LM2854-1000EVAL

User's Guide SNVA358B – August 2008 – Revised May 2013 AN-1880 LM2854 1MHz Buck Regulator Demo Board 1 Introduction The LM2854 PowerWise™ SIMPLE SWITCHER® buck regulator demonstration board is a 1 MHz stepdown voltage regulator capable of driving 0A up to 4A load current with excellent power conversion efficiency. A typical schematic of an LM2854 application is given in Figure 1. The LM2854 pin-out and pin description are given in Section 5. The LM2854 demonstration board is designed to accept an input voltage rail between 2.95V and 5.5V and deliver a fixed and highly accurate output voltage of 1.2V. The output voltage level can be changed by modification of one feedback resistor value. Externally established soft-start with a small value capacitance facilitates a controlled, well-defined and monotonic start-up output voltage characteristic. In addition, the LM2854 is capable of starting monotonically and glitch free into a pre-biased load. With some of the required voltage loop compensation components integrated in the regulator, the number of external passive components and PC board area typically necessary in a voltage mode buck converter application are reduced. An LM2854 based regulator design with type III loop compensation can be implemented with as few as eight external components. Only two small size external compensation components are required, similar to that commonly involved with current mode control compensation. Unlike a compensation solution where all the compensation components are integrated, the LM2854 has the flexibility to deal with ceramic and/or electrolytic based load capacitance spanning a wide range of capacitor values. Lossless cycle-by-cycle peak current limit is used to protect the load from an overcurrent or short-circuit fault, and an enable comparator permits system sequencing or increase of the input UVLO above the nominal 2.7V level. The device is available in a power enhanced HTSSOP-16 package featuring an exposed die attach pad that improves the thermal performance of the regulator. VIN VOUT LO LM2854 SW PVIN CO AVIN CIN FB EN SS AGND PGND RFB1 CSS RFB2 RCOMP CCOMP Figure 1. Typical System Application Using LM2854 Synchronous Buck Regulator PowerWise is a trademark of Texas Instruments. SIMPLE SWITCHER is a registered trademark of Texas Instruments. All other trademarks are the property of their respective owners. SNVA358B – August 2008 – Revised May 2013 Submit Documentation Feedback AN-1880 LM2854 1MHz Buck Regulator Demo Board Copyright © 2008–2013, Texas Instruments Incorporated 1 LM2854 Demo Board Details 2 www.ti.com LM2854 Demo Board Details This application solution relates to the bill of materials shown in Section 6 and references the schematic diagram in Figure 2. The board contains the LM2854MHX-1000 buck regulator IC with nominal switching frequency, fSW, of 1MHz. LM2854MHX-1000 5,6,7 VIN Lf PVIN 12,13 VOUT SW 2.95V ± 5.5V Rf + Cin Ren 11 Cin2 10 U1 FB AVIN AGND SS Cf GND 1.2V, 4A EN 14 15 Css Co2 16 Co + PGND Rfb1 1,2,3, 4,8,9 Rc GND Cc Rfb2 Figure 2. LM2854 Demo Board Schematic Diagram 3 Quick Step Procedure Step 1: Set the power supply current limit to 3A. Turn off the power supply. Connect the power supply to the VIN and GND terminals. Step 2: Connect the load with a 4A capability to the VOUT and GND terminals. Step 3: The EN terminal can be left open for normal operation as there is an on-board pull-up resistor. Step 4: Set VIN to 3.0V with no load applied. VOUT should be in regulation with a nominal 1.2V output. Step 5: Slowly increase the load while monitoring the output voltage. VOUT should remain in regulation with a nominal 1.2V output as the load is increased up to 4A. Step 6: Slowly sweep the input voltage from 2.95V to 5.5V. VOUT should remain in regulation with a nominal 1.2V output. Step 7: Temporarily short the EN terminal to GND to check the shutdown function. Step 8: Increase the load beyond the normal range to check current limit. The output current should limit at approximately 5.6A. Short the VOUT and GND terminals to verify short circuit protection. 2 AN-1880 LM2854 1MHz Buck Regulator Demo Board SNVA358B – August 2008 – Revised May 2013 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated Component Selection www.ti.com 4 Component Selection 4.1 Filter Inductor The selection of the output filter components, Lf and Co, are intrinsically linked as both of these parameters affect the stability of the system and various characteristics of the output voltage. First, a 0.82 µH inductor is chosen to allow stable operation (per datasheet recommendations) over the entire input voltage range from 2.95V to 5.5V. The inductance also directly affects the amplitude of the inductor current ripple which flows in the output capacitor. The filter inductance is given by: Lf = VOUT(1-D) 'iLfSW = VOUT 1 - VOUT VIN 'iLfSW (1) where the variable D refers to the duty cycle and can be approximated by: D= VOUT VIN (2) From this, it follows that the inductor ripple current, ΔiL, reaches a maximum when duty cycle is minimum or input voltage is maximum, i.e. VIN = 5.5V. Under these conditions, the inductor peak to peak ripple current is given by: 1.2V 1 'iL = 1.2V 5.5V 0.82 PH x 1 MHz = 1.14A (3) or approximately 29% of full load current. It follows that the peak inductor current at full load is: ILpk = IOUT + 'iL 2 = 4A + 1.14A = 4.57A 2 (4) and this level is adequately below the peak inductor current associated with current limit, specified in the datasheet as 6.7A maximum. This implies that an inductor must be selected with a saturation current higher than 6.7A to ensure that the inductor will never saturate during normal or fault operating conditions. This evaluation board uses the Vishay IHLP2525 series 0.82 µH inductor to provide the necessary current handling capability with low DC resistance in a relatively small footprint and profile. 4.2 Output Capacitor The output capacitance and its equivalent series resistance (ESR) affect both the ripple voltage at the output and the overall stability of the loop. The output capacitor provides a low impedance path for the inductor ripple current and presents a source of charge for transient loading conditions. In this example, one 47 µF 1206 multi-layer ceramic capacitor (MLCC) was selected. Ceramic capacitors provide very low ESR but can exhibit a significant reduction in capacitance with applied DC bias. Using manufacturer’s data, the ESR at 1 MHz is 3 mΩ and there is approximately 40% reduction in capacitance at 1.2V. This is verified by measuring the output ripple voltage and frequency response of the circuit. The fundamental component of the output ripple voltage amplitude is calculated as: 'VOUT = 'iL 2 1 2 RESR + 8fSWCO (5) and with typical values from this example: 'VOUT = 1.14A (3 m:)2 + 1 8 1 MHz 30 PF 2 = 5.8 mV (6) Because the load could transition quickly from no load to full load, it is sometimes common to add output bulk capacitance in the form of aluminum electrolytic (Al-E), tantalum (Ta), solid aluminum, organic polymer, and niobium (Nb) capacitors. This is largely unnecessary with the LM2854 as the loop crossover frequency can be made sufficiently large to accommodate high di/dt load transients. SNVA358B – August 2008 – Revised May 2013 Submit Documentation Feedback AN-1880 LM2854 1MHz Buck Regulator Demo Board Copyright © 2008–2013, Texas Instruments Incorporated 3 Component Selection 4.3 www.ti.com Input Filter The necessary RMS current rating of the input capacitor can be estimated by the following equation: ICin(RMS) = IOUT D(1-D) (7) From this equation, it follows that the maximum RMS current will occur at full 4A load current with the system operating at 50% duty cycle. However, with the specified output voltage, the worst case occurs at minimum input voltage of 2.95V. Hence, the relevant duty cycle is 0.41 and the maximum RMS current is given by: ICin(RMS) = 4A 0.41 (1 - 0.41) = 1.97A (8) Ceramic capacitors feature a very large RMS current rating in a small footprint making them ideal for this application. Choosing a 47 µF 10V ceramic capacitor to provide the necessary input capacitance and assuming 50% capacitance voltage coefficient, the input AC ripple amplitude, neglecting ESR, is: 'VIN = IOUTD(1-D) 4A 0.41 (1 - 0.41) = = 10 mV fSWCin 1 MHz 100 PF (9) When operating near the minimum input voltage, an electrolytic input capacitor is helpful to damp the input for a typical bench test setup. Essentially, a resonant circuit is formed by the line impedance and input capacitance. The 6TPE100MPB by Sanyo has 100 µF capacitance and an ESR of 25 mΩ. The associated ESR is stable relative to temperature, and capacitance change is relatively immune to bias voltage. For improved performance, an 0603 1 µF ceramic AVIN filter capacitor is placed adjacent to the AVIN pin and referenced to AGND. Together with a 1Ω series resistor from PVIN (optional), this small capacitor helps to filter high frequency noise spikes on the supply rail and prevent these pulses from disturbing the analog control circuitry of the chip. 4.4 Soft-Start Capacitor A 10 nF soft-start capacitor has been chosen to provide a soft-start time of roughly 4 ms. This will allow the LM2854 to start up gracefully without triggering over-current protection irrespective of operating conditions. 4.5 Feedback and Compensation Components The voltage loop crossover frequency, floop, is usually selected between one tenth and one fifth of the switching frequency: 0.1 fSW ≤ floop ≤ 0.2 fSW (10) The complex double pole related to the LC output filter and zero due to the output capacitor ESR are as follows: fLC # 1 1 = 32.1 kHz = 2S LfCo 2S 0.82 PH 30 PF fESR # 1 1 = 1.7 MHz = 2S RESRCo 2S 3 m: 30 PF (11) A simple solution for the required external compensation capacitor, CCOMP, with type III voltage mode control can be expressed as follows, where the constant α is nominally 0.075 for the 1MHz option. Cc(pF) = D Lf(PH)Co(PF) floop (kHz) VIN (V) (12) Selecting a loop crossover frequency of 100 kHz yields: Cc = 0.075 4 0.82 30 100 = 33 pF 5.5 AN-1880 LM2854 1MHz Buck Regulator Demo Board (13) SNVA358B – August 2008 – Revised May 2013 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated Component Selection www.ti.com The upper feedback resistor, Rfb1, is selected to provide adequate mid-band gain and to locate a zero at or below the LC pole frequency. The series resistor, Rc1, is selected to locate a pole at the ESR zero frequency. Thus: Rfb1 = Rc = 1 1 = = 150 k: 2SCcfLC 2S 33 pF 32.1 kHz 1 1 = = 2.8 k: 2SCcfESR 2S 33 pF 1.7 MHz (14) Rfb1 and Rc are chosen as 150 kΩ and 2 kΩ in the demo board. With Rfb1 defined based on the voltage loop requirements, Rfb2, the lower feedback resistor, is then selected for the desired output voltage by: Rfb2 = 150 k: Rfb1 = 301 k: = VOUT 1.2V -1 -1 0.8V 0.8V (15) Note that Rfb2 has no impact on the control loop from an AC standpoint since the FB pin is the input to an op-amp type error amplifier and effectively at AC ground. Hence, the control loop can be designed irrespective of output voltage level. The only caveat here is the necessary derating of the output capacitance with applied voltage. The compensation was optimized to work over the full input voltage range. Many applications have a fixed input voltage rail. It is possible to modify the compensation to obtain a faster transient response for a given input voltage operating point. SNVA358B – August 2008 – Revised May 2013 Submit Documentation Feedback AN-1880 LM2854 1MHz Buck Regulator Demo Board Copyright © 2008–2013, Texas Instruments Incorporated 5 LM2854 Pin-Out 5 www.ti.com LM2854 Pin-Out NC 1 16 FB PGND 2 15 AGND PGND 3 14 SS PGND 4 PVIN 5 13 SW EXP 12 SW PVIN 6 11 EN PVIN 7 10 AVIN NC 8 9 NC Figure 3. LM2854 Pin-Out Table 1. LM2854 Pin Descriptions Pin Number 6 Name Description 1 NC 2,3,4 PGND No connect. This pin should be tied to ground in the application. Power ground. 5,6,7 PVIN Power voltage input. 8,9 NC 10 AVIN No connect. This pin should be tied to ground in the application. 11 EN Enable input. 12,13 SW Switch node. 14 SS Soft-start pin. 15 AGND 16 FB EXP Exposed Pad Analog voltage input. Analog ground. Voltage feedback pin. Exposed pad should be connected to ground. AN-1880 LM2854 1MHz Buck Regulator Demo Board SNVA358B – August 2008 – Revised May 2013 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated Bill of Materials www.ti.com 6 Bill of Materials Table 2. LM2854 Bill of Materials (VIN = 2.95V to 5.5V, VOUT = 1.2V, IOUT (MAX) = 4A) 7 Ref. Des. Function Description Case Size Manufacturer Manufacturer P/N U1 Buck Regulator Synchronous Buck Regulator HTSSOP-16 Texas Instruments LM2854 Cin Input Filter 100 µF, 6.3V B2, 3.5 x 2.8 x 1.9mm Sanyo 6TPE100MAPB Cin2 Input Filter Not Assembled 1210 - C3216X5R0J476M Co Output Filter 47 µF, X5R, 6.3V 1206 TDK Co2 Output Filter Not Assembled 1210 - - Lf Output Filter 0.82 µH, 14 mΩ 6.9 x 6.5 x 1.8 mm Vishay Dale IHLP2525AHERR82M01 Rfb1 Upper FB Resistor 150 kΩ 0603 Vishay Dale CRCW06032493F-e3 Rfb2 Lower FB Resistor 301 kΩ 0603 Vishay Dale CRCW06034993F-e3 Rc Compensation Resistor 2.0 kΩ 0603 Vishay Dale CRCW06031001F-e3 Rf AVIN Filter Resistor 1.0Ω 0603 Vishay Dale CRCW06031R0F-e3 Ren Enable Resistor 100 kΩ 0603 Vishay Dale CRCW06031003F-e3 Cc Compensation Capacitor 33 pF, ±5%, C0G, 50V 0603 TDK C1608C0G1H330J Css Soft-start Capacitor 10 nF, ±10%, X7R, 50V 0603 TDK C1608X7R1H103K Cf AVIN Filter Capacitor 1.0 µF, ±10%, X7R, 16V 0603 TDK C1608X7R1C105K Performance Characteristics 95 VIN = 3.3V EFFICIENCY (%) 90 85 VIN = 5.0V 80 75 70 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 IOUT (A) Figure 4. LM2854 Demo Board Efficiency VOUT = 1.2V SNVA358B – August 2008 – Revised May 2013 Submit Documentation Feedback Figure 5. LM2854 Transient Response IOUT = 0.4A - 4.0A - 0.4A AN-1880 LM2854 1MHz Buck Regulator Demo Board Copyright © 2008–2013, Texas Instruments Incorporated 7 Performance Characteristics 8 www.ti.com Figure 6. LM2854 Startup Characteristic Figure 7. LM2854 Turn On via Enable Figure 8. LM2854 Turn Off via Enable Figure 9. LM2854 Pre-Biased Turn On and Off via Enable VOUT(PRE-BIAS) = 0.6V AN-1880 LM2854 1MHz Buck Regulator Demo Board SNVA358B – August 2008 – Revised May 2013 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated LM2854 PC Board Layout www.ti.com 8 LM2854 PC Board Layout The demo board is based on a small 1.15” × 0.64” × 0.062” (29 mm × 16 mm × 1.6 mm) FR4 laminate PCB with two layers of two ounce copper. The top and bottom side layouts can be seen in Figure 10 and Figure 11. When looking at the top layer, pin 1 of the LM2854 is on the upper left. The PCB layout of the LM2854 evaluation board was designed to occupy as little board space as possible, while still following sound layout guidelines and techniques. The input capacitor, Cin, is placed as close as possible to the PVIN and PGND pins to minimize stray resistance and inductance between Cin and the LM2854. Likewise, the AVIN bypass capacitor is placed as close as possible to the AVIN and AGND pins. PGND and AGND are connected to each other and the ground plane at a single point, the exposed pad of the LM2854. Also, in order to help conduct heat to the ground plane and away from the LM2854, a 3 × 3 via array is used to electrically and thermally connect the exposed pad to the ground plane (instead of a single via). Additional ground plane vias are located close to the three PGND pins and in the localized ground plane emanating away from the exposed pad. Finally, the FB pin trace is intentionally kept as short as possible and routed away from the SW node to minimize any EMI pickup. Figure 10. LM2854 Top Side PCB Layout Figure 11. LM2854 Bottom Side PCB Layout, Viewed from Top SNVA358B – August 2008 – Revised May 2013 Submit Documentation Feedback AN-1880 LM2854 1MHz Buck Regulator Demo Board Copyright © 2008–2013, Texas Instruments Incorporated 9 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. 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