2013 Submit Documentation Feedback Copyright © 2007–2013, Texas Instruments Incorporated AN-1691 LM20143 Evaluation Board 3 Component Selection 5 www.ti.com Component Selection This section provides a walk-through of the design process of the LM20143 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. 5.1 Input Capacitor The required RMS current rating of the input capacitor for a buck regulator can be estimated by Equation 1: 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 Equation 3, it follows that the maximum ICIN(RMS) requirement will occur at a full 3A load current with the system operating at 50% duty cycle. Under this condition, the maximum ICIN(RMS) is given by: ICIN(RMS) = 3A 0.5 x 0.5 = 1.5A (3) Ceramic capacitors feature a very large IRMS rating in a small footprint, making a ceramic capacitor ideal for this application. A 47 µF X5R ceramic capacitor from Murata provides the necessary input capacitance for the evaluation board. For improved bypassing, a small 1 µF high frequency capacitor is placed in parallel with the 47 µF bulk capacitor to filter high frequency noise pulses on the supply. 5.2 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. 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 1 Ω resistor is used for RF ensuring that UVLO will not be triggered after the part is enabled. A recommended 1 µF CF capacitor coupled with the 1 Ω resistor provides greater than 16dB of attenuation at the set 1.5 MHz switching frequency. 5.3 Inductor As per the datasheet recommendations, the inductor value should initially be chosen to give a peak-topeak ripple current equal to roughly 30% of the maximum output current. The peak-to-peak inductor ripple current can be calculated by Equation 4: 'IP-P = (VIN - VOUT) x D L x fSW (4) Rearranging this equation and solving for the inductance reveals that for this application (VIN = 5 V, VOUT = 1.2 V, fSW = 1.5 MHz, and IOUT = 3A) the nominal inductance value is roughly 0.68 µH. However, a final inductance of 1.2 µH was selected to accommodate the full output voltage range of the device. This results in a peak-to-peak ripple current of 507 mA and 623 mA when the converter is operating from 5 V and 3.3 V, 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 LM20143 evaluation board, a Coilcraft DO1813H-122ML inductor offers a good balance between efficiency (17 mΩ DCR), size, and saturation current rating (5.3A ISAT rating). 5.4 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 Equation 5: 'VOUT = 'IP-P x RESR + 4 1 8 x fSW x COUT AN-1691 LM20143 Evaluation Board (5) SNVA277B – October 2007 – Revised May 2013 Submit Documentation Feedback Copyright © 2007–2013, Texas Instruments Incorporated Component Selection www.ti.com The variable RESR above refers to the ESR of the output capacitor. As can be seen in Equation 5, 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, a Murata 47 µF 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 3 mΩ, and the effective in-circuit capacitance is approximately 32 µF (reduced from 47 µF due to the 1.2 V DC bias). With these values, the peak-to-peak voltage ripple on the output when operating from a 5 V input can be calculated to be 3 mV. 5.5 CSS A soft-start capacitor can be used to control the startup time of the LM20143 voltage regulator. The startup time of the regulator when using a soft-start capacitor can be estimated by Equation 6: tSS = 0.8V x CSS ISS (6) For the LM20143, ISS is nominally 5 µA. For the evaluation board, the soft-start time has been designed to be roughly 5 ms, resulting in a CSS capacitor value of 33 nF. 5.6 CVCC The CVCC capacitor is necessary to bypass an internal 2.7 V sub-regulator. This capacitor should be sized equal to or greater than 1 µF, but less than 10 µF. A value of 1 µF is sufficient for most applications.. 5.7 CC1 The capacitor CC1 is used to set the crossover frequency of the LM20143 control loop. Since this board was optimized to be stable over the full input and output voltage range, the value of CC1 was selected to be 4.7 nF. 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 Section 5.8. 5.8 RC1 Once the value of CC1 is known, 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 Equation 7: RC1 = CC1 COUT x IOUT VOUT + 15 x D 1-D + VIN fSW x L -1 (7) For stability purposes the device should be compensated for the maximum output current expected in the application. 5.9 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 LM20143 evaluation board, the CC2 footprint is unpopulated, as 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 Equation 8: CC2 = COUT x RESR RC1 (8) SNVA277B – October 2007 – Revised May 2013 Submit Documentation Feedback Copyright © 2007–2013, Texas Instruments Incorporated AN-1691 LM20143 Evaluation Board 5 PCB Layout www.ti.com 5.10 RT The value for RT will set the operating frequency of the device. For this evaluation board, a value of 49.9 kΩ was chosen that sets oscillator frequency to 1.5 MHz. The value of RT can be adjusted to support operating frequencies from 500 kHz to 1.5 MHz by using Equation 9: RT = 154750 fSW - 55 (9) Where, fSW is the switching frequency in kHz, and RT is the frequency adjust resistor in kΩ. 5.11 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 LM20143 evaluation board is set to 1.2 V, giving resistor values of RFB1 = 4.99 kΩ and RFB2 = 10 kΩ. If a different output voltage is required, the value of RFB1 can be adjusted according to Equation 10: RFB1 = VOUT 0.8 - 1 x RFB2 (10) RFB2 does not need to be changed from its value of 10 kΩ. 6 PCB Layout Figure 2. Top Layer 6 AN-1691 LM20143 Evaluation Board SNVA277B – October 2007 – Revised May 2013 Submit Documentation Feedback Copyright © 2007–2013, Texas Instruments Incorporated PCB Layout www.ti.com Figure 3. 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